The Ancient Stone Implements, Weapons, and Ornaments of Great Britain/Chapter 25
ANTIQUITY OF THE RIVER-DRIFT.
In order to discuss this subject, it will be necessary to enter into some geological details; as it is evident that the least antiquity that can be assigned to the implements is that of the beds of gravel, sand, and clay in which they occur, and of which, in fact, they may be regarded as constituent portions. Whether they may not in some instances have been derived from beds of even greater antiquity than those in which they are found, is another question, which will subsequently be dealt with; but any one examining the condition of the beds in which the implements occur, will have no difficulty in seeing that they have not been disturbed since their deposit; while in most cases, the colouring of the worked and of the unworked flints they contain is similar, and affords proof of their having long lain together under the same conditions.
That the containing beds have, at all events in most cases, been deposited by fresh water, and not by the sea, is proved by the occasional abundance in them of land and freshwater shells, and the absence of those of marine origin; while their general analogy with the flood deposits of existing rivers, and their almost universal contiguity to them, raises the strongest possible presumption of their existence being due to river action. At the risk of being thought to have prejudged the question, I have, therefore, made no scruple in treating them hitherto as being River-drift. To show that for the most part they are so in reality, and to enable the reader to form some opinion of the manner in which deposits originally formed in and about the beds of streams or lakes, now in some cases occupy the tops of hills, and cover the slopes of valleys, far above the level of any existing neighbouring river, or even at a considerable distance from any stream, it will, I think, be well to state a hypothetical case; and then to compare the actual phenomena with it, and see how far they correspond.
Should it appear that with a certain given configuration of the land surface, a certain character of rock, a certain climate, and a certain number of years, certain effects must, judging from all analogy, have been produced; and should we in the case of these ancient Drifts find some of the conditions to have existed, and all the phenomena to be in accordance with the hypothesis, we may with some confidence assume that the other original conditions existed also; and build up a connected theory which will account for the whole of the observed results, and will also throw light on their causes, as well as on the duration of time necessary for their operation to have produced such effects. In stating the case, I lay no claim to originality, and do little more than follow in the steps of Sir Charles Lyell, Sir Joseph Prestwich, and others who have made a study of the character and effects of fluviatile action.
As it is in the gravels of Chalk districts that Palæolithic implements have been chiefly, though by no means exclusively, found, let us base the hypothesis on the assumption that an extensive and almost horizontal area of Upper Chalk, covered for the most part with beds of marine clay and shingle, gradually rose from beneath the sea, to an elevation of 200 feet above its level. Let us also assume that the land was elevated at a rate far in excess of that at which any subaërial action, such as rain, frost, or snow, would enable a river flowing over it to excavate its valley to the depth of 200 feet in the space of time required for its elevation to that height. Let us further assume, that the winter climate was some- what more rigorous than that which at present prevails in this country, and that there was a considerably greater annual rainfall. We may also, for the purposes of the argument, take the position of the coast-line as permanent, instead of its constantly receding in consequence of the eroding power of the sea upon the cliffs.
Let us now see what would theoretically be the effect produced by subaërial causes on the river-valleys in this area during an indefinite number of centuries.
Under ordinary circumstances, and with our present amount of rainfall, there is no geological formation less liable to floods than the Chalk, or at all events, its upper portion. It is of so absorbent a nature that it is only in the extraordinary event of the ground being hard frozen at the time of a heavy fall of rain, or of a rapid thaw of snow; or of some inches of rain falling in the course of a few hours, that the soil is unable to absorb the water as fast as it is delivered upon it. The moisture when once in the soil is either carried off again by evaporation and vegetation, or descends to a point at which the chalk is saturated with water, which is, however, constantly being drained off by springs along the valleys. This body of water has been termed "the subterranean reservoir" in the Chalk. The consequence of this absorbent power of the soil is that the streams and rivers in a Chalk country are not liable to floods, and moreover that their flow is but little affected at the time by rain; they being almost entirely dependent on perennial springs, which, during the driest of summers, still continue to deliver the water that in the course of the preceding winter, or even previously, has accumulated in the body of the Chalk.
The surface of the "subterranean reservoir" in the Chalk is by no means level, but always presents a gradient towards the point at which the springs are delivering its contents, so that within a chalk-hill forming a watershed between two streams there is what may be termed a hill of subterranean water, the summit of which need not, and often does not, correspond with the apparent watershed on the surface. The angle of the water-surface gradient depends principally on two factors, the degree of friction in passing through the chalk, and the amount of rain that finds its way down from the surface.
The height of saturation varies much in different seasons, as is evinced by the intermitting streams, often known as bournes, which perhaps only flow for a few months once in every six or seven years. Near the Chalk escarpment in Hertfordshire, at a spot several miles distant from any stream, I have known this height of saturation, as shown by the level of water in a deep well, to vary as much as 70 feet in the course of a single year. But with a greater rainfall than at present, the Chalk might at all times be in a state of saturation up to within a few feet of the surface; and this would be materially assisted, were there no deep valleys in existence into which the subterranean water could be delivered; as, of course, if the outfall were raised, the level of permanent saturation would be raised also. Were the Chalk in a less porous condition than at present, of course also its absorbent powers would not be so great. Under the circumstances, therefore, which have been supposed, the river- and spring-water from a Chalk district would be delivered in a manner very materially differing from that which at present prevails. The delivery of water by springs would be but small in shallow valleys; and, indeed, the only important springs would be those along the sea-shore; while irrespective of this, the greater rainfall would keep the soil so saturated, that floods would be as readily produced by heavy storms of rain as if the soil were the most unabsorbent of rocks. If after some lapse of time the rainfall diminished, and the valleys were deepened, so that the outlets for the springs were at a considerably lower level than that of the principal area of the country, the case would be altered, and the tendency to floods would be immediately reduced.
At the commencement of the state of things supposed in our hypothesis, these outlets, with the exception of those on the sea-shore, would be but little lower than the general surface of the country, which, however, would not be perfectly plane. For it seems probable that the waters of the retreating sea would, during the elevation of the tract of land, form shallow channels, cutting down some little distance into the clay or chalk; and thus, as it were, mark out a course along which streams or rivers would flow, after the land was completely free from the sea. In some places, perhaps, shallow lakes might be left, but these also would have channels draining off their waters when they rose above a certain elevation.
With a bare surface, such as a newly-elevated tract would expose, there can be no doubt that the eroding power of heavy rains would be highly effective; as may be seen at the present day in the far greater effects of heavy showers on bare soil than on that which is protected by turf and vegetation. At the same time, with a rigorous climate, such as that supposed, the winter accumulation of snow and ice would be great, and its thawing during the summer months would add enormously and rapidly to the streams draining the area, which would in consequence have great power to deepen and widen their channels. The outflows from the lakes, if any such existed, would also be enlarged, while their upper portions would be filled with material brought down by the streams, and eventually they would be drained, with the exception of some channels in their beds through which the streams would pass.
We may therefore readily suppose that in the course of no very great interval of time, geologically speaking, a river-system for carrying off the waters falling from the heavens, analogous in character to those of the present day, but with shallower valleys, would be formed on the surface of the elevated tract. Let us suppose that while this, as it may be termed, preliminary configuration of the surface has been taking place, the land has become tenanted by various trees, shrubs, and plants affording means of subsistence to different forms of animal life; while the streams also have been occupied by colonies of freshwater testacea; and let us now trace what would be the action of the rivers. To use the words of Sir Charles Lyell, "when we are speculating on the excavating force which a river may have exerted in any particular valley, the most important question is, not the volume of the existing stream nor the present levels of its channel, nor even the nature of the rocks; but the probability of a succession of floods at some period since the time when the valley may have been first elevated above the sea."
Now in the first place, all rivers whose banks are not artificially protected, and whose channels are not kept clear, are of necessity more liable to floods than those in civilized countries, which bear much the same relation to rivers flowing through uncultivated lands, as domesticated animals do to wild. We have, moreover, ex hypothesi, a fruitful source of floods in a greater rainfall and in a more rigorous winter climate. The marvellous effects of such floods in excavating channels, and in transporting materials, can only be estimated by those who have seen their results, or have studied the accounts given of them. When we read of a small rivulet on the Cheviots, swollen by heavy rain, having transported several thousand tons of gravel and sand into the neighbouring plain, and having carried blocks of stone, weighing upwards of half a ton, two miles down its course, while another block weighing nearly two tons was transported the distance of a quarter of a mile, we may form some conception of the effects of even a flooded brook. The blocking of a stream by ice or fallen trees, so as to keep back its waters, and thus form a lake, which is suddenly drained by the breaking of the barrier; a heavy fall of rain; or a rapid fall of snow on ground hard frozen, and therefore impervious, are common causes of floods; and such as we may presume to have prevailed in our hypothetical case. What, therefore, would be the effect of such floods?
The first effect would no doubt be to cause the streams to overflow their banks, and spread over the bottom of the valleys in which they usually flowed. The shallower the valley the greater probably would be the sinuosities of the stream, and the wider would its waters spread. The greater also would be the probability of the stream, on the cessation of the flood, not returning to its original channel, which might have become obliterated or filled up, but of its flowing along some new course, it may be miles away from its former channel. Even when not flooded so as to overflow their banks, rivers along which a larger body of water flowed than there does at present, would, so long as they were not confined within deep valleys, have a tendency to wander over a much wider tract of country than that now occupied by their valleys. The tendency of all rivers to produce sinuosities in their course is well known; but Mr. Fergusson, in his excellent paper on recent changes in the Delta of the Ganges, has called attention to the fact that all rivers oscillate in curves, the extent of which is. directly proportionate to the quantity of water flowing through them.
But rivers in a state of flood, or passing even at a moderate speed over soft or incoherent soil, are always turbid, owing to the presence in their waters of earthy matter which they are transporting towards the sea. The character of the solid matter thus transported by water in motion is entirely dependent on its velocity. A velocity of 300 yards per hour is sufficient to tear up fine clay; of 600 yards, fine sand; of 1,200 yards, fine gravel; and of a little over two miles per hour, to transport shivery angular stones of the size of an egg. Considering the small velocity requisite to remove the finer particles of the soil, and to retain them in suspension, a river such as has been supposed, must have been excessively turbid, so long as any fine earthy particles were accessible to its waters, or to those of the streamlets delivering into it.
The amount of solid matter suspended in turbid water is. greater than might be imagined. Mr. A. Tylor has calculated that the detritus carried down by the Ganges is equivalent to what would result from the removal of soil a foot in depth over the whole of the area which it drain sin 1,791 years, and that brought down by the Mississippi to one foot in 9,000 years. Other estimates fix this at one foot in 6,000 years, while the sediment contained in its stream has been estimated at from 1 to 1 of the weight of the water. Taking this latter proportion, an inch of rain falling on a square mile of ground, and flowing off it in a turbid state, would carry with it at least forty-three tons of sediment; and were we to assume an annual rainfall of fifty-four inches—which, though exceptional, is by no means unknown even in the British Isles—about 2,300 tons of fine earthy matter would be removed from a square mile of country in a single year. Taking a cubic yard of solid ground as equal to a ton in weight, this would involve the removal of one foot in depth from the surface in about 450 years. If, however, a portion of the rainfall were delivered by springs, or fell on hard or rocky ground, so as not to be rendered turbid, of course the effect would be proportionally diminished. Sir Archibald Geikie has estimated that practically, at the present day, the Thames (apart from about 450,000 tons of chalk and other matter carried away annually in solution), lowers its basin at the rate of one foot in 11,740 years; the Boyne, one foot in 6,700 years; the Forth, one foot in 3,111 years; and the Tay, one foot in 1,482 years. It is, however, with water moving with far greater velocity than that merely sufficient to keep fine sediment in suspension, that we have to deal in this hypothetical case; and we may readily suppose the streams, at more or less regular intervals, liable to violent floods, eroding the chalk and the superimposed clays and gravels, and carrying with them not only the finer particles and sand, but the pebbles, large and small, of the gravel, and the flints washed out of the chalk.
