Drift (geology)

“ We may be sure, then, that the commencement of the drift period, and the deluge of Noah, cannot have been synchronous. But the drift agency, connected, as nearly all geologists seem now to be ready to admit, with the vertical movements of continents, may have operated, and undoubtedly has, at various periods, and very possibly, in some parts of the world, long posterior to the period usually called the drift period. ”

“ In many places the upper surface of the country rock beneath the drift has been swept clean of residual clays and other waste. All rock rotten has been planed away, and the ledges of sound rock to which the surface has been cut down have been rubbed smooth and scratched with long, straight, parallel lines. The agent which laid the drift can hold sand and pebbles firmly in its grasp and can grind them against the rock beneath, thus planing it down and scoring it, while faceting the pebbles also. ”

“ Wear of drift in transit.—Drift carried at the bottom of the ice is subject to notable wear. The materials in transportation abrade one another and are abraded by the bed over which they pass. Englacial drift is subject to less wear because it is commonly more scattered. Superglacial drift is worn little or none while it lies on the surface of the ice; but in so far as superglacial or englacial drift is derived from the basal load, it may show the same evidences of wear as the basal drift itself. ”

“ The preservation of such surfaces after a second ice invasion must be regarded as the exception rather than as the rule. There is always the possibility, too, that an oxidized and weathered zone marking the surface of an older drift sheet exists, where excavations have not opened full sections of drift to view. The absence of weathered zones of drift beneath the surface, or the absence of knowledge of their existence, is therefore at best no more than negative evidence. ”

“ Where portions of the till have been removed by the sea on the shores of the Forth, or in the interior by railway cuttings, the boulders imbedded in what remains of the drift are seen to have been all subjected to a process of abrasion and striation, the striæ and furrows being parallel and persistent across them all, exactly as if a glacier or iceberg had passed over them and scored them in a manner similar to that so often undergone by the solid rocks below the glacial drift. ”

“ THE THICKNESS OF THE DRIFT. The drift is far from uniform in thickness. It is comparatively thin and scanty over the Laurentian highlands and the rugged regions of New England, while from southern New York and Ontario westward over the Mississippi valley, and on the great western plains of Canada, it exceeds an average of one hundred feet over wide areas, and in places has five and six times that thickness. It was to this marginal belt that the ice sheets brought their loads, while northwards, nearer the centers of dispersion, erosion was excessive and deposition slight. ”

“ Over much of the northern states there is spread an unstratified stony clay called the drift. It often rests on sound rocks. It contains grains of sand, pebbles, and bowlders composed of many different minerals and rocks that the country rock cannot furnish. Hence the drift cannot have been formed by the decay of the rock of the region. A shale or limestone, for example, cannot waste to a clay containing granite pebbles. ”

“ Under these conditions, the climate which would have obtained in the area of the drift exposed during the minor retreat would have been cold, and oxidation, leaching, and disintegration would have proceeded slowly. If they reached considerable depths, the time involved must have been very long. If this surface of oxidized and leached and disintegrated drift were found to reach far to the northward beneath the layer of newer and upper drift, it would indicate a great recession of the ice. ”

“ But if the stratified deposits subsequent [Pg 123] to the drift present us with beings no longer alive on the globe, much more would the drift. Now, the presumption is, that extinct animals and plants belong to a creation anterior to man, especially if they exhibit a tropical character,—as those do which are usually assigned to the drift,—since we have no evidence of a tropical climate in northern latitudes till we get back to a period far anterior to man.
Secondly. No remains of man or his works have been found in drift, nor indeed till we rise almost to the top of the alluvial deposit. ”

“ In fact they are practically confined between the Rocky Mountains and the Missouri River on the west, and the Ural Mountains on the east; and with a little common sense infused into the foundation principles of the science we will cease to be tormented with a "Glacial Nightmare." Much of the Drift phenomena with the raised beaches are certainly later events than most of the other geological work, but are inseparably connected with the general problem in their explanation. ”

“ This possibility makes care necessary in the application of the criterion, but does not destroy its value. When not itself conclusive, this criterion may be so associated with differential weathering, differential erosion, forest beds, etc., that their combined testimony makes but one conclusion possible.
The absence of evidence of radically diverse directions of movement during the time of deposition of the various sheets of drift, would be no proof that there were not distinct epochs. ”

“ The former is but slightly weathered, while the latter is so old that it is deeply reddened by oxidation and is leached of its soluble ingredients such as lime. The younger drift is bordered by a distinct terminal moraine, while the margin of the older drift is not thus marked. Moreover, the two drift sheets are somewhat unlike in composition, and the different proportion of pebbles of the various kinds of rocks which they contain shows that their respective glaciers followed different tracks and gathered their loads from different regions. ”

“ And, if physical science shows that no such Deluge as that described in Genesis could have occurred, geology is equally positive that it never did occur. The drift and boulders which cover a great part of Europe and North America are beyond all doubt glacial, and not diluvial. They are strictly limited by the extension of glaciers and ice-sheets, and of the streams flowing from them. The high-level gravels in which human remains are found in conjunction with those of extinct animals, are the result of the erosion of valleys by rivers. ”

“ It is no answer to this argument to say that the materials lying at the very front of the drift deposits contain boulders which were derived from the disintegrated rock over which the ice has passed, and that they were therefore in a less firm state at the outset. In many cases these boulders have come from great distances, and coming from great distances they must have come in a firm and solid state, else they could not have suffered such extensive transportation, except indeed their position was superglacial throughout their whole journey. ”

