Quarterly Journal of the Geological Society of London/Volume 32/Notes on the Physical Geology of East Anglia during the Glacial Period
[Plate XV.].
Introduction.
In submitting to the consideration of the Society the following observations, I would remark that they are intended to form, not a full description, but a sketch of the physical geology of East Anglia during the glacial period. The evidence upon which they are founded is derived from deposits that either have been, or will be, mapped by the officers of the Geological Survey. Eull descriptions will appear in their published maps and memoirs; therefore the subject will here be treated generally, and all details omitted.
The object I have more especially in view, is to offer an explanation of the origin of a somewhat puzzling series of gravels and sands, classed by Mr. S. V. Wood, Jun., as "Middle Glacial," to give a reason for their occurrence in certain areas and non-occurrence in others, and to account, on mechanical grounds alone, for the almost total absence therefrom of any (except derived) fossil remains. I wish also to remark briefly on the probable means of formation of the so-called "Denudation gravels."
The subject is divided, for reasons that will appear, into two parts; the first relates to the district south and east, and the second to that north and west, of the great Chalk escarpment.
By the term "East Anglia" is meant a tract of country, north of the Thames, which may be considered to be bounded on the west by a line passing from London through Hertford, Royston, and St. Ives to the estuary of the " "Wash," and which includes all Norfolk, Suffolk, and Essex, with part of Cambridgeshire, Hertfordshire, and Middlesex. The general contours of the southern part of the district at every hundred feet above the sea are shown by the dotted lines on the map (fig. 1, Pl. XV.), the smaller and later-formed features being omitted as immaterial to the argument.
The lowest tract in the district may conveniently be termed the "Cambridge Valley:" a large portion of it is fen-land bounded on the south and east by Gault, which is, with trifling exceptions, the lowest geological formation that comes to the surface in the area in question. On the right side of the Cambridge valley runs the chalk escarpment, divided as usual into two main lines, one of the Lower and one of the Upper Chalk. The latter forms some of the highest ground in the district[1], and may be traced as an uneven ridge, with rounded hills and hollows, from Buntingford, by Saffron Walden, Haverhill, Thetford and Swaffham, to the sea. The ground slopes gradually away from this ridge towards the London Basin, where the Tertiaries set in, one of the series (the London Clay) forming an escarpment roughly parallel to that of the Chalk. The northern and eastern portions of the district are in part occupied by deposits of later Tertiary age— he Crags, the Chillesford beds, and the Forest-beds.
The general altitude of the line of escarpment of the Upper Chalk may be taken as from 300 to 500 feet, that of the London Clay from 200 to 300 feet above the sea.
Part 1.—The Drift-deposits to the South and East of the Chalk escarpment.
Pliocene.—Along the coast of Norfolk, Suffolk, and part of Essex, as well as for some distance inland, occur the "Crag" deposits. Of these sands, the older or "Coralline Crag" contains the remains of a deep-sea fauna; in the " Red Crag," immediately succeeding, littoral forms prevail; while the newer or "Norwich Crag" also contains littoral forms with land and freshwater shells in addition. A gradual rising of the coast during the Crag period is thus indicated; and this continued until the land stood slightly higher than at present, admitting the growth of the Cromer "Forest-bed."
These Crag-beds perhaps scarcely belong to the drift-deposits that present the evidence on which the following arguments are founded, although I believe that there is a gradual passage up from them to those of recent date. This gradual passage will probably be found to exist stratigraphically, as well as in regard to their enclosed fossil remains and to the climate[2] that prevailed during the different periods of their deposition.
Lower Glacial.—After the formation of the Cromer Forest-bed the land again sank beneath the water, gradually and to a depth of certainly not less than 400 or 500 feet[3]: during the progress of the submergence the advancing shore-line gave rise to the "pebbly sands" which, according to Messrs. Wood and Harmer, "form the base of the whole glacial series, indicate shore-conditions and the first setting-in of the great glacial subsidence"[4]. Arctic conditions of climate began to prevail; and here and there patches of clay were dropped by icebergs, heralds of those to follow in such vast numbers during the Upper Glacial epoch. The contorted drift, which occupies a large area in the north-east portion of the district, was deposited during the early part of this submergence of the land; its mode of formation I consider to have been much the same as that of the Upper Boulder-clay presently to be considered. The contortions sometimes exhibited by this deposit are probably due to agencies acting on the clay at a subsequent period.