Let us now consider what would be the condition of the surface of a broad shallow valley, on the cessation of a flood such as that which has been supposed. In certain parts removed from the main current, and where the water had been nearly stationary, we should find deposits of fine mud or clay; in others, where the water had still moved with sufficient velocity to retain the clay and fine silt in suspension, the heavier particles of sand would have accumulated; in others, again, the smaller stones and pebbles; while near the main current, especially on the inner side of any curves which it had made, and where of course its velocity had been diminished, we should find the larger flints and pebbles, probably to some extent intermixed with part of the finer materials. In the beds of mud and sand, we should probably find the shells of some of the molluscs inhabiting the waters, and also those of terrestrial species, washed in from the inundated land surface, or brought down from the banks of the tributary rivulets; while mixed among the larger pebbles we might expect to find any animal bones that had been lying on the land contiguous to the stream, or any of the larger and heavier objects of human workmanship, that would have been carried off by such an inundation, had mankind been living on the banks of the river.
Were men, or any of the larger animals overwhelmed and drowned by the flood, it seems probable that, owing to the slight difference between their specific gravity and that of water, they would eventually have been carried down to the sea, unless by some means accidentally arrested in their course, or carried into the more stagnant waters. In either case, they would, on the waters subsiding, probably be exposed on or near the surface, and not be imbedded in any of the deposits of the stream. Assuming the existence at that time of a respect for the dead, such as may be regarded as almost instinctive in man, any human remains would be buried or otherwise disposed of, while the bones of the other carcases would be left within reach of the waters, should another flood occur.
At the mouth of the river, where it joined the sea, its excavating power would be considerably greater than farther inland; for at first, on account of the land having—as was presumed, in this hypothetical case—risen faster than the river could excavate its valley, the stream must have fallen as a cascade into the sea. This, by the cutting back of the lip in such a soft rock as the Chalk, would soon be converted into a rapid, where the greater velocity of the water would much add to its erosive power; and, ere long, a mouth to the river would be formed, which would soon become tidal. Before tracing the results that would be due to this greater declivity of the river-bed in the immediate neighbourhood of the sea, it will be well to consider what would be the results of successively recurring floods, in the less inclined broad shallow valley, on which we have been speculating.
There can be no doubt that with each succeeding flood the valley would be deepened; and the fact of its being thus deepened would tend to make it narrower, by restricting the windings of the river. We can, however, hardly imagine that in this deepening process the whole of the deposits spread by the former floods over the bottom and slopes of the valley would be removed, but must acknowledge the extreme probability of some portions of them having remained intact, especially those which were left at the greatest distance from the course eventually taken by the river during its period of flood. When once they had been thus left, the chances of their being again assailed by the stream would become more and more remote with each successive flood; and though the waters might reach some deposit of the larger pebbles formerly carried down by the main stream, but now at a distance from it, yet they would only belong to the more sluggish portions of the flood, and at first might envelope them in beds of sand; and subsequently, when they were only accessible to the more stagnant turbid waters, leave layer upon layer of muddy silt or clay upon them. In forming the more loess-like beds the action of the wind in transporting sand and dust might also assist. In some cases, and especially at the extremity of curves, and at the end of the tongue between two streams, the accumulation of one period, though at a lower level than that of earlier date, might abut upon it, or even become mingled with it, so that an almost continuous coating of Drift-deposits might extend from the highest level to the lowest.
The bulk, however, of the deposits of one inundation would be moved by the next, or by one of those which subsequently recurred; and stones, and pebbles, and other objects might thus be transported down stream, from place to place, an indefinite number of times, and form constituent parts of an indefinite number of gravelly beds along the bottom of the flooded stream. They might, under some circumstances, lie for a long period of years in some particular bed, in which they would become stained by salts of iron or otherwise, and subsequently be transported and re-deposited among unstained, or differently stained pebbles. The angles of any flints thus transported from place to place would also become rolled, as would, in like manner, those of bones or teeth. In the same way, assuming, as we have done, that the surface of the Chalk in the district was in part, or wholly, covered with beds of marine clay and shingle, it is evident that in the earlier deposits, when the river flowed at the higher level, and was, as it were, commencing to excavate its valley, the proportion of the pebbles derived from these beds to the flints washed out from the Chalk, would be much greater than at a later period. For in the course of time the river would have worked its way below the level of these upper beds, and many of the pebbles at first deposited in its gravels would have been disturbed, again and again, in their beds; on each disturbance carried farther down the stream, and eventually so far as the sea or the tidal portion of the river. At the same time the river itself would be principally excavating the Chalk which had been freed from the marine shingle, and would therefore be forming the gravel in its bed, for the most part, from flints derived from the Chalk.
In the same manner, pebbles brought from a distant part of the country, and higher up the river, would eventually become more abundant in the deposits near its mouth, than they were at the first. Still no amount of transport of this kind could bring any pebbles into the bed of the river, which did not, in some form or other, exist within its drainage area.
Besides the transporting power of water, which by itself is, under favourable circumstances, capable of producing considerable excavations in a comparatively short period, there is another force at work, where, as has been supposed in this case, the climate is severe, which not only aids in the transport of pebbles and blocks of stone from one part of the bed of a river to another, but is a fertile source of floods. This is the formation of ground-ice. Sir Joseph Prestwich, in his second "Memoir on the Flint Implement-bearing Beds," has given numerous instances of the transporting power of this agent, and shown the method of its occurrence in running streams, when the cold suffices to reduce the temperature of the water, and of the bed of the river itself, to the freezing point. Under such circumstances a gravelly river bed—and on mud alone, ice rarely forms—may become coated with ice, which being lighter than water will, on acquiring certain dimensions, overcome the forces which keep it at the bottom, and rise to the surface, carrying with it all the loose materials to which it adhered.
M. Engelhardt, director of the forges at Niederbronn, in the Vosges, has, perhaps, more minutely than any one else investigated the causes of the formation of ground-ice; and to prevent its effects in causing floods, actually removed each year from the bed of the stream supplying the motive power to his works, the stones and other extraneous bodies round which it was likely to form. His account of the effects of ground-ice in causing floods in the upper part of the Rhine and the Danube is worth transcribing. These two rivers having "a rapid current, do not freeze, like the Seine, by being covered with a plane and uniform stratum; they bear along large blocks of ice, which cross and impinge upon one another, and becoming thus heaped together, finally barricade the river. It is a grand spectacle, when the Rhine is thus charged, to see these countless drifts adjust themselves in their relative position, where they unite by congelation, and convey the idea of the fall of some mountain which has covered the plain with rocks of every dimension. But it is not this accumulation of ice-drifts in the Rhine which is of itself the cause of danger; it is, on the contrary, the débâcle, or breaking-up, which is often productive of calamitous consequences. When this débâcle commences in the upper part of the river, above the point where the latter is completely frozen, the masses of ice, drifting with the current and unable to pass, are hurled upon those already soldered together; thus an enormous barrier is formed, which the water, arrested in its course, cannot pass over, and hence overflows to the right and left, breaking the dykes, inundating the plains, and spreading devastation and suffering, far and near. The disasters caused by the débâcles of the Rhine have taught the riparian inhabitants to observe attentively the facts which may serve them as a prognostic, and put them on their guard against the irruption of the ice. It is thus that they have been led to observe the grund-eis—that is to say, the ice formed at the bottom of the rivers—for it is this ice which, in becoming detached from the bottom and rising towards the surface, unites itself to the under surface of the masses already in place, and by further embarrassing the discharge, exposes the country to inundation."
Another most effective agent in transporting the pebbles and larger blocks of stone along the course of rivers is shore-ice. During a severe winter masses of thick ice are formed which enclose the larger stones on the bottom of the river towards its edge; these masses are dislodged and carried away by subsequent floods, whether arising from rapid thaws or from rain higher up the river, or from accumulations of ice, such as those described, having formed a temporary barrier across the stream through which the pent-up water eventually burst and carried all before it. The lateral pressure of such dams of ice, with a large body of water behind, must be enormous; and we can readily conceive their crumbling-up any beds of gravel on the banks of the rivers against which they might happen to abut.
But there is still another way in which a severe climate, such as has been supposed, would act upon the rocks, namely, by their being rent and disintegrated by frost. This has been well pointed out by Sir Joseph Prestwich, who has cited numerous instances of its effects, and mentions having seen a low cliff of chalk, 15 feet high, form a talus or heap of fragments at its foot, 6 feet broad and 4 feet high, in the course of an ordinary winter.
As I am by no means attempting an exhaustive geological essay on this subject, which is indeed hardly needed, I think that enough has been said to show that under conditions such as have been supposed in this hypothetical case, the great subaërial agents—rain and snow, ice and frost—would, in the course of time, enable rivers to excavate their valleys to an almost indefinite extent. Indeed, one can conceive the process being carried on, until what had been rivers became estuaries or arms of the sea; or, until a large island once traversed by rivers became converted into several smaller islands, by the cutting back, and subsequent junction, of its various river-valleys.
Without, however, carrying the excavatory process to such an extreme, let us now consider what would be the condition of our hypothetical river-valley when excavated to a depth of say 100 feet, at a point about midway between its source and the sea. We have already seen that at an earlier period—when the river ran at a higher level by 100 feet than that it is now supposed to occupy—its valley must have been broader, and its bottom strewn with detritus of various kinds, in the shape of gravel, sand, and clay, and, it may be, some larger blocks of stone. In the further process of excavating by agents such as have been described, it has also been seen, that it is in the highest degree improbable that the succeeding floods and other transporting agents should have entirely removed and obliterated the deposits left by those of earlier date. We should, therefore, expect to find, at various heights on the slope of the valley, remains of such beds of detritus, and especially at points such as the junctions of affluents with the river, and the inner side of the bends it makes in its course, which would naturally be the least exposed to the violent invasion of the stream. In these beds we might reasonably search for the remains of the surface and freshwater life of the period; and had there been any amelioration of climate during the process of excavation, a larger proportion of silt and clay, and less of coarse gravel, in the lower and more recent deposits, would testify to the fact. Looking also at the power possessed by rivers of levelling the bottoms of their valleys, during their successive changes of course, we might expect to find in places, tracts of these old valley-bottoms left as terraces on the slopes of the more deeply excavated valleys. The upper surface of any such relics of a former condition of things would, of course, be covered with débris and rain-washed clay, brought down from a higher level on the slopes, but on digging into them their true nature might be recognized.
Nearer the sea, and farther up the valleys, the state of things would be somewhat different. At the mouth of the river, as has already been pointed out, the declivity of the stream would have been greater, audits excavating power therefore increased. If, as originally assumed, the bed of the river, when the land was first elevated, was, at a mile distant from the sea, 200 feet above its level, the declivity would be 200 feet to the mile; when the 200 feet level was 4 miles from the sea, the slope would still be 50 feet to the mile; at 10 miles distance it would still be 20 feet, and it would not be until the 200 feet level was 15 miles from the sea that the ordinary slope of the bottom of the Chalk valleys of Hertfordshire, which is about 13 feet 6 inches to the mile, would be attained. In the meantime, however, if the sea were encroaching on the shore, or were, owing to the nature of the rocks, widening and extending that portion of the river subject to tidal influences, the actual point of contact with the sea would be carried far inland, and—assuming the rock traversed to be of one uniform nature and hardness—it would be long before the river towards its mouth ceased to have a greater declivity than nearer its source. We see, then, that the amount of excavation effected by the river, during the time necessary for the deepening of the valley by 100 feet, at a point midway in its course, would, near the sea, have been twice as great, or 200 feet. We should, therefore, expect to find beds of the same age as those which, at the middle of its course, were 100 feet above the river, at relatively twice that elevation near the mouth; and any intermediate beds would also be proportionally higher above the then existing stream, than contemporary beds farther up the valley.