“ The weather destroys all smoothed, polished, and scored surfaces which are not protected beneath glacial deposits. The oversteepened sides of the trough are graded by landslips, by talus slopes, and by alluvial cones. Morainic heaps of drift are dissected and carried away. Hanging valleys and the irregular bed of the trough are both worn down to grade by the streams which now occupy them. The length of time since the retreat of the ice from a mountain valley may thus be estimated by the degree to which the destruction of the characteristic features of the glacier trough has been carried. ”

“ The evidence of the marine origin of the highest drift is no doubt inconclusive in the absence of shells, so great is the resemblance of the gravel and sand of a sea beach and of a river’s bed, when organic remains are wanting; but, on the other hand, when we consider the general rarity of shells in drift which we know to be of marine origin, we cannot suppose that, in the shelly sands of Moel Tryfaen, we have hit upon the exact uppermost limit of marine deposition, or, in other words, a precise measure of the submergence of the land beneath the sea since the glacial period. ”

“ It is true that at each oscillation of level the solid rocks composing the original continent suffer some fresh denudation, and do not remain unimpaired like the wooden and glass framework of the hour-glass, still the wear and tear suffered by the larger area exposed to subaërial denudation consists either of loose drift or of sedimentary strata, which were thrown down in seas near the land, and subsequently upraised, the same continents and oceanic basins remaining in existence all the while. ”

“ It is evident that the greater the distance north of the overlying drift remains of temperate plants are found, the more conclusive becomes the evidence. Plant remains indicating temperate climate at the very margin of the drift sheet which overlies them, would be less conclusive than similar evidences one hundred miles to the northward. It might be difficult to prove in any given instance that the ice which deposited the drift overlying plant remains advanced one hundred miles, or any other specific distance, south of any particular underlying forest bed. ”

“ We hold that these phases of glacial drainage deposits may be so related to each other, to the valleys in which they occur, and to more or less distinct bodies of glacier drift, as to prove so great a recession of ice between the diverse phases of drainage deposition, as to constitute the second advance a distinct ice epoch.
The absence of evidence that the land stood at different elevations during different parts of the period of drift deposition, does not in any way militate against the theory of recurrent and distinct ice epochs. ”

“ The auriferous quartz of these drifts is derived from veins apparently due to hydrothermal agency, proceeding from granite and penetrating strata supposed to be of Jurassic and Triassic date. The fossil wood of the drift is sometimes beautifully silicified, and occasionally the trunks of trees are replaced by iron pyrites, but gold seems not to have been found as in the pyrites of similarly petrified trees in the drift of Australia. ”

“ The drift is also deeply weathered. The till, originally blue in color, has been yellowed by oxidation to a depth of ten and twenty feet and even more, and its surface is sometimes rusted to terra-cotta red. To a somewhat less depth it has been leached of its lime and other soluble ingredients. In the weathered zone its pebbles, especially where the till is loose in texture, are sometimes so rotted that granites may be crumbled with the fingers. The Kansan drift is therefore old.
Fig. 363. Plain of Wisconsin Drift, Iowa
The Wisconsin formation. ”

“ In glaciers such as those of Greenland, whose basal layers are well loaded with drift and whose surface layers are nearly clean, different layers have different rates of motion, according to the amount of drift with which they are clogged. One layer glides over another, and the stones inset in each are ground and smoothed and scratched. Usually the sides of glaciated pebbles are more worn than the ends, and the scratches upon them run with the longer axis of the stone. ”

“ If the cold of the glacial epoch came on slowly, if it was long before it reached its greatest intensity, and again if it abated gradually, we may expect to find the earliest and latest formed drift less barren of organic remains than that deposited during the coldest period. ”

“ I need hardly say that drift is the term applied by geologists to those superficial deposits of sands, gravels, clays, and loams which we find to have been spread out over the older rocks in many districts by the driving action of currents of water, whether salt or fresh, or by the drifting action of ice. ”

“ The term drift is now applied generally to the Quaternary deposits, which consist for the most part of gravel, sand, loam or brickearth and clay; it naturally refers to strata laid down at some distance from the rocks to whose destruction they are largely due; but, although applied to river deposits, the word drift is more appropriately used in reference to the accumulations of the Glacial period. ”

“ Here and there on the chalk may be noticed outlying patches of Tertiary sand and clay, the remains of a once extensive formation, the denudation of which has chiefly furnished the Drift gravel material in which the relics of man and the bones of extinct animals lie buried. The Drift alluvial deposit of the Somme valley exhibits nothing extraordinary in its stratification or outward appearance, nor in its composition or organic contents. The stratum in which the bones of the Drift fauna are found intermingled with the relics of man is partly a marine and partly a fluviatile deposit. ”

“ Explorers in search for gold have made extensive cuttings through the underlying drift-gravel, down to the slate rock upon which it rests; and during one of these excavations, at a depth of nine feet, intermingled with the gravel and boulders of the drift, three large implements were found, nearly resembling the rude flint hatchets of the drift type. Examples of this class, however, though repeatedly noted, have been too isolated to admit of their use for any such comprehensive inductions as the disclosures of the glacial drift of north-western Europe have justified. ”