A mastodon (p. [68]) which was found at Greendell in Sussex County quite certainly lived there after the last glacial stage.
Berry (Torreya, vol. X, p. 261) has studied a collection of nine species of plants which had been obtained in peat from near Long Branch. Only three of these now range north of Long Branch. He concluded that the last glacial stage had been followed by a period of climate warmer than the present climate. This is in accord with views which the present writer has held. It ought not, however, to be assumed with too much confidence that the peat-bed is of Late Wisconsin origin.
PENNSYLVANIA.
About half of the area of Pennsylvania lies outside of the region which was glaciated. Figure [10] is a map taken from Folio 172 of the U. S. Geological Survey, published in 1910 and compiled by Dr. W. C. Alden in 1901. A broad strip along the southern part of the State, being non-glaciated, is not represented. The areas shaded by parallel ruling and stippling are those which present evidences of glacial action.
The glaciated area consists of two principal portions. One of these, that subjected to the action of the Wisconsin ice-sheet, is represented on the map by means of oblique parallel lines coming down to an interrupted heavy line. This line, representing the Wisconsin terminal moraine, starts on Delaware River north of Easton, runs northwestward to Potter County, thence into New York, thence back into Pennsylvania, in Warren County, and then enters Ohio north of the Ohio River. The course of this moraine was worked out especially by H. C. Lewis and G. F. Wright and was described in report L of the Pennsylvania Geological Survey, in 1881. The moraine crosses the Delaware at Belvidere, New Jersey, and passes through the following counties: Northampton, Monroe, Carbon, Luzerne, Columbia, Sullivan, Lycoming, Tioga, Potter, Warren, Crawford, Venango, Butler, Lawrence, and Beaver.
South of this moraine are two areas which, on this map, are represented by stippling. These are occupied by drift materials, usually forming a considerably thinner covering, which are believed by most glaciologists to belong to an older Pleistocene stage, probably about as old as the Kansan. Especially in the valleys these older drift deposits may reach thicknesses of 200 or 300 feet. These old glacial deposits are represented also by terraces along the margins of the valleys. Some of these in the vicinity of Warren stand at a height of about 1,400 feet above the sea. Figure 17 is taken from Shaw and Munn (Folio 178, U. S. Geol. Surv., p. 12). The uppermost gravels are supposed to represent the Kansan stage. A few small patches lying in the angle of the unglaciated area are of doubtful age, as indicated on the map. It must be stated, however, that there is some dissent from this conclusion as to the age of this outer drift. Professor E. H. Williams has published a number of papers in which he takes the position that this drift is a deposit laid down by the same ice-sheet that later on built up the Wisconsin moraine (Amer. Jour. Sci., vol. XLVII, 1894, pp. 32–36; Science (n. s.), vol. XXXVII, pp. 447–450; Pennsylvanian Glaciation, first phase, 1917, pp. 1–101). Professor G. F. Wright appears to take the same view. The writer sees no sufficient reason for distrusting the opinions of Dr. Alden and his colleagues.
It must not be assumed that an animal whose remains have been found within the area occupied by the Wisconsin drift lived during or after that stage. Even within this area there may occur fossil-bearing deposits of an older Pleistocene time. These older deposits may underlie the Wisconsin drift or they may occur as old terraces high up on the sides of the valleys of rivers. Cases of the latter kind are found along Allegheny River (Leverett. Monogr. XLI, U. S. Geol. Surv., pp. 229–252; Shaw Jour. Geol., vol. XIX, 1911, pp. 140–156; folio 178, U. S. Geol. Surv., p. 8). On the other hand, an animal of very late Pleistocene age, or even of the Recent, may be buried in deposits which overlie an old Pleistocene deposit. It is necessary, if it can be done, to determine the actual age of the deposit containing the remains; otherwise one must depend on the geological age of the species involved, or be content to wait for further information. Unfortunately, but few of the quadrangles in the glaciated area have had their geological structure studied and reported on. At present the U. S. Geological Survey has published only Folios 92 (Gaines) and 93 (Elkland and Tioga), lying mostly in Tioga County, partly in Potter; also Folio 172 (Warren), occupying a part of Warren County. Information may sometimes be secured from the numerous volumes which have been published by the Geological Survey of Pennsylvania and from articles in the scientific journals.
Fig. 10.—Glaciated areas of northern Pennsylvania. From W. C. Alden.
The Pleistocene deposits which lie outside of the glaciated areas have been mostly laid down along rivers. Some of the materials were transported by the streams which carried away the drainage from the glaciers; in other cases the materials were brought down from the higher lands and laid down along the lower and less sloping parts of the streams. In the unglaciated area many of the quadrangles have been surveyed by the U. S. Geological Survey and the folios aid in determining the age of deposits which contain fossil vertebrates.