Middle Glacial.—The Lower Glacial beds are overlain and considerably overlapped by a series of gravels and sands called the "Middle Glacial." These beds occupy, or have occupied, almost all the area covered by the Lower Glacial, and extend far beyond it in a southerly direction. It will be seen on reference to the map (fig. 1, Pl. XV.), that many of the valleys running S. and E. from the chalk escarpment to the sea are, almost to their source, cut through the Upper Boulder-clay, and expose these gravels beneath it. They are thus proved to be very persistent over a definite area, within which it is a rare occurrence for the clay to be seen resting on the older geological formations. It is probable that the gravels extend beneath the clay over nearly all the intervening area, in greater force along the lines of the larger valleys (which I believe to have been carved into nearly their present form in preglacial times), and in a more attenuated, or even patchy condition under the higher grounds.
But the gravels can in no instance of which I am aware be traced up to the escarpment of the Chalk, or, in other words, beyond a certain definite level. It is not that they disappear beneath a great thickness of Boulder-clay to reappear at its opposite boundary; on the contrary, it is evident that they gradually thin out; and a few miles before the escarpment is reached we find the Boulder-clay overlapping them and resting directly on the Chalk (fig. 2, Pl. XV.). This is well seen in many sections, also along the N.W. face of the scarp, where the junction of the Chalk and the clay forms for many miles a well-marked line, the absence of any intervening sand or gravel being constant and remarkable. The fact of the gravels not running up to the escarpment was noticed, but no inference drawn therefrom, in 1835, by Mr. Caleb Burrell Bose, who writes[5]:— "The general surface of the chalk must have suffered prodigious abrasions from the violence of the elements, as evidenced by the immense quantity of gravel formed and collected in various situations, as well as by the different altitudes at which the chalk is found, it appearing immediately beneath the vegetable soil, even on the highest ground ; and at a level of not less than 50 feet lower it may be found covered by more than 150 feet of sand and clay containing boulders."
In the thinning-out of the Middle Glacial beds against the Chalk, and their not rising beyond a certain height, we have a clue to the conditions under which they were formed. The great chalk escarpment, as a long and narrow ridge, standing at this time well above the sea, was an important feature in the physical geography of the period; and the Middle Glacial sands and gravels, as such, are entirely owing to its existence. For it formed a barrier opposing itself to the strong current which must at that time have been sweeping round from the North Sea to the Atlantic Ocean. By this current were brought down the materials of which the gravels are composed, and which consist of pebbles derived from the rocks of the northern and eastern coast along which it travelled, mixed with a large percentage of flints from the chalk barrier itself.
In the gravels are intercalated occasional masses of Boulder-clay which were brought down by icebergs, and which, heavily descending from them, have distorted the gravels wherein they now lie without any approach to order or arrangement. Further evidence of icebergs during this period is found in the associated series, or parts of series, of fossil remains taken from the gravels—as, for instance, at Bishop Stortford, where a good number of vertebræ and other bones of Pliosaurus, parts of the same animal, were discovered. These of course were derived from some of the Secondary rocks to the northward, and must have been transported in a large mass of the matrix in which they had been previously fossilized.
The submergence which began at the commencement of the Lower Glacial series went on until the waters af the North Sea were again united to those of the Atlantic[6], when a strong current was set up in that direction. It steadily continued during the accumulation of the Middle Glacial beds, bringing them up to the level at which they are now found. But while yet the higher ranges of the Chalk were above the water, the lower parts of the escarpment had gone down sufficiently to admit of the passage over them of the sea; and this of course began to give rise to a set of very different physical conditions. In place of the strong southerly current confined to the eastern side of the chalk ridge, the waters having admission to a much larger area, their power (within that area) of transporting gravels was materially lessened, and, as submergence went on, absolutely lost. For this reason the gravels are found running not quite although nearly up to the lower levels of the chalk escarpment.
