Popular Science Monthly/Volume 43/June 1893/The Bay of Fundy Tides and Marshes
|THE BAY OF FUNDY TIDES AND MARSHES.|
CONCERNING the Bay of Fundy the school-books generally note the single fact that "here the tides rise higher than anywhere else in the world." But so meager a reference to what is in itself an imposing exhibition of gravitational energy, helpful as it may be in a mnemonic way to the learner of geographical catalogues, gives no hint either of the extraordinary series of physiographical conditions which are the cause of this phenomenon or of those which it creates. The Bay of Fundy is remarkable not only for the grandeur of its tidal phenomena, but equally so for the exquisitely picturesque sculpturing of its coast line, and the diversity, range, and richness of geological evidence thereby revealed; for the unique character of the extensive alluvial tracts that skirt its head waters, and for the wealth of legend, tradition, and romantic incident embodied in the early history of the people that dwell about it.
What is the cause of the extraordinary height of the Bay of Fundy tides? What part have they played in the creation of the Acadian marshes? Whence have been derived the materials for this enormous alluvial deposit? And what is the source of its haustless fertility? These are questions often asked by tourists, and which are answered, imperfectly no doubt, in the following pages.
North, of Cape Cod the continental coast line recedes abruptly westward, and then sweeps in a long curve northeastwardly till the head waters of the Bay of Fundy are reached. Turning again on itself, its course is westward to Cape Sable, from which it again stretches away toward the east as the southern shore of Nova Scotia. Thus, between Capes Cod and Sable lies the long, narrow, open Bay of Maine, which terminates toward the north and east in the landlocked Bay of Fundy. In the shallow waters of this larger open bay the tidal impulse, which over ocean depths moves only as a wave of vertical oscillation, is changed into one of translation. As the effect of this transformation the whole body of water moves first landward, and then, sweeping round with the curving coast line, skirts the southern shores of Maine and New Brunswick, till it reaches the narrow strait between Briar Island and Grand Manan. Compressed between these closer limits the water is forced onward with increasing velocity into the Bay of Fundy. Part finds its way into the Annapolis Basin and its tributary rivers, while the main current moves onward till it meets the tongue of land which terminates in Cape d'Or. Here it divides, the northern portion filling Shepody and Cumberland Basins; while the southern half rushes onward through the narrow entrance to the Basin of Minas. As it passes Cape Blomidon this swirling, eddying, foaming torrent reaches its greatest velocity—a rate of ten or twelve miles an hour.
Thus it is that the long, sickle-curved Maine coast gradually gathers up the water rolled upon it twice a day by the rhythmic ocean movements, and, throwing it backward, presses it at last into the funnel-shaped Bay of Fundy and its adjacent basins, covering with a semi-daily flood the low and unprotected marshlined shores and filling the channels of the tributary rivers for many miles inland to a height of ten, twenty, or thirty feet above their fresh-water levels. Such, in a general way, is the set of conditions under which the spectacular and physiographical effects of ordinary tidal phenomena are exaggerated in the Fundy tides far beyond their normal limits. At some points the extreme elevation of the flood tide above low-water mark is as great as seventy feet. In some of the rivers, particularly in the Peticodiac of New Brunswick and in the Shubenacadie of Nova Scotia, the upward flow against the fresh-water current forms a rapidly moving wall or bore several feet in height, the rushing sound of which can be heard at a considerable distance, while in others the two currents meet and mingle so quietly that an observer can hardly tell where the backward flow begins.
Lining the shores of the head waters of the bay and spreading far inland up the valleys of its river tributaries are extensive tracts of alluvial marsh land of remarkable fertility, and differing in their origin from other so-called marshes. In general, alluvial deposits are formed in river basins, by materials washed down from higher levels by fresh-water floods; but here the whole deposit is of tidal origin, the result of a landward rather than a seaward transportation. Every incoming tide is freighted with a finely comminuted sediment, the product of the wearing action of the currents upon the sides and bottom of the bay. During the interval between the flood that covers the undiked river and basin margins and the ebb that leaves them bare again, the sediment is deposited as a film of soft and glistening mud upon the somewhat hardened material left by previous tides. Thus layer after layer accumulates, until the flat becomes too high for any but extraordinary tides to cover.
Instructive illustration these marsh flats often give of Nature's methods in the preservation of the records by which the geologist reads the physical history of the earth. So plastic and impressionable is the mud which an outgoing tide has left that it easily takes and holds the tracings of any disturbing contact. A wind-blown leaf, a resting insect, a drop of rain, may make in it a tiny mold which, hardened somewhat before the next incoming flood, receives thereafter successive linings to which it gives its form and markings. In this way even the rain-prints of a passing shower have been fixed, and then completely covered up; and yet when subsequently exposed, so perfectly were the spatter marks preserved, that one could tell in which direction the wind was blowing when the shower fell.