At the heads of the valleys, the excavation would, on the contrary, have been less than towards the middle of the course of the river; partly owing to there always being less water present, partly to the reduced liability to floods, and partly to other causes. The heads of the valleys would, however, be constantly receding in all cases, and their retrogression would in most instances be aided by springs issuing from them. In cases where, from some geological cause, the heads of two valleys running in opposite directions receded in the same line, we can readily imagine their meeting eventually at the watershed, and cutting through it so as to form apparently but a single valley, though on either side of the highest portion of its bottom, the waters flowed in opposite directions.
The mention of springs recalls another denuding agent, which has been already discussed in connection with caverns, and seems to have assisted in moulding the surface of the country and in excavating the valleys. It is well known that the water flowing in the streams of a chalk-country contains, in solution, a considerable amount of chalk, or rather, of bi-carbonate of lime; the water on entering the ground deriving a certain amount of carbonic acid from the decaying vegetable matter contained in the soil, and when thus charged, becoming capable of dissolving a corresponding quantity of the chalk. The amount is usually 17 or 18 grains in the gallon; and even in the Thames at London, not a purely chalk - stream, there are about 14 grains. Taking the proportion of 17 grains to the gallon, it will be found by calculation that every inch of rain which falls over a square mile of chalk-country, and passes off by springs, carries with it, in solution, and without in the slightest degree interfering with its brightness, no less than from 15 to 16 tons of solid chalk. The quantity of rain which thus finds its way to the springs has, as already stated, been ascertained by experiment to be as much as 9 inches per annum in average seasons, giving an amount of about 140 tons of chalk thus annually carried away from each square mile of country at the present day; so that the loss is still going on at the rate of 140,000 tons of dry chalk to each square mile in every ten centuries.
The lowering of level from this cause is probably not uniform over the whole surface. For the acidulated water sinking into the chalk on the top of a hill, and descending one or two hundred feet before reaching the surface of "the subterranean reservoir," might, in its almost vertical passage, become saturated with carbonate of lime, and only render the chalk through which it passed somewhat more porous, without materially affecting the level of its surface. On the other hand, that absorbed in a valley would probably, to some extent, acquire the chalk which it eventually held in solution during its almost horizontal passage to the point of its delivery by springs; and as this would be at no great depth, the abstraction of solid matter would become more perceptible on the surface, so that the level of the valley would be lowered more rapidly than that of the hill. With an increased rainfall, such as we have supposed, this removal of solid matter by solution must have been considerable; but still nothing in comparison with that effected by the other denuding agencies which have been mentioned. It is, moreover, to be borne in mind that, as will shortly be seen, until the valleys had been excavated to a considerable depth, the amount of water delivered by the springs would, with the same rainfall, have been far less than at present. The springs would also, to some extent, have been affected by the chalk being in a less porous condition than it now is, owing to its not having lost so much of its substance by the chemical action which has just been described.
Before comparing the actual phenomena with the results of the conditions which have been assumed, it will be well to say a few words as to the probable effects of an amelioration of climate, and a diminution in the rainfall, upon a valley already excavated to an average depth of 100 feet, such as has already been described. It is evident that any transport of materials due to the action of ice, by floating loose stones and pebbles from one part of the bed of the stream to another, would be materially diminished; as would also the number of floods resulting from the thawing of the winter accumulation of ice and snow, and from rain falling on frozen ground. The only remaining principal cause for floods would be the heavy fall of rain during storms or wet seasons; but here, a comparatively slight alteration in the conditions will have made a vast difference in the results. When the valleys were once excavated to a certain depth, the level of the springs or outfalls carrying off the accumulation of water in the absorbent soil, would be proportionally reduced, as would also be the line of permanent saturation in the chalk. The effect of this would be that during any dry interval, the water contained in the upper part of the chalk would gravitate downwards, until it reached the subterranean reservoir of water saturating the chalk; and thus leave the surface soil in the same absorbent condition as it is at present, and capable of receiving a much greater amount of rain than formerly, before any would flow from off its surface.
Even with a constant and excessive rainfall, the result of the continued deepening of the valleys would be to cause more and more to flow off by the springs, and less from the surface; but with the valleys once deepened, a small diminution in the rainfall, or its more even distribution over the whole year, might cause the flow from the surface almost entirely to cease, and allow the whole to be carried off by the springs. Whenever this was the case, any great and rapid excavation of the valleys from rain alone would be rendered almost impossible; and with no extreme reduction in the total amount of annual flow of the rivers, yet by their originating in perennial springs subject to but slight variations, and from their being no longer to any extent immediately connected with the surface drainage, there would cease to be that immense difference between their maximum and minimum volume, which must have formerly existed. The result of this comparatively uniform flow would be a great diminution in the tendency of any river to change its bed, and even if it occasionally received a great accession of water, it would find relief by overflowing into the wide valley due to its former more violent action. In the less inclined portions of its valley, the parts now almost deserted by the stream would be favourable for vegetation, such as would result in the formation of peat, and any occasional overflowing of the banks might, owing to the less torrential character of the inundations, have a tendency to fill up and level these marginal spaces rather than to excavate them deeper. The deposits of gravel, sand, and clay at the low levels would also be more continuous than those at the higher.
In tracing the effects of subaërial action in forming valleys, I have assumed the subsoil or rock in which they were formed to have been chalk, as it is principally in valleys in the Chalk that the gravels containing Palæolithic implements are known to occur. This is probably on account of the greater natural abundance of flints in such valleys, which of course led to implements being there chipped out in greater numbers, as well as to their being less cared for, from their being more easily replaced than they would be where flint was scarce. The effects on other soft and absorbent soils would not materially differ from those on chalk. On clay, the general amount of denudation would perhaps be greater, but the valleys broader, and with less inclined slopes on their sides. In a clay country we might, I think, expect to find the old river-gravels not unfrequently at greater distances from the existing streams than in a chalk-district.
It must, however, be borne in mind that in such a country the materials from which river-gravels can be formed are usually absent, and can only have been derived from older superficial beds, or brought from Chalk higher up the valley. In some valleys, partly or almost entirely excavated in Pre-Glacial times, gravels belonging to the Glacial Period exist, and tend to complicate the question of the more recent River-drifts.
Any theory of the valleys having been excavated at some remote period in some unknown manner, and then having been filled with gravels derived from an unknown source, and again re-excavated, presents such difficulties that, to my mind, it cannot well be entertained. If, however, such a view be accepted, it seems to add to the time necessary for the excavation of the valleys; as much of the rainfall might find a subterranean vent at a low level through the gravel lining the bottom of the filled-up valleys, and thus keep the upper soil in a more absorbent condition and therefore less liable to erosion.
I must not, however, dwell too long upon this hypothetical case, which perhaps is such as may not have found an absolutely exact analogue in nature, but which may yet, I think, be accepted as a fair typical example of the results which, under the supposed conditions, must, judging from what we know of the action of subaërial causes, in all probability have ensued.
Let us now compare the phenomena as we find them in the gravel-beds of our present river-valleys, with those of the hypothetical case, and we shall, I think, find them coincide in a remarkable manner.
In the first place, the constituent parts of the gravels of the beds of Drift containing Palaeolithic implements are always, petrologically, such as are to be found in the existing river-basins, as they must also of necessity have been in the hypothetical case. This fact, which holds good both in France and England, has been insisted on by Sir Joseph Prestwich, and such insistency cannot be too often reiterated. Where old superficial marine deposits of the Glacial or any other period, consisting of pebbles of various ages and origins, exist within a river-basin, there also will such pebbles be found in its gravels, but the originally derivative character of the pebbles prevents any strong argument being founded upon their presence. Where, however, no such beds exist, the case can clearly be made out. Unless a river traverses a granite or slate country, no granite or slate is found in the Quaternary gravels of its valley: unless it passes over Oolite, Purbeck, or Greensand, no blocks or pebbles of these rocks occur. This fact suffices to prove that the gravels are due to some local cause, such as river-action, and not to any general submergence or supposed "wave of translation," which would of necessity bring in materials not to be found in the existing basins.
That the various deposits resulting from a flooded river, should contain some of the land and freshwater shells, and animal bones of the period, is, as has been shown, most natural. Such shells and remains are of constant occurrence in the Quaternary gravels. If they prove nothing else, their evidence as to the freshwater origin of the beds must be accepted as conclusive. It is true that in all cases such land and freshwater remains have not as yet been found; but if in a dozen instances we find beds of a certain character containing these remains, and also flint instruments wrought by the hand of man; and in a dozen other instances, similar beds in analogous positions, also containing implements of the same kind, but, so far as is known, no such organic remains; we are justified in regarding both sets of beds as due to the same original cause, and in believing that the organic remains, if actually absent, are so from some accidental circumstance. We may indeed accept the implements as being truly characteristic fossils of a certain class of deposits. The character of the beds, consisting as they do, of gravel, sand, and fine silt, brick-earth or loess, and their manner of deposition, are also absolutely in accordance with the river-hypothesis.
On the higher levels above but near the valleys, we frequently find these beds at a considerable distance from the existing stream; we find them at all levels on the flanks of the valleys, and occasionally almost at their bottom, or even below it. In these lower beds, the implements, if of the same form and character as those in the upper beds nearer the source, are, in accordance with what would be the case under the hypothesis, very frequently much rolled and water-worn. The beds at the low level are also usually, so far as the gravel is concerned, of a finer character than those at the high level, and present a greater abundance of sand and brick-earth. They seem, in fact, indicative of some such amelioration of climate as that supposed.
Looking again at the position of the deposits with regard to the neighbouring rivers, we find them, as a rule, exactly in such positions as might have been expected, had their presence been due to the action of a stream in the process of excavating its valley, in such a manner as that described. So constantly is this the case, that a practised geologist, from a mere inspection of the Ordnance map, could with almost certainty predict where deposits of River-drift would occur, of such an age and character as to be likely to contain Palæolithic implements. In more than one instance, indeed, as has already been mentioned, the probability of certain gravels containing these relics of human art, was pointed out before their actual discovery.
These are some, but by no means all, of the points in which the actual phenomena agree with those which must have resulted from river-action such as suggested in the hypothesis, and they are alone sufficient to raise the strongest presumption that the phenomena are due to such action, and that the theory that would account for them in this manner, cannot be far from the truth.
I will, however, now pass in review some of the principal localities where Palæolithic implements have been found in Drift-deposits, and see what other points of accordance, and what difficulties, if any, they present.
Taking first the basin of the Ouse and its tributaries, we find at Biddenham, near Bedford, one of the principal localities for Drift-implements, the gravel on the inner side of a bold sweep made by the river, and from forty to fifty feet above it. Its constituent stones are all derived either from the rocks in the neighbourhood, or from the Glacial beds which cap them, and which have evidently been cut through by the river. Throughout the beds are seams containing numerous freshwater shells, mixed with some derived from the land and from marshy places; numerous bones of terrestrial mammals also occur. In the valley of the Lark remains of such shells occur at Bury St. Edmunds, in the same beds as the implements. Farther down, at Icklingham, the beds at Rampart Field cap a rounded knoll on the inner side of a curve of the river, which appears, however, to have somewhat straightened its course since they were deposited. Below Icklingham, the whole surface of the country, and its drainage, have been so much modified by the invasion of the sea, which produced the wide level of the Fens, that we should expect to find any deposits of an ancient river, which existed before that great planing down of the adjacent country, in somewhat anomalous positions.
