58 NEW YORK STATE MUSEUM 



and Europe. This event takes rank as one of the most interesting 

 and remarkable occurrences of geological time. 



Some of the proofs for the former presence of the great ice sheet 

 are the following: (i) polished and scratched rock surfaces which 

 are precisely like those produced by existing glaciers, and which 

 could not possibly have resulted from any other agency; (2) glacial 

 boulders called " erratics " which are often somewhat rounded and 

 scratched, and which have often been transported many miles from 

 their parent ledges; and (3) glacial deposits, including true moraines 

 (see below) and the widespread heterogeneous glacial debris, both 

 stratified and unstratified, which is clearly ice transportated material 

 usually resting by sharp contact upon the bedrock. 



An area of nearly 4,000,000 square miles of North America 

 was covered by ice at the time of maximum glaciation and also 

 there were three main centers of ice accumulation and dispersal, 

 namely, the Labradorean, Kewatin and Cordilleran. It was the 

 Labradorean ice sheet which spread southward to cover all of the 

 Adirondacks. The directions of flow of the ice have been determined 

 by noting the directions of the glacial scratches (so-called " striae "). 



When the Labradorean ice sheet spread out southward as far as 

 northern New York, the Adirondack mountains stood out as a 

 considerable obstacle in the path of the moving ice. Hence the 

 tendency was for the ice to divide into two currents or portions, one 

 of which passed southwestward up the low, broad St Lawrence 

 valley, and the other due southward through the deep, narrow Cham- 

 plain valley. As the ice kept crowding from the rear, part of the 

 St Lawrence ice lobe pushed into the Ontario basin, while another 

 portion pushed its way up the deep Black River valley and finally 

 into the Mohawk valley in central New York. At the same time the 

 Champlain ice lobe found its way into the Hudson valley and sent 

 a branch lobe westward up the Mohawk valley. The two Mohawk 

 lobes, the one from the west and the other from the east, met in 

 the Mohawk valley not far from Little Falls. As the ice sheet con- 

 tinued to push southward, all the lowlands of northern New York 

 were filled, and finally the whole Adirondack region was buried 

 under the ice. The highest, or central to east-central, Adirondacks 

 were the last to become submerged under the ice. The general 

 direction of movement at this time of greatest ice extent was south- 

 ward to southwestward with perhaps some undercurrents determined 

 by the larger topographic features. 



The accompanying maps (figure 11) depict three important 

 stages in the retreat of the ice sheet from northern New York. It 