It is obvious that the deposition of tidal sediment can, in general, be made only between the lower and the higher limit levels of the ebb and flow. The accumulation of greater depths of mud than such a range permits can only be accounted for by the supposition of a gradual subsidence of the littoral areas—a movement which would also widen the area of tidal inundations. That such a steady and prolonged subsidence of the Fundy marsh-lined shores has been in progress since the marsh began to form is attested to not only by the surprising depths of mud accumulated, but also by the occurrence in many places, especially along the shores of the Cumberland Basin, of deeply buried forests which were clearly once above the coexistent tidal levels.
A general idea of the geological features of the great depression in which the Bay of Fundy lies is necessary to a fuller understanding of the nature of these Acadian marshes, and especially of the sources of their wonderful fertility. In early geological times, and until long after the close of the Carboniferous period, the bay was much, wider and somewhat longer than it is now. The long ridge of trap rock, known as the North Mountain, which stretches as a huge wall between the Annapolis Valley along its southern, and the waters of the bay along its northern base, did not then exist, and the waters of the bay extended uninterruptedly over the whole of the Annapolis Valley to the base of the Silurian hills which, under the name of the South Mountain, now form the southern inclosure of the valley. Eastwardly the head waters of the ancient bay washed the Devonian and Carboniferous rocks of the Cobequid Hills, while the northern shore line of the present bay, skirting the southern limit of the Palæozoic rocks of New Brunswick, is substantially identical with that of the original bay.
In general character the tidal movements of this larger Atlantic inlet were the same as in the smaller modern bay. And the semidaily ebb and flow of the waters produced, by their incessant attrition with the carboniferous limestones, shales, and sandstones, and the other ancient rocks that formed the bed and margins of the bay, immense quantities of sand and mud—sediment that was redistributed over the greater part of the Fundy Valley. Subsequent changes of level caused a recession of the waters to within their present limits, and brought to view, as the Triassic, or new red sandstone, extensive areas of these deposits. These red sandstone strata are still to be seen in shreds and patches at various points in the Annapolis Valley and on the shores of the Minas and Annapolis Basins. Their general dip toward the north indicates that the epoch-closing movement which narrowed the Bay of Fundy within its present confines was a sinking of the bed along its northern or New Brunswick border.
Following this subsidence, and as the concluding events in the series of seismic convulsions by which the region gained its present topographical features, occurred the volcanic eruptions in which the North Mountain had its origin. This long, trappean wall forms the southern boundary of the bay from Cape Split to the extremity of Digby Neck, a distance of one hundred and twenty-five miles, the only interruption to its continuity being the singular gap called Digby Gut, which gives an entrance into the beautiful Annapolis Basin. Though there were probably many volcanic vents along this extended line of fracture, yet the scene of greatest activity was undoubtedly near Cape Split, at the entrance to Minas Basin, scattered along the shores of which on either side are isolated patches of amygdaloidal trap. Transverse ridges of the same volcanic rock run at intervals, also, across the bottom of the bay.
It is the grinding action of the Fundy waters upon these two Triassic rocks, the trap and its underlying sandstone, that provides the tidal currents with an unfailing supply of muddy sediment. It is mainly in the erosion, transportation, and reprecipitation of these two rocks, and especially of the latter, that the process of marsh formation consists. The incessantly destructive tide-work may be seen at many points along the shore line, perhaps most conspicuously at the base of Blomidon. Here the sandstone foundation is continuously being cut away from under the super-incumbent columnar trap; and at intervals, especially in the spring time, large masses of the igneous rock are loosened from the precipitous mountain side and crushed upon the beach below, where the solvent and abrading action of the waters can reach them. It is after one of these spring slides that the richest harvests of amethystine and zeolitic crystals, for the beauty and abundance of which the Minas shores are noted, can be secured. But it is along the bottom of the bay that the destructive tidal work is most extensive and effective. Here exist great troughs, furrowed out of the soft sandstone, many fathoms deep along the channel bed, with here and there the interruption of the transverse trappean dikes already spoken of.
The sandstone yields, of course, the greater part of the marsh-creating sediment. Its detritus consists of a large percentage of silica, a little clay, the iron which mainly determines its reddish color, and the calcareous matter which served as cement in the parent rock. This material, in the extremely comminuted form in which it occurs in marsh-land soil, would itself afford conditions highly favorable to the support of vegetable life. But an additional cause of the wonderful fertility of the Acadian marshes is the richness of the trap rock in various salts of potash, lime, and alumina which the action of the water mingles freely with the sandstone mud. The plant-supporting power of this complex soil is increased still further by contributions from the upland soils through the medium of the streams and rivers flowing toward the bay.