I need not here enter into the history of the origin of the Fens; it is enough to say that the subsoil of almost the whole district consists of clays, belonging either to the Oolitic or Cretaceous series, and unprotected by any rocks of a more durable nature towards the sea, which has thus been enabled to invade it. The presence of the sea is attested in various localities by marine remains. Buccinum, Trophon, Littorina, Cardium, and Ostrea are abundant in the gravel at March. In the valley of the Nene, near Peterborough, oysters and other marine shells occur, mixed with those of land and freshwater origin. In Whittlesea Mere, remains of walrus and seal, and sea shells are found; while so far south as Waterbeach, less than ten miles from Cambridge, remains of whale have been discovered.
The old land-surface having been thus destroyed, we cannot with certainty trace the course of the ancient representative of the river Lark, below Mildenhall; it seems, however, to have proceeded northwards by Eriswell and Lakenheath, to join the Little Ouse. At Eriswell, a gravel of the same character as that near Mildenhall, occurs on the slope of the hill towards the Fen; but in it, as yet, few implements are recorded to have been found. At Lakenheath, however, they occur in the gravel now capping the hill overlooking the Fen, as well as on the slope.
Owing to the distance of these beds from any existing rivers, the late Mr. Flower found great difficulty in reconciling them with any theory which would account for their presence by the action of rivers. If, however, we regard the great denudation of the Fen, country as subsequent in date to the deposit of the gravels, it appears to me that any difficulty on this point vanishes. That this denudation was in fact, at all events in part, subsequent to the deposit of the gravels, is proved by the position of the beds at Shrub Hill, which there cap a small area of Gault, and which, being above the general level of the Fens, can hardly have been deposited in the position they now occupy, when the configuration of the country was at all like what it now is. Such beds must, on the contrary, have been deposited in the bottom of a valley; and it appears as if in this case, by their superior hardness to the clay around them, or from some other accidental cause, they had protected this small spot from tidal action, which in the adjacent river, previously to the construction of Denver Sluice, extended nearly as far as Brandon.
The rolled condition of so many of the implements found at Shrub Hill, proves that they must have been transported some distance by water, from beds of a higher level.
Turning now to the existing valley of the Little Ouse, we find, at Brandon Down, the gravel occupying the summit of a high ridge of land almost at right angles to the present course of the river. It is difficult to account for its occurring in this position, unless we are to suppose that at an early period before the complete denudation of the Fen country, and while the Boulder Clay still covered the surface of the Chalk, and the level of saturation was higher in the latter than at present, a tributary stream, possibly the old representative of the Lark, flowed into the Little Ouse near this spot, and the gravel was deposited on the tongue of land near the confluence. The country drained by the Little Ouse seems at one time to have been almost covered by Glacial deposits, including beds of shingle, composed for the greater part of quartzite pebbles. The beds at Brandon Down are nearer the sea than any analogous beds towards the source of the stream, and occupy a higher position relatively to the existing river, being 90 feet above it. If they resulted from river-action, they would, in accordance with the hypothesis, be among the oldest of the river- deposits; and would, as indeed they do, consequently contain a far larger proportion of the quartzite pebbles than those of somewhat later age and farther up the valley.
At Bromehill, where the drift is but a few feet higher than the present level of the stream, and would, in accordance with the hypothesis, belong to a later period, there are but few of these quartzite pebbles, but the gravel contains a very large proportion of rolled fragments of chalk, which, so far as I have observed, are absent in the probably older beds, at Brandon Down; the implements also are frequently much rolled and water-worn. This fact is also in accordance with the hypothesis, for the river at the time of the formation of these lower beds would, in the lower part of its course, have completely cut through the Glacial deposits above the Chalk, and would have been attacking the Chalk itself. There is also an abundance of rolled chalk in the Shrub Hill beds, which seem to be of much the same age. In the valley of the Lark, the rolled chalk pebbles occur in gravels at a somewhat greater elevation. Higher up the Little Ouse, the gravel at Santon Downham occupies the slope of a hill on the inner side of a great sweep of the river, while at Thetford, the beds form a long terrace by the side of the stream, with a rather abrupt slope towards it. Here also, land and freshwater shells have been found in the gravel, but neither these nor implements have as yet been observed in the gravels of the valley of the Little Ouse, or of its tributaries, above Thetford.
Tracing the main stream back to its source, we find that both the Little Ouse and the Waveney, the one flowing westward, and the other eastward, take their rise in the same valley, and within a few hundred yards of each other, at Lopham Ford. With regard to the elevation of this spot above the sea-level, there has been some diversity of opinion. On the Greenough map, published by the Geological Society, it is erroneously stated at 15 feet; and Mr. Flower, in arguing in favour of his views, that the beds at Brandon are not connected with any river-action, assigns it a height of only 23 feet above high-water mark. That this also is erroneous can be readily shown, for Sir Joseph Prestwich has recorded the level of the Waveney at Moor Bridge, near Hoxne, ten miles below its source, as being 59 feet 9 inches above high-water mark at Yarmouth. Mr. Alger, of Diss, who has surveyed the district, informs me that the level at Lopham Ford is 75 feet 3 inches above high-water mark; and as by actual survey he found the fall, from the head of the Waveney to Hoxne Mill, to be upwards of 15 feet, there can be little doubt of this level being approximately correct. Still, the gravel beds at Brandon being upwards of 90 feet above high-water mark, there can be no doubt of their being at an elevation actually above the source of the present stream; and at first sight, this fact appears difficult of reconciliation with the view that they are due to fluviatile action. Without, however, calling to aid any possible oscillations in the level of the land, varying in amount at different parts of the course of the stream, an examination of the local geological conditions suffices to throw light on the causes, why the erosion of the land at the sources of the Little Ouse and Waveney has been abnormally great; so that not only have the streams excavated back the heads of their respective valleys until they have met, but their inclination at the upper part of their course, instead of being as usual in chalk countries at the rate of 12 to 18 feet in a mile, is only about 18 inches.
The general level of the country for some distance around Lopham Ford is at least 100 feet above it, and the Chalk and the superimposed beds are for the most part covered with a deposit of impervious Boulder Clay, through which the valleys of the Little Ouse and of the Waveney have been cut. But, at the time of the last emergence of this district of country from beneath the sea, this clay must have been continuous across the tract since excavated, so that at that time the sources of the streams flowing in either direction must have been at least 100 feet above their present level, and 80 feet above the gravels at Brandon Down, and probably at some distance apart. That the heads of the two streams should have cut back their valleys, and at last have met, appears to be due to the fact that, previously to the covering of Boulder Clay being deposited, there existed an old depression in the Chalk, which had been filled with laminated sandy clays, either Glacial or belonging to what is known by geologists as the Chillesford series. These being more easily acted on than the chalk by running water, led the streams to follow the course of the old depression which they filled, and it is to their presence that the small inclination of the upper part of the valley of the Waveney appears to be mainly due. Another cause is to be found in the country near Lopham Ford being coated with clay, so that the streams, even at the present day, exhibit the remarkable phenomenon of being liable to floods at their source. An isolated hill, about 30 feet high, formed of the laminated beds, and with a slight capping of gravel, still remains in the valley of the Waveney, near Redgrave, to show the nature of the beds which have been removed.
The only spot in the valley of the Waveney, where as yet Palæolithic implements have been found, is at Hoxne, where the summit of the beds is about 111 feet above high-water mark at Yarmouth, and though at a higher level than the existing source of the Waveney, probably much below the level of its earlier source. Since the beds were deposited, the surface of the ground in the neighbourhood has been completely remodelled by sub-aërial denudation, and they now lie in a trough on the summit of a hill, both sides of which slope down to small streams which are tributary to the Waveney, and are still at work cutting out their valleys in the Boulder Clay. The beds in which the implement occur are beyond all doubt of freshwater origin, being full of freshwater shells. The trough in which they lie, has much the appearance of the deserted bed of a river, silted up under more lacustrine conditions. Such a change in the position of a river-bed, and its subsequent infilling, is quite in accordance with the hypothetical case of river-action, especially when, as here, its eventual valley had not been distinctly carved out.
The phenomena at Hoxne have lately been more fully examined by Mr. Clement Reid, by means of grants from the British Association and the Royal Society; and the views that I expressed in 1872 have been in the main corroborated. The deposits are proved to be distinctly more recent than the Chalky Boulder Clay of the district, and there is evidence of oscillations in climate since the valley was formed in which the lacustrine beds were laid down, and before any Palæolithic implements or the brick- earth containing them had been deposited.
The beds at High Lodge, near Mildenhall, are of somewhat similar character to those at Hoxne, though occupying a depression on the slope of a hill, instead of a trough on the summit; and were probably deposited under nearly the same circumstances, though as yet no testaceous remains have been found in them.
Turning south, to the valley of the Thames, we find the gravel-beds at Acton and Ealing, though occasionally at a higher level, forming a terrace 80 or 90 feet above Ordnance Datum, along the side of the broad valley, at a height of some 50 feet above the general surface of the valley. In the bottom of this are spread out other beds of gravel, sand, and brick-earth, exactly as might be expected on the river-hypothesis; while at Highbury New Park, and Hackney Down, we have beds of the same character, which contain land and freshwater shells and flint implements, at a height, in some cases, of 100 feet above Ordnance Datum. The presence of these beds in such a position, consisting, as they do at Highbury, of sand and brick-earth, such as can only have been deposited in comparatively tranquil water, involves the necessity either of a large lake having existed at the spot, or of its having been within access of the flood-waters of the river. But either of these conditions is impossible, unless we are to suppose that the lower part of valley of the Thames, in which London now stands, was at that time non-existent. It must, therefore, have been subsequently excavated. But again, at lower levels at Hackney Down, and in Gray's Inn Lane, we have gravels of a more distinctly fluviatile character, and also containing palæolithic implements. The existence, character, and position of all these beds is, therefore, perfectly in accordance with the theory of the excavation of the valley by the river, and it is extremely difficult, if not impossible, to account for them satisfactorily in any other manner.
At Hitchin beds of much the same character occur, which there also are newer than the Boulder Clay of the district.
At Caddington the discoveries are quite consistent with the hypothesis, but point to a period when the excavations of the existent valleys had made but little progress.
Higher up the Thames valley at Reading and at Oxford the phenomena are all in accordance with the hypothesis; at the former place the river has deepened its valley to the extent of at least 100 feet.
The discoveries in the gravels capping the North Downs and those made near Ightham and Limpsfield in the transverse valley at the foot of the Downs, seem at first sight difficult to reconcile with any river-theory. But assuming that the beds capping the hills were at one time continuous with others in the Wealden area, and that the transverse valley was produced by denudation at a later date, the difficulties disappear, though the time requisite to effect such superficial changes may seem to be immense.
