Page:Popular Science Monthly Volume 44.djvu/800

This page has been proofread, but needs to be validated.

concludes that several hundred feet of the ridge have been worn away by the ice.

The crowning example of bowlder transportation is, however, afforded by the blocks of light gray gneiss discovered by Prof. Hitchcock on the summit of Mount Washington, over six thousand feet above sea-level, and identified with Bethlehem gneiss, whose nearest outcrop is in Jefferson, several miles to the northwest, and three or four thousand feet lower than Mount Washington.

These varied phenomena of erratic blocks and rock striations, together with the enormous quantity of bowlder clay and glacial drift spread over the whole of the Eastern States, terminating southward in a more or less abrupt line of mounds having all the characteristics of an enormous moraine, have led American geologists to certain definite conclusions in which they all practically agree. It may be well first to give a notion of the enormous amount of the glacial débris under which a large part of the Eastern States is buried. In New England these deposits are of less thickness than farther south, averaging from ten to twenty feet over the whole area. In Pennsylvania and New York east of the Alleghanies the deposits are very irregular, often sixty or seventy feet thick and sometimes more. West of the Alleghanies in New York, Pennsylvania, and Ohio the thickness is much greater, being often one hundred and fifty or two hundred feet in the wide valleys, and forty or fifty feet on many of the uplands. Prof. Newberry calculates that in Ohio it averages sixty feet deep over an area of twenty-five thousand square miles.

The direction of the striæ and of the traveled bowlders together with the form of the great terminal moraines show that there must have been two main centers of outflow for the icesheet, one over Labrador, the other over the Laurentian Highlands north of Lake Superior. The southern margin of the drift may be roughly represented by portions of circles drawn from these two points as centers. The erratics on the summit of Mount Washington show that the ice-sheet must have been a mile thick in its neighborhood, and much thicker at the centers of dispersion, while the masses of drift and erratics on plateaus two thousand feet high near its southern boundary indicate a great thickness at the termination. The Laurentian plateau is now about two thousand feet above the sea-level, but there are numerous indications from buried river channels, filled with drift and far below the sea, which lead to the conclusion that during the Ice age the land was much higher. That snow can accumulate to an enormous extent over land of moderate height when the conditions are favorable for such an accumulation is shown by the case of Greenland, the greater part of whose surface is a vast