[The ground to the N. and W. of the escarpment (fig. 2, Pl. XV.) being at even a lower level than where the gravels occur, was of course also under water; it was, however, land-locked on every side but one, and formed a bay in which no current of any extent was possible; consequently we find in it but few (if any) deposits that undoubtedly belong to the Middle Glacial series. The waters from the bay flowed outward through the "Wash" to join the southerly current, and contributed their share of pebbles to the gravel that it deposited.]
It is almost impossible to locally classify in detail these drifts; but, speaking generally, the coarser gravels are found, as might naturally be expected, in positions nearest to the chalk ridge, as in the neighbourhood of Clare, the finer gravels in the intermediate country between it and the most distant deposits at Hertford, where they take the form of brick-earth. Throughout are interspersed masses of current-bedded sand and occasional patches of iceborne clay.
The general absence from these beds of fossil remains may be ac- counted for by the conditions of deposit; they were formed in a strong current, which would be (except, perhaps, at the commencement and end of the period) unfavourable to the existence of animal life, while the remains of any plants or animals that may have come within its sweep would speedily be reduced by attrition, except of course those which were previously fossilized, and all of which are much rolled and waterworn. In the exceptional cases where shells do occur, they are found to be of such species as would perhaps indicate a climate warmer than that which prevailed before and after the Middle Glacial period. This fact is probably due not so much to a real difference of climate as to the temperature of the current, which would be governed by its direction and the conditions affecting it before reaching these islands.
If these views be correct, there is between the Lower and Middle Glacial no definite line of demarcation.
Upper Glacial.—The Great Chalky Boulder-clay in East Anglia extends in mass, or in patches that once formed part of the main mass, over the northern half of the London Basin, the Pliocene area, and the dip slope of the Chalk, rises over the escarpment of the same formation, and plunges down into the Cambridge valley. It now caps the highest hills; and it occupies the deepest valleys, except where it has been removed by recent denudation.
The submergence which began with the Lower Glacial and continued during the Middle Glacial periods still proceeded; consequently the strong northern current was, as we have seen, gradually replaced by a more open sea. During the succeeding era the bottom of this sea became covered with a thick deposit of ice-transported clay. This clay, made up of chalk and the débris of other rocks also found to the northward of the area, having been brought down and dropped in masses, presents within itself no traces of stratification. Still it is seen in most instances resting evenly on the Middle Glacial beds; it is, indeed, as a bed, stratified with them, although its own structure is not the result of stratification. There are no signs of grinding or thrusting of their surface, such as must have been apparent had the clay been formed beneath a covering of ice sliding over the land. This latter mode of formation is accepted by many as a true explanation of the phenomena presented by the Boulder-clay; but for this reason especially I differ from that conclusion. Moreover, if the clay be a direct result of ice moving over the land, an emergence after the deposition of the Middle Glacial gravels must have intervened; but of this there seems to be no evidence beyond the disputed point of the method of the clay's formation.
In this district we have no indications of the greatest depth of the glacial submergence; it was probably many hundreds of feet. That it continued for a lengthened period is certain, judging from the great thickness in many places of the Boulder-clay. Whatever oscillations of level may have occurred elsewhere during the Glacial period, there appear to be in East Anglia no marks of any but one, and that a gradual and long-continued movement of depression succeeded by another, equally gradual, of reelevation.
Postglacial.—When at length the land again assumed an upward movement, and as it rose from beneath the sea, every part in turn, as a receding shore-line, would be subject to the action of the waves, and the surface of the Boulder-clay thereby eroded and to some extent reassorted. A clayey gravel would naturally result; and the whole of the uneven surface of the clay would be more or less covered by such material, although on the flats, in the hollows, and in the channels the larger portions of it would be accumulated.
As it invariably happens that subaerial erosion acts most powerfully along an anticlinal line, the gravels in the hollows and channels would be the longest-preserved from destruction. And we find here and there on the surface of the Boulder-clay patches of this clayey gravel or loam generally capping the higher grounds, frequently many of them in a curved or straight line, showing the direction of the old channel in which they were originally formed. These so-called "Denudation gravels" are never of any great extent or thickness, nearly all having been removed by erosion. Where they exist it may be inferred that there the Boulder-clay still retains its original thickness; and a line drawn from point to point where they occur would give a rough measure of the minimum amount of Postglacial denudation.
Part 2.—The Drift-deposits in the Cambridge Valley.