The great fertility of this alluvium may be inferred from the fact that portions of the Annapolis, Cornwallis, Grand Pré, and Cumberland marshes have been producing annually for nearly two centuries from two to four tons per acre of the finest hay. Besides, it is a common practice, after the hay has been removed, to convert the marshes into autumn pastures, on the luxuriant tender after-growth of which cattle fatten more rapidly than on any other kind of food. Thus, virtually, two crops are annually taken from the land, to which no fertilizing return is ever made. The only portions of the Acadian marshes that have as yet shown signs of exhaustion are those about the Chiegnecto branch of the bay, on the cliffs and bed of which the Triassic rocks do not occur, but in their stead a series of blue and gray " grindstone grits" of an earlier formation. In this region the marshes situated well up toward the head of the tide, where the red soil of the uplands has been mingled with the gray tidal mud, are good, while those lower down are of inferior quality and less enduring. Efforts are being made to renew and improve these inferior tracts by admitting the tide upon them.
In general, however, the necessity for periodic inundations by the muddy waters of the bay in order to maintain the productiveness of the marshes, as implied in the passage from Evangeline—
"Dikes that the hand of the farmer had raised with labor incessant,
Shut out the turbulent tides; but at stated seasons the flood-gates
Opened and welcomed the sea to wander at will o'er the meadows"—
not only does not exist, but, on the contrary, some two or three years are required for the grass roots to recover from the injury done them by the salt water, when, as occasionally happens, an accident to the protecting dikes admits the uniwelcome flood.
The exceedingly fine texture of the soil, and its consequent compactness and retentiveness of moisture, render it for the most part quite unsuitable for the production of root crops, and at the same time adapt it admirably for the growth of hay and of cereals, especially oats, barley, and wheat. As a rule, however, the succession of grass crops is interrupted only at intervals of from five to ten or more years by a single crop of grain. The reproductive power of the grass roots declines perceptibly with long-continued cropping, so that a renewal of the stock by reseeding is occasionally necessary. For this purpose the marsh is plowed in the autumn or spring and new seed sown; but to avoid the loss of a season, since grass does not mature for harvesting the first year, grain is also sown and a large yield usually obtained. This plowing and reseeding at intervals often of many years is the only cultivation the soil receives or requires. There is no reason to suppose that abundant harvests of grain might not be obtained annually for an indefinite period, but, as this would involve annual tilling, the hay crop is more profitable.
Along the river estuaries the encroachment of the land upon the sea is in continual progress, so that there are always considerable areas of unreclaimed salt marsh, the lower portions of which are flooded every day, while the higher portions are covered only by the highest tides. The reclamation of such new marsh is effected by building around its seaward margin a wall or dike of mud to prevent all tidal overflow. After two or three years the salt will have sufficiently disappeared to permit the growth of a crop of wheat, and in a year or two more the best quality of English grass will grow.
At the head of Cumberland Basin an interesting experiment in the reclamation of worthless land has been successfully tried. Large areas of swamp and in some instances shallow lakes have been connected with the tidal waters of the neighboring rivers by channels cut through intervening ridges of upland, thus effecting the double purpose of draining and of admitting the mud-laden tides. In this way, in five or ten years, many acres of worthless swamp have been converted into valuable dike-land.
The use of marsh mud as a fertilizer is very general among farmers to whom it is accessible. It is taken in the autumn or winter from the bank of some tidal creek or river, where the daily depositions can soon replace it, and spread directly on the upla,nd. Its effects are twofold: it enriches with valuable supplies of plant food the soil to which it is applied, and it greatly improves the texture of all light and open soils, making them more compact and firm, and so more retentive of moisture and of those ingredients which are otherwise easily washed away. This permanent effect upon the physical character of the soil which the marsh mud produces renders undesirable its application to clayey soils already compact and firm and moist enough; for it makes them more difficult to work, and more impervious to atmospheric influences. To well-drained hay fields, however, which need but little cultivation, the mud may be advantageously applied, even though the soil be naturally stiff and heavy.
The French settlers were the first Acadian dike-builders. They brought the art from the Netherlands; and to this day no other class of provincial workmen is as skillful in the often difficult work of dike construction as the Acadian French. It was no doubt the existence of these vast areas of marsh land, whose potential value was even then clearly seen, that induced the first New World immigrants to settle about the Bay of Fundy shores; and it was these same broad, fertile marshes left unoccupied by the expulsion of the Acadian French that attracted the New England settlers, whose descendants now derive from them an income aggregating not less than a million dollars annually.