GEOLOGICAL HISTORY OF NEW YORK. 649 



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 sub- 

 tropical 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 moun- 

 tains of Labrador, the Canadian highlands, and the hills of New Eng- 

 land 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 strong- 

 est 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 debris, 

 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 diago- 

 nally, and probably, because this had been filled with transported ma- 

 terial, was by it little deflected from its course. In other localities the 

 old river-valleys were sometimes completely obliterated, and the drain- 

 age 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 pres- 

 ent 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 