Passing by other localities where implements have been found in the valley of the Thames, such as Swanscombe and Northfleet, though it may be observed that the gravels in which they have occurred are, on the river-theory, exactly where they might have been expected to be present, we come to the beds near Reculver, where they have been found in large numbers. Looking, however, at the enormous encroachment of the sea, even within the last few centuries, upon the soft cliffs of sand and clay at that spot, it is difficult to form any satisfactory idea of the conditions under which a river may have flowed near the spot at a remote period, or of the position of the coast at the time. Where, however, as is here the case, a large tract of land has been washed away, which must of necessity have had its system of superficial drainage by streams, and may possibly have had rivers passing through it, which now, owing to the altered conditions, find their way into the sea at a point much nearer their source than formerly, we should expect to find on the top of the cliffs traces of the former state of things; and where any portion of the slope of an old valley remained, to see its gravels, though now so close to the sea, at a height far above its level. Still, it is hard to say whether the implement-bearing beds at Reculver are connected with the old valley of the Thames, or with that of some other stream which has now disappeared, but of which the upper portion is to be traced in the Swale, which now separates the Isle of Sheppey from Kent, and which appears to afford, in its junction with the West Swale and Long Reach, an instance of two valleys being gradually eroded inland until they met. The beds may even be connected with the valley of the Stour; for it is by no means impossible that the present second and northward mouth of that stream may run along the valley of an old river, which originally flowed southward past Reculver, and joined the old representative of the Stour, somewhere to the south of where is now the village of Sarre.
The great tract of gravel which at some little distance inland fringes the East Essex coast, between Shoeburyness and the Blackwater estuary, may also be connected with some old river; but as yet no well-defined implements or freshwater shells have been found in it, though Mr. Whitaker has discovered shells near Southend. The fluvio-marine deposits at a lower level at Clacton, just north of the Blackwater, like those at Chislet, in Kent, seem to belong to a somewhat later period, when the rivers had so far deepened their beds as to have become tidal.
Though no land or freshwater shells have as yet been found in the gravel beds near Canterbury, yet their position is quite in accordance with the theory of the excavation of the valley by river-action; and here as elsewhere the implements from the lower beds are often much water-worn.
The superficial deposits of the south of Hampshire and the Isle of Wight, and in a lesser degree those of the neighbouring counties, have been fully discussed in an able paper by Mr. T. Codrington, F.G.S., though since it was published a large number of implements has been found near Bournemouth, Barton, and Hordwell. He has pointed out that the whole of the New Forest, between Poole and Southampton Water, appears at one time to have been an extensive plain, with a gradual slope to the south, very generally covered with gravel and brick-earth. This has since been in great part cut up, and over large areas entirely removed by the action of the streams and rivers, which latter flow in well-defined valleys.
The formation of this table-land and the overlying deposit of gravel which, in places far inland, is found at a height of more than 420 feet above the present sea-level, appears to be due to marine action, though as yet no marine remains have been discovered in it. Sea-shells have, however, been found by Sir Joseph Prestwich in an old sea-beach at Waterbeach, near Goodwood, and similar beds, at Avisford Bridge, near Arundel, occur at a height of 80 or 100 feet above the sea. We seem, then, here to have evidence of a considerable elevation of the land from beneath the sea; and as the gravel in places overlies late Tertiary beds, this must have taken place at a comparatively late geological epoch. When rivers run through a tract of country covered with a marine gravel of this kind, itself apparently deposited in a somewhat contracted area, it is, in the absence of organic remains, difficult to distinguish the reconstructed gravels resulting from fluviatile action, from the older beds. Any one, however, who is acquainted with the country, or who will examine Mr. Codrington's map, will see what an enormous denudation has been effected in this great sheet of gravel, by rivers and streams, and by subäerial action. When once the protecting gravel has been cut through, and the soft Tertiary beds of sand and clay below have been reached, the process seems to go on with great rapidity, A large tract of land west of Southampton appears to have been in this way almost cleared of its gravel, of which but patches are left. Even the principal portion of the old table-land which has survived, that to the east and south-east of Fordingbridge, is deeply cut into by numerous valleys, many of a depth of 200 feet. The existence of these valleys is clearly in accordance with the river theory.
Let us now examine the discoveries in the valleys of the Test and of the Itchen from this point of view. Looking at the numerous instances of the finding of flint implements in gravels containing terrestrial and freshwater remains, and looking at the improbability of their occurring in a purely marine deposit, I venture to regard them as being equally characteristic of freshwater deposits as any organic fossils, and to claim the beds in which they occur as being of freshwater origin.
At Southampton several implements have been found in the pits upon the Common at heights ranging from 80 to 150 feet above the sea-level. The gravel there slopes at a considerably greater inclination than that of the table-land nearer Chilworth, with which it is continuous, and from which it would appear to have been in part derived. It occupies a tongue of land between the valley of the Itchen and that of the Test, now widened out by tidal action. It is in places covered by brick-earth, and its position and character are quite in accordance with a fluviatile origin. If, from their proximity to the apparently marine gravels, we assume these beds to belong to an early period in the history of the excavation of the valley, their high position above the present tidal stream is such as, according to the hypothesis, was to be expected.
The gravels found lower down the course of the river, at Hill Head, Brown Down, and Lee on the Solent, appear to belong to a somewhat later period; and to bear much the same relation to those of Southampton Common, as do the beds at Shrub Hill to those of Brandon Down. As I pointed out long ago, "There can be but little doubt that these gravel beds are merely an extension of the valley-gravels of the rivers Test, Itchen, Hamble, and other streams, which at the time they were deposited, flowed at this spot in one united broad stream, at an elevation of some forty feet above the existing level of their outfall, over a country which has since, by erosive action, been in part converted into the Southampton Water." We shall shortly have to revert to this circumstance; but before returning to the coast, we must take a short glance at the features of the discoveries near Salisbury.
In the neighbourhood of this city there can be no doubt of the deposits being thoroughly in accordance with the river theory. The Fisherton and Milford Hill beds occupy points or spurs of land, in the forks above the junction of streams, or precisely those spots in which their presence was to be expected. There are the usual beds of gravel, sand, and clay, the usual bones of the Quaternary fauna, some representing what are now Arctic species, and therefore presumably indicative of a severer climate than at present; and the usual land and freshwater shells. Though the valleys, being confluent, are excavated to the same depth, yet, on examination, their sectional areas will be found to be approximately proportional to the extent of country drained by the rivers still flowing through them. At Milford Hill, the deposit is cut off from the main spur of land by a kind of transverse valley, about thirty feet in depth, besides having on either side a valley some 100 feet deep. On any hypothesis of the beds having been deposited by aqueous action—and no other can for a moment be entertained—these valleys must have been mainly excavated since the deposition of the gravels. For had the valleys at that time existed, we can conceive of no conditions under which a body of water sufficient to fill the valleys to their summit, and able to carry along detrital matter with it, would leave its heavy contents at the top of the hills instead of at the bottom. The old fluviatile beds occur also at various levels on the slopes, in complete accordance with the theory of gradual excavation; and farther down the valley, at Fordingbridge, we find them again occurring with remains of Elephas primigenius at about forty feet above the river.
The circumstances of the discoveries at Bournemouth seem at first sight almost irreconcilable with any river-hypothesis; as it is difficult to conceive how gravels capping the cliffs along the sea-shore for miles, and at an elevation of from 130 to 90 feet above its level, can have been deposited in such a position by the agency of a stream. And yet on a closer examination of the case, all such difficulties vanish, and the ancient existence of a river at such an elevation, and running in such a direction that it would leave these gravels to testify to its former course, seems absolutely demonstrable. Without being aware of the results at which others had arrived, I came, after due consideration of the facts of the case, to the conclusion that, as has already been mentioned in an earlier page, there must in ancient times have existed a river draining an extensive tract of country along the southern coast, and flowing in an easterly direction; and that of this river a portion still survives in an altered and enlarged condition as the Solent Sea, which separates the Isle of Wight from the mainland. Mr. Codrington, whose paper I have already so often quoted, arrived on independent grounds at substantially the same conclusion. But at an earlier epoch still—in 1862—before any flint implements had been found at Bournemouth, or indeed in any of the gravels of the South of England, the late Rev. W. Fox, of Brixton, in the Isle of Wight, published nearly similar views as to the origin of the Solent. As his opinions cannot by any possibility be supposed to have been influenced by preconceived views as to the antiquity of man, I prefer stating the case, in the first instance, in his words rather than in my own:—"The severance of this island (the Isle of Wight) from the mainland, it appears to me, was effected under very unusual circumstances, and at a very distant period. The present channel of the Solent, being pretty nearly equally deep and equally broad throughout its entire length of twelve or fourteen miles, proves at once that it was not formed in the usual way of island-severing channels, that is, by gradual encroachments of the sea on the two opposite sides of a narrow neck of land" . . . "it is to be accounted for, therefore, not by the excavations of a gradually approaching sea, but, as I shall hereafter have to attempt to show, by its being originally the trunk or outlet of a very considerable river." . . . "Whoever, as a geologist, examines the vertical strata of the Chalk at the Needles, nay, and throughout the whole length of the Isle of Wight, and the strata of the same rock in exactly the same unusual position on the bold white cliff on the Dorsetshire coast some twenty miles westward of the Needles, will not doubt but that the two promontories were once united, forming a rocky neck of land from Dorset to the Needles. This chain of chalk might, or might not, be so cleft in twain as to allow the rivers of Dorset and Wilts to find a passage through them to the main ocean. My opinion, however, is that they had no such outlet, but that at that far distant period, the entire drainage of more than two counties, embracing the rivers that join the sea at Poole and Christchurch, flowed through what is now called Christchurch Bay, down the Solent, and joined the sea at Spithead."
"According to this theory, the Solent was at that time an estuary somewhat like the Southampton Water, having but one opening to the British Channel, but of so much more importance than the latter as it was fed by a vastly greater flow of fresh water." "Of course, according to this view, the sea would lose its original condition as an estuary at the time when the British Channel had so far made a breach through the chain of rocks connecting the Isle of Wight with Dorsetshire as to give an opening into itself for the Dorsetshire rivers, somewhere opposite to the town of Christchurch. From that time forth the Solent would become what it is at present, losing its character as an estuary, and assuming that of a long narrow sea." . . . "The distant period at which such changes took place it would be hopeless to guess at, amid the dimness of the data on which calculations could be founded. It could not be less, however, than many thousands of years, seeing that since that time, the British Channel has not only made a broad breach of twenty miles through a chain of slowly yielding rocks, but has also pushed its way gradually across the broad extent of the Poole and Christchurch Bays."
Such is the theory of Mr. Fox, which places the probable course of events fully and fairly before our view. I see in it but little on which to comment, except that it does not appear to have sufficiently taken into account the widening of the Solent subsequently to the time of its becoming a channel of the sea; and that in a passage, which I have not quoted, Mr. Fox estimates the drainage area of the ancient river as but little inferior to that of the Thames, or Humber. Taking the basins of all the streams discharging into the sea between Ballard Down, near Poole, on the west, and Calshot Castle and the Medina on the east, but not including the latter river, I find that, according to the Ordnance Map, the present land area which would have drained into an ancient river such as that supposed, is 1,617 square miles. To this may be added another 100 square miles, representing the area included between the present coast and an extension of the chalk downs from Ballard Down to the Needles, the whole of which has been washed away; though within this large area, the present depth of the sea attains in but very few places to ten fathoms. The drainage area of the ancient river Solent can therefore have been but about one-third of that of the Thames and its affluents, unless we are to suppose that, as is the case in the neighbourhood of Carisbrooke Castle and with the Medina, a portion of land to the south of the old chalk downs drained northward through some gap in the range of hills. That such land existed seems probable, from the occurrence of gravels with elephant remains along the south-west coast of the Isle of Wight at an elevation of 80 feet and upwards above the sea, which, Mr. Codrington has suggested, may have been deposited by tributary streams of a river flowing northwards through the chalk range to the Solent. But even with any such addition the area drained by the old River Solent can hardly have been half that of the basin of the Thames.