The title of "Cambridge Valley" I consider to be most applicable to that main branch of the valley which is occupied by the river Cam, or Rhee, a stream that rises on the Chalk a few miles from the town of Royston (fig. 1, Pl. XV.) This stream runs in a N.E. direction along a line roughly parallel to the Chalk escarpment, until, south of Cambridge, it is joined by the Cam, or Granta. Thence the united streams, under the name of the river Cam, run nearly due north, still parallel to the escarpment. By Ely their waters flow into the Ouse, a river which continues the course hitherto taken by the Cam, and falls into the "Wash" at King's Lynn. The valley occupied by the Cam and its extension the Ouse thus lies parallel to and near the base of the Chalk; it has been in fact cut back, and is still being cut back into the escarpment. In this operation the river is aided by several streams having their source well up in the Chalk district and running across the strike into the Cambridge valley. These are:—the Cam, or Granta, which rises near Saffron Walden, and has a branch from Bartlow; the Lark, from Bury St. Edmunds and Mildenhall; the Little Ouse, from Brandon; the Wissey, from Watton; and the Nar, from the west of Swaffham.
The valley has been, along the greater part of its course, cut down to the horizon of the Gault (fig. 2, Pl. XV.)—its upper portion being enclosed on either side by gentle slopes of Chalk-marl, succeeded by hills of the Lower Chalk. North of Cambridge it opens out into a broad expanse of fen-land, overlooked from the east side only by the high chalk range. Rarely do any beds lower than the Neocomian crop out; but by these and the Gault the Fen is skirted all the way to the sea.
The Chalk escarpment receded to its present position or thereabouts in the Pliocene period: during the progress of its submergence for, and reelevation after, the Eocene deposits it had been subjected to some disturbance, many small faults, flexures, and contortions being the result. To a considerable flexure of this period is owing the present position of an upper extremity of the valley.
That the Cambridge valley, as such, is mainly preglacial is evident from the consideration of several reasons, amongst others the position which the Boulder-clay now occupies therein[7]. It rests on the escarpment at an elevation of more than 500 feet; it is found here and there on the flank at lower and lower levels; and it occurs in the bottom of the valley at but a small height above the sea. It is true that in those instances where Boulder-clay is found on the flank it is in small outlying patches only (fig. 2, Pl. XV.); but this is inevitable from the progress of recent erosion; and they are quite sufficient to prove the former extension of the mass. These patches now cap small elevations resulting from denudation; and a line drawn from the escarpment to the low ground would intersect them all.
Although the escarpment had been cut back to nearly its present position during the Pliocene period, and the land was somewhat submerged during the deposition on the east coast of the Pliocene beds, we find in this valley no signs of their present or former existence. For the valley was not excavated to its present depth, by perhaps 50 or 100 feet; consequently any deposits that may have been left in it during the Pliocene era have long been swept away, for the same reason, perhaps, we have no beds of Lower Glacial age, which are, so far as I have seen, confined to areas of comparatively slight elevation. It is not difficult to suppose that these Lower Glacial deposits would be excluded from this valley by its then height relatively to that at which they are now found, or that any which may possibly have once occupied the area have been subsequently removed.
But it is far more difficult to account for the apparently total absence of the Middle Glacial deposits, which, just over the Chalk to a height of 300 feet or more above the sea, and to at least 200 feet above what must have been the bottom of this valley at the time of their deposition. The subsequent physical conditions were not favourable to, nor was the time sufficient for, the removal from the valley of beds of any extent (assuming them to have been therein deposited) before the deposition of the Great Chalky Upper Boulder-clay. We should find them still, however great the subsequent denudation, between the older rocks and the Boulder-clay in some at least of the many instances in which it occurs. It cannot be assumed, as in the case of the Pliocene and the Lower Glacial, that the Middle Glacial beds at one time occupied the valley, and were afterwards, and before the Upper Boulder-clay period, removed; yet in no instance do we find the Boulder-clay resting on any other than preglacial formations. As suggested in the first part of this paper[8], the currents from the north that- formed the Middle Glacial gravels and sands were confined to the seaward side of the Chalk range, which was not wholly submerged, and were entirely excluded from the Cambridge valley; for, as we have seen, if they had had access to the valley, and had not been excluded therefrom as suggested, some undoubted traces of the series of gravels and sands would remain and testify to the fact. There are certain gravels which, I admit, bear a striking resemblance to those of undoubted Middle Glacial origin; but my reasons will presently be given for believing them to be of much more recent date. In some cases the Boulder-clay has a gravelly base; but it is that and nothing more, representing the gradual change of physical conditions which occurred when the Chalk-ridge was nearly submerged. At this time, although the southerly set of the water still continued[9], its currents possessed much less power of transporting and rearranging gravels, and their deposition ceased. But the change was gradual; and we may readily conceive that during the transition -period the surface of the Chalk, being slowly encroached upon, would be partly covered over with a wash from the rock itself, and perhaps also from the gravels. This wash is not a clean gravel; and it gradually passes up into the Boulder-clay, which then began to be deposited.
