Man and the Glacial Period

by G. Frederick Wright

Speaking a little later of taking the present rates of river erosion as a standard to estimate the chronology of the Glacial period, the same high authority remarks: "It no more affords a true and sufficient guide than it would be to take the tottering paces and weakened force of an old man as the measure of what that individual was, and what he could do, in his robust and active youth. It may be right to take the effects at present produced by a given power as the known quantity, a, but it is equally indispensable, in all calculations relative to the degree of those forces in past times, to take notice of the unknown quantity, x, although this, in the absence of actual experience, which cannot be had, can only be estimated by the results and by a knowledge of the contemporaneous physical conditions. It may be a complicated equation, but it is not to be avoided.[ER]

[ER] Prestwich’s Geology, vol. ii, pp. 520, 521.

“In this country and in the north of France broad valleys have been excavated to the depth of from about eighty to a hundred and fifty feet in glacial and post-glacial times. Difficult as it is by our present experience to conceive this to have been effected in a comparatively short geological term, it is equally, and to my mind more, difficult to suppose that man could have existed eighty thousand years or more, and that existing forms of our fauna and flora should have survived during two hundred and forty thousand years without modification or change.”[ES]

[ES] Ibid., p. 533.

The discussion of the age of the high-level river gravels of the Somme and other streams in northwestern Europe is not complete, however, without considering another possibility as to the mode of their deposition. The conclusion to which Mr. Alfred Tylor arrived, after a prolonged and careful study of the subject, was that the main valleys of the Somme and other streams in northern France and southern England were preglacial in their origin, and that the accumulations of gravel at high levels along their margin were due to enormous floods which characterised the closing portion of the great ice age, which he denominated the pluvial period.[ET] The credibility of floods large enough to accomplish the results manifest in the valley of the Somme is supported by reference to a flood which occurred on the Mulleer River, in India, in 1856, when a stream, which is usually insignificant, was so swollen by a rainfall of a single day that it rose high enough to sweep away an iron bridge the bottoms of whose girders were sixty-five feet above high-water mark. One iron girder weighing eighty tons was carried two miles down the river, and nearly buried in sand. The significance of these facts is enhanced by observing also that for fifteen miles above the bridge the fall of the river only averaged ten feet per mile. Floods to this extent are not uncommon in India. During the Glacial period spring freshets, must have been greatly increased by the melting of a large amount of snow and ice which had accumulated during the winter, and also by the formation of ice-gorges near the mouths of many of the streams. It is probable, also, that the accumulation of ice across the northern part of the German Ocean may have permanently flooded the streams entering that body of water; for it is by no means improbable that there was a land connection between England and France across the Straits of Dover until after the climax of the Glacial period. In support of his theory, Mr. Tylor points to the fact “that the gravel in the valley of the Somme at Amiens is partly derived from débris brought down by the river Somme and by the two rivers the Celle and the Arve, and partly consists of material from the adjoining higher grounds washed in by land floods,” and that the “Quaternary gravels of the Somme are not separated into two divisions by an escarpment of chalk parallel to the river,” but “thin out gradually as they slope from the high land down to the Somme.”

Mr. Tylor’s reasoning seems especially cogent to one who stands on the ground where he can observe the size of the valley and the diminutive proportions of the present stream. Even if we do not grant all that is claimed by Mr. Tylor, it is difficult to resist the main force of his argument, and to avoid the conclusion that the valley of the Somme is largely the work of preglacial erosion, and has been, at any rate, only in slight degree deepened and enlarged during post-Tertiary time.

[ET] Proceedings of the Geological Society, London, November 8, 1867, pp. 103-126: Quarterly Journal of the Geological Society, February 1, 1869, pp. 57-100.

Summary.

In briefly summarising our conclusions concerning the question of man’s antiquity as affected by his known relations to the Glacial period, it is important, first, to remark upon the changes of opinion which have taken place with respect to geological time within the past generation. Under the sway of Sir Charles Lyell’s uniformitarian ideas, geologists felt themselves at liberty to regard geological time as practically unlimited, and did not hesitate to refer the origin of life upon the globe back to a period of 500,000,000 years. In the first edition of his Origin of Species Charles Darwin estimated that the time required for the erosion of the Wealden deposits in England was 306,662,400 years, which he spoke of as “a mere trifle” of that at command for establishing his theory of the origin of species through natural selection. In his second edition, however, he confesses that his original statement concerning the length of geological time was rash; while in later editions he quietly omitted it.