Man and the Glacial Period

by G. Frederick Wright

[PREFACE TO THE SECOND EDITION.]

Since, as stated in the Introduction ([page 1]), the plan of this volume permitted only “a concise presentation of the facts,” it was impossible to introduce either full references to the illimitable literature of the subject or detailed discussion of all disputed points. The facts selected, therefore, were for the most part those upon which it was supposed there would be pretty general agreement.

The discussion upon the subject of the continuity of the Glacial period was, however, somewhat elaborate (see pages [106-121], [311], [324], [332]), and was presented with excessive respect for the authority of those who maintain the opposite view; all that was claimed ([page 110]) being that one might maintain the unity or continuity of the Glacial period “without forfeiting his right to the respect of his fellow-geologists.” But it already appears that there was no need of this extreme modesty of statement. On the contrary, the vigorous discussion of the subject which has characterized the last two years reveals a decided reaction against the theory that there has been more than one Glacial epoch in Quaternary times; while there have been brought to light many most important if not conclusive facts in favour of the theory supported in the volume.

In America the continuity of the Glacial period has been maintained during the past two years with important new evidence, among others by authorities of no less eminence and special experience in glacial investigations than Professor Dana,[A] Mr. Warren Upham,[B] and Professor Edward H. Williams, Jr.[C] Professor Williams’s investigations on the attenuated border of the glacial deposits in the Lehigh, the most important upper tributary to the Delaware Valley, Pa., are of important significance, since the area which he so carefully studied lies wholly south of the terminal moraine of Lewis and Wright, and belongs to the portion of the older drift which Professors Chamberlin and Salisbury have been most positive in assigning to the first Glacial epoch, which they have maintained was separated from the second epoch by a length of time sufficient for the streams to erode rock gorges in the Delaware and Lehigh Rivers from two hundred to three hundred feet in depth.[D] But Professor Williams has found that the rock gorges of the Lehigh, and even of its southern tributaries, had been worn down approximately to the present depth of that of the Delaware before this earliest period of glaciation, and that the gorges were filled with the earliest glacial débris.

[A] American Journal of Science, vol. xlvi, pp. 327, 330.

[B] American Journal of Science, vols, xlvi, pp. 114-121; xlvii, pp. 358-365; American Geologist, vols, x, pp. 339-362, especially pp. 361, 362; xiii, pp. 114, 278; Bulletin of the Geological Society of America, vol. v, pp. 71-86, 87-100.

[C] Bulletin of the Geological Society of America, vol. v, pp. 13-16, 281-296; American Journal of Science, vol. xlvii, pp. 33-36.

[D] See especially Chamberlin, in the American Journal of Science, vol. xlv, p. 192; Salisbury, in the American Geologist, vol. xi, p. 18.

A similar relation of the glacial deposits of the attenuated border to the preglacial erosion of the rock gorges of the Alleghany and upper Ohio Rivers has been brought to light by the joint investigations of Mr. Frank Leverett and myself in western Pennsylvania, in the vicinity of Warren, Pa., where, in an area which was affected by only the earliest glaciation, glacial deposits are found filling the rock channels of old tributaries to the Alleghany to a depth of from one hundred and seventy to two hundred and fifty feet, and carrying the preglacial erosion at that point very closely, if not quite, down to the present rock bottoms of all the streams. This removes from Professor Chamberlin a most important part of the evidence of a long interglacial period to which he had appealed; he having maintained[E] that “the higher glacial gravels antedated those of the moraine-forming epoch by the measure of the erosion of the channel through the old drift and the rock, whose mean depth here is about three hundred feet, of which perhaps two hundred and fifty feet may be said to be rock,” adding that the “excavation that intervened between the two epochs in other portions of the Alleghany, Monongahela, and upper Ohio valleys is closely comparable with this.”