We have seen that the Cambridge Valley was excavated in preglacial times, and that in all probability no Lower or Middle glacial beds were formed within its area. But when the Chalk was wholly submerged[10], the sweeping currents, hitherto confined to the east side of the range, were replaced by an open sea extending over every part of East Anglia that does not now attain to an elevation of 500 feet. In this sea the icebergs laden with rocky débris from the north were slowly melted; and their freights, descending in mass, formed on the bottom the unstratified Boulder-clay. There had been icebergs borne along in the Middle-glacial currents also; but they were swept off more quickly to the southward, and probably a great majority of them may have melted over an area still beneath the waters of the Atlantic.
The Upper Boulder-clay is found all along the top of the Chalk range; it caps the minor elevations on the flank, and it occupies some of the lower ground in the valley. At one time it doubtless spread as a sheet over the whole area, from the highest point of the 'scarp down to, if not below, the present sea-level. It has since been so much denuded that its main mass on the hills is disconnected from the remainder, which now exists merely as outliers on the smaller hills and ridges.
Noting that the Boulder-clay is found on the back and top of the escarpment, as well as on the low ground beyond the foot of it, we might assume that there is no "horizontality" in its mode of occurrence, and that its present boundary-lines would ignore all the contours, and features even, of the country. To some extent, and in small areas, this may be so; but looking at the Boulder-clay as a whole, there is a striking regularity in its occurrence. When viewed on a true scale, in the exceptional case of the Chalk escarpment it is seen to plunge down about 500 feet; but this being in a distance of not less than 10 miles, represents a fall of 1 in 100 only, or an angle of about half a degree. It touches, as any other formation would be expected to do, the outliers and hills within the larger valley which are sufficiently high to reach its horizon ; and its boundary-line conforms generally to the present features of the district.
As the surface of the Chalk, when gradually encroached on by the glacial sea, became partly covered by a gravelly wash, so also did the Boulder-clay on its emergence[11]. Of this gravel or loam but little remains, and that invariably on the higher grounds, in patches which once formed portions of extensive plateaux. I think it can scarcely be assumed that so large a mass of material as the Boulder-clay which once filled the valley could have been removed by denudation, marine or subaerial, or by both combined, without its component particles having been more or less reassorted and re- deposited on the denuded surface. This may have been the origin of certain deposits of doubtful age which occur, here in the form of loam, there as an impure gravel.
Besides the high-level deposits, there are in the valley broad sheets of much newer valley-gravel, enclosing the remains of recent shells and of extinct mammalia. They occur in three or more terraces at different levels, marking as many points in the progress of the valley's formation, the higher and older terraces having been already reduced to mere patches, indicating a former extension, and testifying to the rapidity of denudation. These gravels have been made up mainly from the waste of the Chalk and superincumbent Boulder-clay, and may be seen in many sections in the neighbourhood of Cambridge. Much has been already written about them; they have now been mapped by the Geological-Survey Officers, and will be fully described by them also in their publications. The same remark applies to the Fen-lands, which are of more recent date than the Drifts proper that form the subject of this communication.
But there are within the area certain other gravels upon which a few remarks are necessary, owing to their resemblance to those of Middle-glacial age. These deposits occur at an elevation of 20 to 60 feet, or thereabouts, above the level of the river, and consist of gravels and sands with intercalated masses of loam and clay, the latter having somewhat the appearance of Boulder-clay, or, rather, of a wash from Boulder-clay in its immediate vicinity. The lines of stratification are irregular, sometimes horizontal, more frequently inclined, and, in the two most noteworthy sections, several miles apart, dip north at an angle of 15°. In one of these sections are two gravels, or, rather, a gravel and a loam, the latter being banked up against a scooped-out edge of the former, thus presenting the appearance of a fault.
