Man and the Glacial Period

by G. Frederick Wright

Something similar to this so often appears, that there can be no question as to its meaning, which is, that during the farthest extent of the ice the front rested for a considerable period of time along the line marked by the terminal moraine. During this period there occurred both the accumulation of the moraine and of the gravel terraces in the valley below, due to the vast flow of water emerging from the ice-front, especially during the period when it was most rapidly melting away. Upon the retreat of the ice, the moraine constituted a dam which has not yet been wholly worn away. For a while the water was so effectually ponded back by this as to form a lake, which has since become filled up with sediment and accumulations of peat. From this it is evident, also, that when the ice began to retreat, the retreat was so continuous and rapid that no parallel terminal moraines were formed for many miles.

Before leaving this section we will summarise the leading facts concerning the glacial phenomena north of Pennsylvania and New Jersey. From the observations of Professor Smock, it appears that, from the southern margin the ascent to the summit of the ice-sheet was pretty rapid; the depth one mile back from the margin being not much less than a thousand feet. “Northward the angle of the slope diminished, and the glacier surface approximated to a great level plain. The distance between the high southwestern peaks of the Catskills and Pocono Knob in Pennsylvania is sixty miles. The difference in the elevation of the glacier could not have exceeded a thousand feet,” [BB] that is, the slope of the surface was about seventeen feet to the mile.

[BB] American Journal of Science, vol. cxxv, 1883, p. 339 et seq.

Professor Dana estimates the thickness of the ice in southern Connecticut to have been between fifteen hundred and two thousand feet. Attempts to calculate the thickness of the ice farther north, except from actual discovery of glacial action on the summits of the mountains, are based upon uncertain data with reference to the slope necessary to secure glacial movement. In the Alps the lowest mean slopes down which glaciers move are about two hundred and fifty feet to a mile; but in Greenland, Jensen found the slope of the Frederickshaab Glacier to be only seventy-five feet to the mile, while Helland found that of the Jakobshavn Glacier to be only forty-five feet.

It is doubtful if even that amount is necessary to secure a continental movement of ice, since, as already remarked, it is unsafe to draw inferences concerning the movements of large masses of ice from those of smaller masses in more constricted areas. We have seen, from the glacial deposits on the top of Mount Washington, that over the northern part of New England the ice was more than a mile in depth. We have no direct evidence of the depth of the stream which surrounded the Adirondack Mountains. Nor, on the other hand, are we certain that the Catskills were not completely enveloped in ice, though most observers, reasoning from negative evidence, have supposed that to be the case. But from the facts stated concerning the boulders along the glacial boundary in Pennsylvania, it is certain that the ice was deep enough to surmount the ridge of the Alleghanies where they are two thousand and more feet in height. At the least calculation the ice must have been five hundred feet thick, in order to secure the movement of which there is evidence across the Appalachian range. Supposing this to be the height of the ice above the sea on the crest of the Alleghanies, and that the slope of the surface of the ice-sheet was as moderate as Professor Smock has estimated it (namely seventeen feet to the mile), the ice would be upwards of six thousand feet in thickness in the latitude of the Adirondacks, which corresponds closely with the positive evidence Ave have from the mountains in New England.

A study of the map of New York will make it easy to understand the distribution of some interesting glacial marks over the State. The distance along the Hudson from the glacial boundary in the vicinity of New York to the valley of the Mohawk is about one hundred and sixty miles. Prom the glacial boundary at Salamanca, N. Y., to the same valley, is not over eighty miles. It is easy to see, therefore, that when, in advancing, the ice moved southward past the Adirondacks, the east end of the valley of the Mohawk was reached and closed by the ice, while at the west end of Lake Ontario the ice-front was still in Canada. Thus the drainage, which naturally followed the course of the St. Lawrence, would first be turned through the Mohawk. Afterwards, when the Mohawk had been closed by ice, the vast amount of ponded water was compelled to seek a temporary outlet over the lower passages leading into the Susquehanna or into the Alleghany.

A number of such passages exist. One can be traced along the line of the old canal from Utica to Binghamton, whose highest level is not far from eleven hundred feet. Another lies in a valley leading south of Cayuga Lake, whose highest point, at Wilseyville, is nine hundred and forty feet above tide. Another leads south to the Chemung River from Seneca Lake, whose highest point, at Horseheads, is less than nine hundred feet above tide. The cols farther west are somewhat more elevated; the one at Portage, leading from the Genesee River into the Canisteo, being upwards of thirteen hundred feet, and that of Dayton, leading from Cattaraugus Creek into the Conewango, being about the same. Of other southern outlets farther west we will speak later on.

Fixing our minds now upon the region under consideration, in the southern part of the State of New York, we can readily see that a glacial lake must have existed in front of the ice while it was advancing, until it had reached the river-partings between the Mohawk and the St. Lawrence Rivers on the north and the Susquehanna and Alleghany Rivers on the south. After the ice had attained its maximum extension, and was in process of retreat, there would be a repetition of the phenomena, only they would occur in the reverse order. The glacial markings which we see are, of course, mainly those produced during the general retreat of the ice.

The Susquehanna River stretching out its arms—the Chenango and Chemung Rivers—to the east and the west, evidently serves as a line of drainage for the vast glacial floods. These floods have left, along their courses, extensive elevated gravel terraces, with much material in them which is not local, but which has been washed out of the direct glacial deposits from the far north. The east-and-west line of the water-parting throughout the State is characterised by excessive accumulations of glaciated material, forming something like a terminal moraine, and is designated by President Chamberlin as “the terminal moraine of the second Glacial epoch,” corresponding, as he thinks, to the interior line already described as characterising the south shore of New England.