With regard, however, to the former existence of this range of chalk hills and the land to the north of them, Mr. Codrington has shown, in the paper already so often quoted, that the spreading out of the marine gravel, and the levelling of the table-lands was probably effected in an inlet of the sea, shut in on the southern side by land which connected the Isle of Wight with the mainland, and opening to the eastward. Assuming, then, the existence of this ridge of high land, it is evident, as Mr. Fox has pointed out, that the only outlet for the rivers now represented by the Frome, the Trent or Piddle, the Stour, Blackwater, Avon, and other streams now discharging into the sea, must have been by an eastward channel, in fact, a continuation of the rivers now discharging through Poole Harbour. The course of such a river would naturally be guided, in the first instance, by the configuration of the surface of the old marine gravels of the sloping tableland. This, as has been shown, slopes upward from the present coast northward, and attains its highest level inland; but traces of the same gravel occur also in the Isle of Wight, though it there slopes upward in a southerly direction, attaining a height of 368 feet at St. George's Down, but being only from 100 to 160 feet above the sea in various places along the northern shore of the island, at a distance of about a mile inland. It appears, therefore, that there must originally have been a valley running east and west in the old marine gravel, forming a natural course for the drainage of the country, and probably finding its way towards the sea, somewhere within the space now occupied by the Solent and Spithead, though not actually discharging into the sea until it had attained some distance eastward.
Evidence as to the highest level at which freshwater action removed and re-deposited the marine gravel on the southern slope of the valley is at present wanting; but, judging from a section across the Isle of Wight from St. George's Down to Norris Castle, given by Mr. Codrington, the declivity is so much more rapid below the 160 feet level than above it, that the ancient river may have commenced its action at about that level. How far eastward the Isle of Wight may have extended at that time it is difficult to say; but from the enormous denudation of land to the west, and the range of the ten-fathom line, there may probably have been land at all events as far east as opposite to Selsey, the extensive estuarine beds at which place, containing remains of Elephas primigenius, are possibly connected with this old river.
The precise manner in which the Foreland gravels and brick-earth, in which Mr. Codrington found a palæolithic implement at 85 feet above the sea-level, were connected with the old river-deposits, is difficult to determine. Mr. Codrington is inclined to think that a rise of land to the extent of 70 or 80 feet must have taken place since the deposition of the brick-earth in which the flint implement was embedded, but this to me seems unnecessary. It is, however, unsafe to speculate on a single specimen found in such a position. The implements found at Seaview and Bembridge may have been washed out of gravel-beds at a lower level than those of the Foreland, or even have been transported for some distance by marine currents.
Turning to Bournemouth, where so many more have been found, the highest and most westerly point at which implements have occurred appears to be about 130 feet above the sea. Farther east, near Boscombe, the level is about 120 feet; midway between that spot and Hengistbury Head, the height of the gravel is 90 feet; at High Cliff, 84 feet; at Hordwell, where implements abound, a short distance inland, 60 feet; and about midway along the northern shore of the Solent, 50 feet. The surface of the ground is, of course, much cut up by the numerous streams coming in from the north; but the general fall of the gravel from west to east is perfectly in accordance with its having been deposited in the valley of an ancient river running in this direction, the whole of the southern side of which has since been carried away by the sea. Whether the old river had become tidal so far west as Hurst Castle, when first it was intercepted by the sea to the south, does not appear to me to be a matter of importance, inasmuch as no doubt a valley was already formed, along the course of which the encroachments of the sea would be more rapid than where the cliffs were higher, and more solid matter had to be removed. That the valley, in which is now Southampton Water, was also originally, for the most part, scooped out by the rivers coming from the north, which in remote times flowed into the old River Solent, is, I think, beyond all reasonable doubt. The increased volume of the ancient river, after receiving so important an affluent, is evinced by the widening of the channel, from Calshot Castle eastward by Spithead, to a full third more than it is to the west, along what is now the Solent Sea.
As to the character of the gravels at Bournemouth, it is, as already observed, hard to distinguish those presumably of fluviatile origin from the older and probably marine beds. In the railway-cutting between Bournemouth and Christchurch, I thought, however, that in places I could trace the superposition of the one upon the other. The more recent deposits contain waterworn fragments of quartz, granite, and porphyry, as was noticed long ago by Mr. Godwin-Austen, who, from this circumstance, saw reason for connecting them with the gravels capping the tabular hills of Devon and Dorset to the west.
It is, of course, evident that at the period when the river ran at this high level, past the spot where now is Bournemouth, all the land to the immediate west must have been far higher than it is at present, and that Poole Harbour could not have existed. In attempting to reconstruct the map of a country, the shores of which have been much wasted by the sea, in order to show what must have been at some remote period the old coast-line, the task is rendered difficult and within certain limits impossible by the absence of any evidence as to the elevation above the sea of the land removed, and as to the channels along which the sea could work. In this case, however, there is a strong presumption as to the unbroken continuity of the chalk-range, and of its elevation having been much the same throughout, as it now is at both ends of the breach. The general character of the beds above the Chalk, so far as their power of resistance to water-action goes, seems also much the same at either extremity; though perhaps the beds at the Isle of Wight end of the breach are somewhat the harder. Assuming nearly equal conditions, and looking at the form of the present coast-line, which is indented by two distinct broad bays, it seems probable that the old course of the river may have been intercepted by the sea at two several points, the one nearer Poole and the other nearer Lymington. Directly this closer communication with the sea was formed for the Dorsetshire rivers, they would, of course, owing to the more rapid fall, excavate their valleys with greater speed at their mouth, and directly they became tidal, the sea would make rapid inroads on the soft sand and clay exposed to its action. So effective is this action, that at Hordwell Cliff the waste of the shore is said to be now going on at the rate of about a yard per annum, or upwards of half a mile every thousand years, though perhaps this is somewhat exaggerated.
In discussing this question, I have purposely avoided complicating the subject with the effects of any general lowering of the surface of the ground by erosion either chemical or mechanical; or of upheavals and depressions of the land during the period of the formation of the valleys, though no doubt this also has taken place, especially along the southern coast of Britain. I must, however, mention the existence of a submerged forest, occasionally visible at low water, at the foot of the cliffs at Bournemouth, which seems to show that there as elsewhere a depression of a former land surface has taken place. The late Mr. Albert Way, F.S.A., who had the opportunity of examining some of the stumps of trees exposed at rare intervals at low water, informed me that they appeared to be those of the true Scotch fir; and also that local tradition speaks of an impassable morass having, so late as the commencement of the present century, intervened between the line of cliffs and the sea. On the occasion of one of my visits to Bournemouth, some of these stumps were fortunately visible, and were pointed out to me by Mr. Way at a spot but a few yards to the west of the pier, and between high and low water-mark. They appear to be of no very remote antiquity, geologically speaking, and to be connected rather with the present valley of the Bourne than with the valley of the old river Solent, as the trees, some of which were fully a hundred years old, grew on the surface of a thick bed of hard peat. Under any circumstances, however, the presence of such remains at the foot of the cliff does not tend to diminish our estimate of the antiquity of the freshwater beds containing the works of man, which we find occupying their summit.
In passing the deposits containing flint implements in different parts of this country under review, enough has, I think, now been said to show that in position, in character, and in the nature of their organic contents, they are perfectly in accordance with what might have been expected from river-action under certain circumstances. The case might indeed have been made much stronger had deposits in other places, in all respects similar, except that the presence of flint implements has not as yet been observed in them, been brought into account; and it must not be forgotten that this might, with perfect propriety, have been done, as there can be no possible doubt that a certain series of gravels, sands, and clays, containing organic remains and flint implements in extremely variable quantity, all belong to one geological period, and owe their existence to similar causes.
But though on no other hypothesis than that of river-action can the phenomena be accounted for, yet, as has already been seen, it is necessary, in order that river-action should have produced such effects, that the streams, during some portion of the year at all events, should have been more torrential in character than they are at the present day. If, however, we see satisfactory grounds for attributing these beds containing land and freshwater shells and remains of terrestrial animals, to rivers formerly flowing at much higher levels than at present, which have since excavated their valleys—and it seems impossible to do otherwise—then we must also accept as a fact that the climatal conditions were such as would enable the rivers to perform the work. It is, as Sir Joseph Prestwich has shown, quite out of the question to suppose that with, the valleys excavated to the present depth, any meteorological causes could fill them to their summits; or even if they could and did, that they would leave such deposits as we find at high elevations on their slopes, or even on detached eminences. It will, however, be well to examine briefly any corroborative evidence that may be forthcoming, as to the probability either of a severer climate involving a greater accumulation of winter snows, or of a greater rainfall, or of both. The one, indeed, seems hardly probable without the other, as a cold land surface "presented to vapour-laden sea-winds, as in the mountainous districts of the north-west of Spain, in our own lake districts, and in Scandinavia," involves of necessity a heavy rainfall.
With regard to climate, we may take into account that which prevailed at a somewhat earlier date; for there appears no doubt that the flint implement-bearing gravels are all of later date than the Chalky Boulder Clay of the Eastern Counties, a deposit which belongs to the so-called Glacial Period, during a portion of which a great part of England and Scotland was submerged beneath the sea, and became coated with masses of Boulder Clay and other deposits, derived for the most part from the moraines of glaciers, sometimes at no great distance, and possibly in the main transported and dropped in their present positions by means of icebergs and coast ice. That they are of later date is proved by more than one of the implement-bearing beds reposing in valleys either in, or cut through, this Chalky Boulder Clay; and at Hoxne the interval between the Glacial deposits and the Palæolithic beds is marked by two sets of lacustrine strata, the lower and earlier with a flora characteristic of a mild climate, and the upper by one which points to the recurrence of Arctic conditions. Prof. Boyd Dawkins has suggested the probability of the higher ground of North Wales and the northern part of England having been still enveloped in an ice-mantle at the time that the mammoth, reindeer, and other post-glacial mammals were living in the lower and less inclement districts. But this view is to some extent founded on negative evidence, and on the assumption that palæolithic implements do not exist in this northern area. I have already commented on the possibility of implements being eventually found in it.
The crumpling and contortion of some of the beds of River-drift, especially at high levels, has been regarded by Sir Joseph Prestwich as possibly resulting from the lateral pressure produced by packing and jamming together of blocks of ice, such as may now be witnessed in rivers like the Danube and the Rhine. The "trail and warp" of Mr. Trimmer, those superficial deposits so common over a large portion of this country, which, indeed, constitute so large a portion of the arable soil, seem also, as the Rev. Osmond Fisher has pointed out, to be significant of a severer climate than at present prevails. The "Palæolithic floors," both near London and at Caddington, are buried under a considerable thickness of this "trail." There is moreover a high probability that, at the time of the deposit of the gravels, Britain was still united to the continent; so that, apart from other causes, there was a tendency for the climate to partake more of a continental character than at present, and to induce greater cold in winter and greater heat in summer.
That the existence of enormous glaciers is as indicative of the action of heat, in order to convert the water of the ocean into vapour, as of cold to condense it, has been insisted on by Professor Tyndall, and even more strongly by Professor Frankland. If at the time of the rivers flowing at the high level, Britain was still connected with the continent, it is by no means impossible that the temperature of the seas on either side of the connecting isthmus may have been different. That connected more immediately with the Southern Ocean would have been the warmer of the two, from which a copious supply of vapour would be carried by the southerly winds, and be condensed as rain in its passage northward.