There is a Middle-glacial appearance about the gravels; but still I think that all the phenomena might occur as well either in an esker or in deposits left by a river of tolerable magnitude. Inclined bedding is not unusual in river-gravels; and the included masses of wash from Boulder-clay are easily accounted for, if we assume that the gravels were formed during the removal by denudation of the Boulder-clay which once filled the valley. The uniformity of level at which they occur would also seem to indicate a fluviatile rather than a glacial origin. But the strongest argument in support of this view is the fact that the gravels in places contain recent land and freshwater shells (Helix, Pupa, Succinea); this would be conclusive evidence if the shells could be more generally found. But it is just possible, although far from probable, that in the few instances only in which (so far as I know) they have been discovered, a newer gravel may have been deposited in channels cut by streams through that which is of older date. In my opinion the parts of the gravel where the shells do occur so exactly resemble the other portions where they do not, that such a proposition cannot be entertained. Therefore without hesitation I refer these deposits to the existence of an ancient river running, as does the present one, along the foot of the escarpment, although, of course, not on the ground occupied by the Cam of our own time. The Chalk 'scarp was then somewhat different from what it is now; it had been cut down to such a level only that its foot was higher by as much as these gravels are now above the river. The line of this old river is now indicated by an elongated series of patches of the gravel in question which formerly occupied its channel. The remaining portions of these gravels now form long, low, and rounded ridges, occupying, probably, slight depressions in the Chalk, which depressions (formerly a part of the old river-course) are now somewhat elevated above the surrounding area, the gravels having partly preserved, as usual, the lines of hollow from denudation. This series commences near Royston, in a hollow formed by the flexure in the Chalk previously mentioned[12], runs in an easterly direction about parallel to the present course of the river, crosses the minor valley of the Cam or Granta, is joined by another line of similar patches from the east, and then sweeps round to the north of Cambridge, being lost in the fens beyond (fig. 1, Pl. XV.).
I am well aware that this solution may not explain the origin of all the patches of gravel that are found in the valley and cannot with certainty be referred either to the Glacial, Postglacial, or Recent period. But I do not think that any of them have been actually traced under the Boulder-clay, as they would have been if of Lower- or Middle-glacial origin. It is possible that during the period in which the drifts were being formed by the Middle-glacial currents traversing the east coast, a stray iceberg may have occasionally found its way into the land-locked inlet of the Wash and its extension the Ouse and Cam valleys, at that time so much broader through greater submergence. But this would be, I apprehend, a very exceptional occurrence; and any deposits dropped from such bergs are not to be classed with the current-formed gravels that occur in such force on the other side of the escarpment.
Taking all the facts into consideration, I feel justified in drawing the conclusion that, strictly speaking, there are no Lower or Middle Glacial deposits within the area of the Cambridge valley[13].
Supplementary Notes.
Since the above notes and the conclusions therefrom were brought before the Society, the completion of the mapping of some of the gravels, mentioned as of doubtful age, seems to have thrown light upon the date and mode of their formation. It has afforded, also, additional evidence in support of the theory advanced respecting those gravels which were thereby asserted to be relics of an ancient river Cam.
There is a hill called "The Rivey," situated about ten miles S.E. of Cambridge; its height above the Cam is about 325 feet; it is, as it wore, an outlier from the escarpment, and it is similarly covered by Boulder-clay. This Boulder-clay rests directly on the Chalk, and is capped by a few feet of gravel; it is supposed that this is a patch of "Denudation gravel," and that the Boulder-clay beneath it retains its full thickness; or it may be a higher example of those to be described. In a direction running away from the escarpment are patches of gravel at a level several feet lower; then, further on, other patches at a still lower level, and others, still further away, with less elevation, the Boulder-clay being continuous beneath them all. At Hildersham, in the valley, 112 mile from the Rivey and 200 feet below the summit of the hill, the gravel and Boulder-clay occur in the same relative positions; and a little further on, the gravel overlaps the clay and rests on the Chalk.
