Page:Popular Science Monthly Volume 20.djvu/249

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237
NORTH AMERICA IN THE ICE PERIOD.

The writings of Moseley and Croll recall the utterances of Agassiz as respects the motion of the glacier, lie himself modestly avows his physical researches upon the glacier as inferior to those of his successors, Forbes, Tyndall, etc.; yet he could not shut his eves to the fact that those physicists had not satisfactorily solved the problem. While ascribing the motion to expansion by freezing, Agassiz insisted that heat was largely concerned, asserting that its presence was more consequential than that there should be an inclined plane—that the ice might move up-hill toward the sun; and particularly that a great thickness of it could make its way over such level territory as the prairies of Indiana and Illinois. The ice along the southern melting edge would be charged with moisture percolating through the pores and capillaries, descending the icy slopes obedient to gravity, and no longer requiring the shearing force. The amount of motion would in these circumstances be like that of the Greenland sheet, sixty feet per diem, rather than the sluggish crawl of one to three feet in the same time of the comparatively poorly developed Alpine glaciers. In agreement with these views we find the motion northward toward the pole to be very slight, though the land may be inclined northerly as in Grinnell Land.

Now, if we apply these principles to the territory in question, we would say that the ice began to accumulate in Canada from an unusual precipitation of moisture, gradually filling up the St. Lawrence Valley, and at first moving southwesterly. But, the supply still continuing plentiful, the valley fills up and runs over. It does not need to accumulate to the thickness of several miles upon the Laurentian highlands so as to have a downward slope all the way to Mount Washington. A mass only a few thousands—possibly hundreds—of feet thick might soften before the southern sun and the influence of the Atlantic Ocean off our coast, and lead the plastic material southeasterly over the Montalban water-shed. Once started upon the seaward slope, the ice could not fail to reach its destination. The southwest motion would likewise continue, and accomplish greater results, transporting blocks much farther because commencing earlier, continuing longer, and pushing forward in a thicker sheet. This Canadian ice would have resembled the present mer de glace of Greenland, confluent over hill, valley, and island, dragging the reluctant erratics up hill and down, accumulating ground moraines and lenticular hills, hollowing out pot-holes and discharging clouds of mud into the edge of the sea. Hence, instead of saying that the land rose three or four miles above its present level in Labrador in order to give the required impetus to the ice-movement in New England, it is easy to see how the same work could be accomplished by the action of much simpler causes.

Messrs. Torell and Dana have advocated the notion that the Greenland and Eastern American areas are one, and that Greenland was the source of the ice that has covered the eastern part of our continent.