Mr. Alfred Tylor, F.G,S., in his profusely illustrated papers on the Amiens gravel, and on Quaternary gravels, contends for the existence of a "Pluvial period" subsequent to the Glacial, in which the rainfall was far greater than at present, and such a view has much to commend it for acceptance. But when he proceeds to assert that the surface of the Chalk in the valley of the Somme, and in all other valleys of the same character, had assumed its present form prior to the deposition of any of the gravel or loess now to be seen there, and to argue that the whole of the gravels at all levels on the slopes are of one age, and due to floods extending to a height of at least 80 feet above the level of the rivers, we may well hesitate before we give in our adhesion to such views. In the first place, it is, to say the least of it, unphilosophical to rely too much on a single example, such as that of the valley of the Somme; and to account for its phenomena by causes which are evidently incapable of producing the effects observable in other localities, as, for instance, at Southampton, close to the sea, and 160 feet above its level. But what shall we say to floods raising the levels of rivers upwards of 80 feet, yet having no erosive power, and the waters of which, regardless of the laws of gravity, tranquilly deposited their solid contents evenly over the slopes, or often in the greatest thickness on their higher part, and in some cases on almost isolated hills, instead of principally on the bottom of the river-valley? Whence all the materials for the gravels are to be derived, how they are to be reduced to a subangular condition by water-wear, especially in the case of the flint implements occurring in the gravels, are points on which further information will have to be supplied, before any such views can be seriously entertained.
I have up to this point almost left out of view any distinctive differences between the deposits at a high level and those at a low level in the river-valleys. That such, however, exist has been pointed out by Sir Joseph Prestwich; and judging from the northern range of the group of shells found in the high-level beds, the absence of southern species, the character of the mammalian and vegetable remains, the transport of large blocks such as could only be effected by ice and the other physical features of the case, he is inclined to assign a winter temperature to the period of their deposit from 19° to 29° Fahr. below that which now obtains in these regions. From a consideration of the features of the low-level deposits he considers that at the time of their deposit, the climate was rather less severe, by about 5°. The presence of the mammoth and woolly-haired rhinoceros, animals specially adapted for cold climates; of the musk-ox, the reindeer, the lemming, and marmot, corroborates the same view; while the hippopotamus, which seems characteristic of the low-level deposits, is suggestive of a somewhat warmer climate. Like the mammoth and rhinoceros, its structure may, however, have been somewhat modified, so as to enable it to occupy colder regions than at present, or it may have been merely a summer visitor ranging northwards before the separation of Britain from the continent. Under any circumstances its presence seems to indicate that the volume of the rivers was probably in excess of what it is at the present time. But whatever may have been the degree of winter cold, or the amount of the snow and rainfall, the one was not so extreme as to prevent there being an abundance of animal life, nor the other so great as to interfere with the growth of a sufficient supply of vegetable food on which it might subsist.
It has, indeed, been supposed by some that the remains of the early mammals occurring in the gravels are derived from older beds, and that their presence in association with flint implements no more proves the contemporaneity of the men who made those implements with the old Quaternary fauna, than their association with Chalk fossils proves that mankind were originally inhabitants of the bed of the Cretaceous ocean. Did the gravels only occur at such levels as are within reach of existing streams, there might be some reason in such a view, which is, moreover, in certain cases and within certain limits, probably correct. For we have seen how in the course of the excavation of a valley, the beds deposited at one time are liable to be disturbed at another, and re-deposited in a fresh place; which could hardly happen without an admixture of fresh materials, some probably of a more recent date. In the process of transport, however, not only the implements but the still softer bones are liable to wear and abrasion of the angles, and it is impossible to conceive that, assuming the Quaternary fauna to have disappeared from this region before the valleys were excavated, and the implement-bearing beds deposited, their bones could still exist in such numbers, and so often in an unrolled condition in the low-level beds.
Had this older fauna disappeared, it is evident that man could not have subsisted here alone, unaccompanied by other animals to furnish him with food; and if these animals belonged to the later or "prehistoric" fauna, where, as Sir John Lubbock pertinently asks, are their bones? If, however, we acknowledge that the pleistocene mammals still occupied this country at the time of the low-level beds being formed, and if we find their remains also in those at a high level, and at all intermediate heights, it is evident that they must have persisted here during the whole period of the excavation of the valleys; while, if we find also flint implements in an unrolled and unworn condition at all heights, it is evident that those who made them must also have been co-occupants of the region during the same period. If, indeed, as appears to be in some valleys the case, the unworn implements occur only in the high-level deposits, while in the lower they are either absent or in a much worn condition, the inference is, that in those particular valleys the occupation by man, though for some time contemporaneous with that of the mammoth and his congeners, ceased before the extinction or emigration of the old fauna. In some cases, however, as at Fisherton, the worked flints have been found below the remains of mammoth; while in the beds at Menchecourt, near Abbeville, in which the implements occur, were found the bones of a hind leg of rhinoceros still in their natural position, so that they must have retained their ligaments when deposited, and could not since have been disturbed. With regard to the amelioration of climatal conditions which led to the cessation of the excavation of the valleys, it may not impossibly have been connected with the insulation of the country, when the isthmus connecting it with the continent was cut through by the sea. But this is hardly the place for such speculations. If, however, we may regard the estuarine deposits at Selsey, in which almost entire skeletons of mammoth occur, as belonging to the period when the deposit of the low-level gravels was ceasing, it would appear from the associated molluscan forms, as interpreted by Mr. Godwin-Austen, that the temperature of the waters of the English Channel was at that time such as may now be met with twelve degrees farther south.
If there was a difference in the climatal conditions of the high and low-level deposits, it might have produced some effect on the method of living, and on the implements of the men of the two periods. At one time I thought it probable that a marked distinction might eventually be drawn between the high- and low-level implements, but so far as Britain is concerned, this can hardly be done. Still the facies of a collection from two different spots is rarely quite the same, and I think there is generally a preponderance of the ruder pointed implements in the high-level gravels, and of the flat ovate sharp-rimmed implements in the low-level. In the valley of the Somme, the broad polygonal flakes are certainly most abundant in the lower beds, as at Montiers, near Amiens.
I would, however, deprecate the introduction of such terms as "Eolithic" and "Mesolithic" in order to distinguish two phases in the Stone Period as being both unfounded and misleading. We know not where or when the dawn of human civilization arose, but it was probably long before the date of our earliest River-Gravels and in some part of the world more favoured by climate than Britain. Why then should we speak of British implements as Eolithic? And how can we apply the term Mesolithic to a period intervening between the Palæolithic and Neolithic Ages, when we know neither when the one ended nor when the other began?
Enough has now been said with regard to the manner in which these beds of River-drift were probably deposited; and the irresistible conclusion is, that, owing to the wasting agency of rain, frost, and rivers, there must have been a vast change in the superficial features of the country, since the time when those who fashioned the flint implements found in the high-level gravels were joint occupants of the land with the mammoth and rhinoceros and the other departed members of the Quaternary fauna. A similar change in the surface of the country has also taken place in the neighbourhood of the caves in which the remains of this same fauna occur, associated also with similar relics of human workmanship.
What length of time it must have taken for such changes to be effected, is a question we must now approach; but before doing so it will be well to say a few more words, in addition to what has already been said, on the almost entire absence of human bones in the beds containing those of the associated mammalia.
In the first place, it is well to repeat that whatever may be the case in the brick-earth, or loess, there have not, as has been pointed out by Sir John Lubbock, been found in the gravel up to the present time any remains of animals so small as man, who, as the same author observes, must of necessity have been few in number in comparison with the animals by the chase of which he must have subsisted. Another cause appears also to have been at work; for however barbarous we may suppose the human race to have been at that remote period, we can hardly believe them to have been so destitute of all natural affection as to deny some rites of sepulture to friends or relatives removed by death. There would, therefore, in all probability, be but few or no human bones exposed on the surface in such a manner as to be carried off by the flooded streams, and imbedded in their gravels; while, in case of any human beings perishing by drowning, their bodies, as I have already shown, would probably either be carried to sea, or left in such a position as to allow of their recovery, at all events before they became disarticulated.
This is, however, a matter of but small importance, as there will be but little difficulty in conceding that an implement fashioned by human agency—and on this point there can be no question, unless we are to assume in ancient times the existence of some other now extinct race of intelligent beings—is as good an evidence of the existence of man, as would be any or all of his bones. Moreover, human bones are reported to have been discovered in these Quaternary beds, both in this country and in France. In England, I have already mentioned a human skull found near Bury St. Edmunds by Mr. Trigg, and the more doubtful skeleton found near Northfleet. I will not, however, insist upon either discovery being beyond all cavil.
Nor will I do more than allude to the too celebrated Moulin Quignon jaw, over which I have already pronounced a Requiescat in pace, but the discovery of portions of the human skeleton by M. Bertrand, and M. Reboux, in the valley of the Seine, at Clichy and elsewhere near Paris, in the same beds in which implements of true Palæolithic types have been found, seems better substantiated.
Whether the Pithecanthropus erectus of Dr. Dubois was human or simian, and what is the date of the beds in which his remains were found, and whether there is evidence of the existence of Miocene or Pliocene Man in Burma, Portugal, France, Italy, or California, are questions which want of space compels me to leave on one side. I have, however, more than once elsewhere expressed my opinion on the subject of Tertiary Man.
I need hardly again repeat that according to my view it is not in Britain, but in some part of the world more favoured by climate that the cradle of the human race is to be sought. And yet the antiquity of Man in Britain seems to extend far beyond any of our ordinary methods of computation. In attempting to estimate it, however vaguely, I must at the outset observe that with our present amount of knowledge, it is hopeless to expect that it can he determined with anything approaching to precision. Not only have we no trustworthy measure of the rate of excavation of the valleys, which might give us an approximate date for the higher deposits in them, but we are at a loss to know at what epoch their excavation in the lower part of their course ceased, and what may be termed the modern alluvial deposits, which to some extent have partially refilled the old channels, began to accumulate.
That the general configuration of the surface of the country, in Neolithic times, when the ordinary forms of polished stone implements were in use, was much the same as it is at present, is proved by the fact of such implements being frequently found in recent superficial deposits. Were we, in defiance of probability, to assume that the use of these polished implements did not date farther back than two thousand years from the period when we are first made acquainted with this country by history, this would give an additional four thousand years beyond the period necessary for the excavation of the valleys for the date of the older River-drift implements. Such a period as two thousand years is in all probability almost ridiculously small to assign for the duration of the Neolithic and Bronze Periods; but however this may be, there appears, in this country at all events, to be a complete gap between the River-drift and Surface Stone Periods, so far as any intermediate forms of implements are concerned; and here at least the race of men who fabricated the latest of the palæolithic implements may have, and in all probability had, disappeared at an epoch remote from that when the country was again occupied by those who not only chipped out but polished their flint tools, and who were, moreover, associated with a mammalian fauna far nearer resembling that of the present day than that of Quaternary times.
So different, indeed, are the two groups of animals that, as has already been observed, Prof. Boyd Dawkins has shown that out of forty-eight well-ascertained species living in the Post-glacial or River-drift Period, only thirty-one were able to live on into the Prehistoric or Surface Stone Period. Such a change as this in the fauna of a country can hardly have been the work of a few years, or even of a few centuries; and yet we must intercalate a period of time sufficient for its accomplishment between the remotest date to which we can carry back the Neolithic Period, and the close of the Palæolithic Period as indicated by the low-level gravels. The antiquity, then, that must be assigned to the implements in the highest beds of River-drift may be represented (1) by the period requisite for the excavation of the valleys to their present depth; plus (2), the period necessary for the dying out and immigration of a large part of the Quaternary or Post-Glacial fauna and the coming in of the Prehistoric; plus (3), the Polished Stone Period; plus (4), the Bronze, Iron, and Historic Periods, which three latter in this country occupy a space of probably not less than three thousand years.