This interesting and well-marked geological feature has been preserved, it may be, by its position in the bend to the north of the escarpment; and it affords a key to many other patches of gravel in similar positions, and heretofore considered as of doubtful age (p. 200). All the patches (many of which are small), and the indications of several other patches removed by recent denudation, seem to form parts of lines that once were continuous, and which are more or less at right angles to the long series ascribed to an old course of the Cam. They continue in a gradually descending order from almost the top of the 'scarp down to the level of that series—that is, from 325 feet down to 20 feet above the present river.
The explanation proposed is, that the slope of the Preglacial 'scarp was entirely covered by Boulder-clay, that the valley was, in fact, almost filled by that deposit, the patches of Boulder-clay which here and there remain marking out the line of that Preglacial escarpment (see ante, p. 198). This Boulder-clay has been removed entirely by postglacial erosion; and the Chalk has been cut back beyond the old line, except where such patches still remain. In the early Postglacial times, when the valley began to be reexcavated, a main stream must have run approximately in the same direction as the present river, but then, of course, at a height considerably above it—as much higher really as the thickness of the Boulder-clay that has since been removed. The escarpment covered by Boulder-clay was cut back then in the same way as it is being cut back now, by minor streams running across the strike (p. 196) into the main river parallel thereto. This operation began immediately upon the reelevation of the land after the great submergence, and has continued up to the present time.
There must have been very considerable accumulation of detritus from the removal of such a mass of Boulder-clay; and that removal would be the more rapid in proportion to the greater steepness of the area subject to denudation. These accumulations would, in turn, be removed; but it is suggested that in the mass of Boulder-clay and overlying patches of gravel leading down from the Rivey to the valley below we have evidence of the later stages of that clay's denudation, and small remnants of the resulting accumulation.
The gravels are made up of exactly such material as would be derived from the waste of Boulder-clay—rolled lumps of hard and soft chalk, flint pebbles, with some fragments of quartz, limestone, and other derived rocks, enclosing broken pieces of derived fossils. They constitute also a gradually descending series, without any very great break in their continuity, either in regard to level or position. All the patches appear to occupy hollows in the surface of the clay, by which circumstance they have, indeed, been preserved—that is, down to the level of the Preglacial valley-system; below it the gravels rest on the excavated chalk. In many instances they occur in hollows along the top of a ridge—the old stream-channels in which they were left now forming synclinals, and thus preserving the gravels and the clay or chalk immediately beneath, while that on either side has been worn away.
The marine Middle-glacial currents could not have formed these gravels, even if they had access to the valley, as some of the ridges run up into coombe-like valleys where such currents would have been impossible. The streams that deposited them were doubtless rapid, and occupied gorges in the clay; this would account for the false-bedding sometimes exhibited.
The series described as indicating the ancient course of the Cam represents the latest stage of all, previous to the present system of drainage; in this series and in patches at or slightly above the samo level occur the recent land and freshwater shells. At one place in this series shells of Cardium edule have been found; and their occurrence would seem to bear out the suggestion made, that during the Glacial period the Cam valley formed a land-locked bay, on the sandy shores of which this mollusk nourished (p. 194). It occurs, but not plentifully, at the base of the deposit, and probably indicates the position of the old shore-line.
If the explanation offered be the true one, we have here a series of river-gravel terraces occurring at all heights, from nearly the bottom of the present valley up almost to the top of the Chalk escarpment.
| Approximate Elevations (above the Cam) of two lines of Gravel Patches. | ||||||
| feet. | feet. | |||||
The Rivey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
325 | Balsham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
370 | |||
Intermediate patches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
270 | Small patch W. of Balsham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
250 | |||
| 200 | Gravel-pits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
205 | ||||
Hildersham . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
125 | (Possible continuation of the above.) | ||||
| (Indications of other patches between the above.) |
Gravel-pits N.W. of Dungate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
175 | ||||
Wratting valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
110 | |||||
| Ancient course of the Cam, 20 to 60 feet. | ||||||
EXPLANATION OF PLATE XV.
Discussion.
