Page:Popular Science Monthly Volume 13.djvu/669

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GEOLOGICAL HISTORY OF NEW YORK.

covered with noble trees, and a dense undergrowth of species, for the most part different from those now living there; and that these were the homes and feeding-grounds of many kinds of quadrupeds and birds, which have long since become extinct. The broad plain which sloped gently seaward from the highlands must have been covered with a subtropical forest of giant trees and tangled vines teeming with animal life. This state of things doubtless continued through many thousands of years, but ultimately a change came over the fair face of Nature more complete and terrible than we have language to describe. From causes which are not yet fully understood, and into the discussion of which we cannot here enter, the climate of the northern hemisphere became gradually more severe, and that of Greenland, from being what it had been for ages, like that of our Southern States, became arctic as we now find it, and its luxuriant forests were replaced by fields of snow and ice. But the change did not stop here, for with increasing cold the ice-sheets spread southward and covered successively the mountains of Labrador, the Canadian highlands, and the hills of New England and New York. At the culmination of the Glacial period the ice reached as far south as Staten Island and Trenton, and all the country north of this line was buried under a great moving mass of ice, in places several thousand feet in thickness. At this time the present climate of Greenland had been transferred to New York; in the strongest possible contrast to that earlier time when the present climate of New York prevailed in Greenland. In the advent of the Ice period not only were all kinds of animals and plants exterminated or driven southward, and thus what had been a paradise was converted into a howling wilderness, but even the topography of the country was greatly modified. The ice-sheet moving from the north ground down or rounded over all projecting rock-masses, and filled up valleys with the d├ębris, producing great abrasion in some places, and accumulations in others, until the whole face of the country was changed. In the vicinity of New York the ice moved from north-northwest to south-southeast, and was of such thickness that it crossed the trough of the Hudson diagonally, and probably, because this had been filled with transported material, was by it little deflected from its course. In other localities the old river-valleys were sometimes completely obliterated, and the drainage of the surface given new channels and even new direction. To this cause we may attribute the blocking up of the old line of drainage from the lake-basin through the Hudson, and its diversion to the present course of the St. Lawrence.

Now that the glaciers have left this region and have retreated again to the far north, we everywhere see evidence of the stupendous changes they wrought in the country over which they moved. North of the line which marks the margin of the glacier, we find the contour-lines rounded over and softened, ridges of granite converted into domes, and the hardest rocks grooved and striated, or ground smooth and even