A single equation, involving so many unknown quantities, is, as already observed, not susceptible of solution; but various attempts have been made to arrive at some approximate idea of the amount of time it represents. One method has been that of assigning a date for the Glacial Period, deduced from astronomical causes, mainly in connection with the eccentricity of the earth's orbit, as pointed out by M. Adhémar and Mr. Croll. From data thus obtained, Sir Charles Lyell inclines to place it at a period of extreme cold about 800,000 years ago, though Sir John Lubbock would rather accept an epoch of somewhat less severity about 200,000 years removed from our time.
Another and more direct method suggested by Sir Archibald Geikie, following in Mr. A. Tylor's track, is that of estimating the time required for the excavation of the valleys by the amount of solid matter carried down in suspension by various rivers at the present day. He estimates that this amount, if spread over the whole area drained by the rivers, represents, on an average, an annual loss of about 1 of a foot; but inasmuch as the erosion of the slopes and watercourses is very much greater than that of the more level grounds, the excavation of the valleys must proceed at a more rapid rate, which he assumes to be about 1 Part of a foot per annum, or one foot in 1,200 years. Such a calculation is, of course, open to various objections, as we may readily conceive the bottom and slopes of a valley to have been so far washed that, under ordinary circumstances, they afford little or no fine earthy matter to be taken up by the rain falling on their surface; and in such a case, the rivers, if turbid, would derive their turbidity from the water delivered from the higher and comparatively unwashed table lands. Or again, the soil may, like the Chalk under ordinary circumstances, be so absorbent that but little of the rainfall flows off from its surface. The calculation has already been made, that a rainfall of 54 inches annually, supposing the whole of it flowed off the land into the sea in a turbid state, containing, like the Mississippi, 1 part of its weight of solid matter, would lower the surface a foot in 450 years; but as has already been observed, we cannot conceive it possible that with such soils as we have here to do with, the constant turbidity should have been anything like so great. And, in fact, the whole system of calculation is one which may be regarded rather as proving the necessity of valleys being in course of time formed by subaërial action, than as giving any definite guide by which to calculate the period requisite for their formation. There can, indeed, be no doubt that the denuding power of the falling rain is greater on the slopes than on the level surfaces; but it seems impossible to assign any proportions to the effects on land lying at different inclinations, of different characters, and under different circumstances as to any vegetable covering. Were the action uniform over the whole surface exposed, of course no alteration beyond a general lowering of the land-surface would result from this cause, and the valleys would remain of precisely the same depth with regard to the adjacent land as they did at remote epochs. Looking at the quantity of brick-earth still left on the slopes of many of our valleys, I am inclined to think that the lowering of the surface has been more general than has been supposed by Sir Archibald Geikie. The presence of these soft and easily denuded beds is also an argument against the excavation of the valleys having progressed in a uniform manner, by heavy rains falling during the period of the year when such beds were soft and unfrozen; and seems rather significant of the excavation of the valley by floods principally occurring at a time when the upper part of the soil was in a frozen condition.
Certainly the whole character of the deposits is more in accordance with their resulting from the occasional flooding of the streams than from any other cause. If this be so, who shall tell at what intervals such floods occurred, and what was the average effect of each in deepening the valleys? That they were of comparatively rare occurrence, and not so frequent that they were foreseen by the men of those days, seems deducible from the number of their implements found in the gravels. For there is much probability that these must have been washed in from settlements on the banks of the rivers, which, notwithstanding previous catastrophes of the same kind, were constantly placed within reach of the stream when flooded.
Sir Joseph Prestwich has suggested as a possible gauge of the antiquity of the deposits, the natural funnels eaten into the chalk by the action of water charged with carbonic acid, and has cited one at Drucat, near Abbeville, which has been formed since the deposit of the gravel containing flint implements, and is upwards of 20 feet in diameter at top, and probably 100 feet in depth; but here also it seems impossible to introduce a factor by which the time represented can be ascertained. There are, however, features in connection with this case which can only be reconciled with the former high level of the bottom of the adjacent valley, and with its gradual excavation. It will be remembered that similar pipes of erosion, leading in some cases to caverns above them, occur in the Drift-beds of the valley of the Little Ouse.
There is yet another means at our command for forming, at all events, an approximate idea of the time that has elapsed since the deposit of the beds containing the remains of the old Quaternary fauna, inasmuch as at the time of their introduction into this country, if not for a lengthened period afterwards, Britain had apparently not become an island, but was still connected by an isthmus of greater or less width with the Continent. To estimate the time, however, that would be required for cutting through this isthmus and widening the Channel to its present dimensions, is a work from which the mind almost recoils. Even the wearing away of that tract of land to the south of the present Hampshire coast, which must almost of necessity have existed at the time when the Bournemouth flint implement-bearing gravels were deposited, taking the present rapid inroad of the sea on the unusually soft cliffs at Hordwell as a guide, would seem to involve a period of not less than 10,000 years; but inasmuch as the cliffs during a considerable portion of the time must have been of chalk instead of sand and clay, and as a chalk cliff 500 feet high, instead of being worn away at the rate of a yard each year, is said only to recede at the rate of an inch in a century, the actual period necessary for the removal of this tract must probably have been many times 10,000 years, and can with certainty be regarded as having been immensely in excess of such a lapse of time.
On the whole, it would seem that for the present, at least, we must judge of the antiquity of these deposits rather from the general effect produced upon our minds by the vastness of the changes which have taken place, both in the external configuration of the country and its extent seaward, since the time of their formation, than by any actual admeasurement of years or of centuries. To realize the full meaning of these changes, almost transcends the powers of the imagination. Who, for instance, standing on the edge of the lofty cliff at Bournemouth, and gazing over the wide expanse of waters between the present shore and a line connecting the Needles on the one hand, and the Ballard Down Foreland on the other, can fully comprehend how immensely remote was the epoch, when what is now that vast bay was high and dry land, and a long range of chalk downs, 600 feet above the sea, bounded the horizon on the south? And yet this must have been the sight that met the eyes of those primeval men who frequented the banks of that ancient river which buried their handiworks in gravels that now cap the cliffs, and of the course of which so strange but indubitable a memorial subsists in what has now become the Solent Sea.
Or again, taking our stand at Ealing, or Acton, or Highbury, and looking over a broad valley fully four miles in width, with the river flowing through it at a depth of 100 feet below its former bed, in which, beneath our feet, are relics of human art deposited at the same time as the gravels; which of us can picture to himself the lapse of time represented by the excavation of a valley on such a scale, by a river larger, it may be, in volume than the Thames, but still draining only the same tract of country? But when, to this long period we mentally add that during which the old fauna, with the mammoth and rhinoceros, and other to us strange and unaccustomed forms, was becoming extinct, so far as Britain was concerned; and also that other, we know not how lengthened period, when our barbarous predecessors sometimes polished their stone implements, but were still unacquainted with metallic tools; and then beyond this, add the many centuries when bronze was in use for cutting purposes; and after all this, further remember that the ancient and mighty city now extending across the valley does not, with all its historical associations, carry us back to the times even of the bronze-using people, the mind is almost lost in amazement at the vista displayed.
So fully must this be felt, that we are half inclined to sympathize with those who, from sheer inability to carry their vision so far back into the dim past, and from unconsciousness of the cogency of other and distinct evidence as to the remoteness of the origin of the human race, are unwilling to believe in so vast an antiquity for man as must of necessity be conceded by those, who however feebly they may make their thoughts known to others, have fully and fairly weighed the facts which modern discoveries have unrolled before their eyes.
- See Trans. Watford Nat. Hist. Soc., vol. i., 1878, p. 137.
- Lyell, "Principles of Geol.," 10th ed., vol. i. p. 354.
- Op. Cit., p. 350.
- Quar. Journ. Geol. Soc., vol. xix. (1863), p. 321.
- "Encyc. Brit."—Art. "Rivers." Lyell, "Princ. of Geol," 10th ed., yol. i. p. 348. Lubbock, "Prehistoric Times," 4th ed., p. 382.
- Quar. Journ. Geol. Soc., vol. ix. (1853), p. 48.
- Lyell's "Princ. of Geol.," vol. i. p. 458. Geikie, Geol. Mag., vol. v. p. 250.
- Geol. Mag. (1868), vol. v. p. 250.
- Phil.. Trans., 1864, p. 293. See also Lyell, "Princ. of Geol.," vol. i. p. 366.
- Annales de Chimie et de Physique, 1866. Trans. in Smithsonian Report, 1866, p. 425.
- Phil. Trans., 1864, p. 296.
- See p. 664.
- H. G. Seeley, Q. J. G. S., vol. xxii. p. 472.
- Q. J. G. S., vol. xxv. p. 465.
- Quar. Journ. Geol. Soc., vol. xxv. p. 453.
- Phil. Trans., 1860, pl. xi.
- Prestwich, Phil. Trans., 1860, pl. xi. See p. 577, supra.
- Brit. Assoc. Report, 1896, p. 400.
- Geol. Mag., vol. iii. p. 348.
- Q. J. Geol. Soc., vol. xxvi. p. 528.
- Q. J. Geol. Soc., vol. xv. p. 219.
- Q. J. Geol. Soc., vol. xx. p. 189.
- Geologist, vol. v. p. 452.
- "Rivers and their Catchment Basins."
- R. A. C. Godwin-Austen, Q. J. G. S., vol. xiii. p. 50.
- There may be some degree of uncertainty whether the gravels at this spot are to be connected immediately with the main stream, or with an affluent running into it approximately by the same course as that of the present Bourne, but this is of little moment.
- Q. J. G. S., vol. xiii. p. 45.
- Q. J. G. S., vol. xxvi. p. 532.
- Phil Trans., 1864, p. 266.
- Phil. Trans., 1864, p. 291.
- Q. J. G. S., vol. xxv. p. 209.
- p. 580 supra.
- Q. J. G. S., vol. vii. p. 31
- Q. J. G. S., vol. xxii. p. 553.
- "Heat considered as a Mode of Motion," p. 182. Lubbock, "Preh. Times," 4th ed., p. 408.
- Q. J. G. S., vol. xxiv. p. 103; xxv. p. 57.
- Phil. Trans., 1864, p. 278, &c.
- "Flint Chips," p. 47.
- Ravin, Mém. de la Soc. d'Emul. d Abbeville, 1838, p. 196. Phil. Trans., 1860, p. 301.
- "Preh. Times," 4th ed., p. 365.
- Athenæum, 1863, July 4.
- Hamy, "Paléontologie humaine," p. 210, et seqq. Bull. Soc. d'Artthrop. de Paris, 2nd S., vol. iii. p. 331. Belgrand, "Bassin de la Seine," pl. xlviii. and xlix.
- Rec. Geol. Sur. of India, vol. xxvii., 1894, p. 101. Geol. Mag., Dec. 4, vol. i., 1894, p. 525. Nat. Science, vol. v. p. 345, vol. x. p. 233.
- Trans. Herts. Nat. Hist. Soc., vol. i. p. 145. Brit. Assoc. Rep., 1890, p. 963. Nature, vol. xlii. p. 50.
- Several writers have attempted to bridge over this gap, or to show that it does not exist. See Journ. Anth. Inst., vol. xxii. p. 66. Cazalis de Fondouce, Cong. Préh. Stockholm, 1874, p. 112. Brown, "Early Man in Midd." Worthington Smith, "Man the Prim. Savage."
- Trans. Preh. Cong., 1868, p. 278. Supra, p. 485.
- "Princ. of Geol.," 10th ed., vol. i. p. 295.
- "Preh. Times," 4th ed. p. 423.
- Geol. Mag., vol. v. p. 249.
- Phil. Trans., 1864, p. 299. Proc. R. S., xiii. p. 135.
- Lubbock, "Preh. Times," 4th ed., p. 430.