Prof. Duncan said that his impressions, formed twenty years ago, agreed with those of the author. It was a very simple case of geology complicated by geologists. He inquired whether the author thought that such valleys as extend up to the Chalk from the German Ocean were excavated before the Glacial period, or whether they were then filled up and afterwards excavated. He also asked whether the author was quite satisfied that all the gravels are waterworn by tidal action. It seemed to him exceedingly interesting to find that Mr. Penning pushed the ice-cap a little further back.
Prof. Morris remarked that the valley of the Cam presents two or three different conditions; near Cambridge the river flows over the Gault, cutting through the old gravel between Barnwell and Chesterton. The valley was probably excavated in the Boulder-clay, which occurs at higher levels, and once no doubt covered the whole district.
Mr. Jukes-Browne thought that the Boulder-clay did fill the valleys, and that the boulders referred to by the author had been derived from the Boulder-clay. One point of special interest in the paper seemed to him to be that the author regarded the whole Glacial series as continuous, in opposition to those who would place the excavation of all the valleys in the Glacial period, while the author considers them preglacial. At Cromer the so-called Middle Glacial is nothing but blown sand.
Mr. George Maw inquired as to the order of sequence of certain beds known as Lower, Middle, and Upper Glacial, what evidence there is with regard to those extensive coast-beds which occur all along the coast from Yarmouth to Cromer, and especially what evidence we have as to the so-called Upper Boulder-clay on the coast being really newer than the great mass of the drift of high Suffolk.
Prof. Ramsay thought that the question raised by Mr. Maw had been thoroughly discussed at the last meeting of the Society. He had had much pleasure in listening to Mr. Penning's paper, because that gentleman seemed to have aimed at getting rid of the infinite refinements in the classification of these beds as Upper, Lower, and Middle Glacial deposits, these things being generally treated in such a manner as to produce, at least in the popular mind, a notion that these deposits are of as great importance as Upper and Lower Silurian, for example, with beds thousands of feet thick. He thought that the effects of Glacial action had been immensely exaggerated, and believed that all the great features of the country existed before the Glacial period.
The President (Mr. Evans) considered Mr. Penning's paper of great importance from two points of view—first, with respect to the sequence of the Glacial beds, and, secondly, with regard to the Cambridge valley. He thought that the author's views as to the southern part of the Cambridge valley would be borne out when he came to examine the portion north of the Lark, and further north in the neighbourhood of Brandon, where he would be able to trace the connexion between the Glacial and Postglacial gravels. With regard to the ice-cap, there were certain points that deserve great attention, especially the question whence are derived the materials of the Boulder-clay. These seem to be in great measure derived from the Upper Cretaceous and Tertiary beds in the neighbourhood. And in some cases unworn flints show glacial scratches; hence he suggested that inquiry should be made whether they were due to local glaciers.
The Author, in reply, stated that the Cambridge valley was excavated in Preglacial times. There is no evidence of extended denudation in Glacial times, but only of local unconformity.
- ↑ At Tharfield (S. of Royston) 550 feet, and in West Norfolk 450 to 650 feet above the sea.
- ↑ Lyell, 'Student's Elements,' p. 177.
- ↑ Lyell, 'Student's Elements,' p. 167.
- ↑ 'Outline of the Geology of the Upper Tertiaries of East Anglia,' p. 16.
- ↑ "Geology of West Norfolk," Phil. Mag. vols. vii. and viii.
- ↑ "England was again joined to the continent during the time that the vegetation of the 'Forest-bed' flourished" (Ramsay, 'Physical Geology,' p. 178).
- ↑ Ramsay, 'Physical Geology,' p. 211.
- ↑ Ante, p. 193.
- ↑ Ante, p. 194.
- ↑ Ante, p. 195.
- ↑ Ante, p. 198 and Part 1. p. 196.
- ↑ Ante, p. 197.
- ↑ Since the above paper was read the author's attention has been called to the fact that in the 'Geological Magazine' for February 1870, Mr. S. V. Wood, Jun., published a statement to the effect that the Middle Glacial beds in East Anglia do not occur at a greater elevation than 250 feet; also, that in a section illustrating a paper by him read before the Society, June 19, 1867, the overlap of those beds by the Boulder-clay, and the occurrence of the latter in the Cambridge valley without any Middle Glacial are represented. Although Mr. Wood did not draw the same inferences from these facts, he is entitled to priority in their observation.—W. H. P.