332 HISTORICAL GEOLOGY 



York state on account of its peculiar relief features. When the 

 Labradorean ice sheet spread southward as far as northern New 

 York, the Adirondack Mountains stood out as a considerable 

 obstacle in the path of the moving ice, and the tendency was for 

 the current to divide into two portions, one of which passed south- 

 westward up the low, broad St. Lawrence Valley, and the other 

 due southward through the deep, narrow Champlain Valley. As 

 the ice kept crowding from the rear, part of the St. Lawrence ice 

 lobe pushed into the Ontario basin, while another portion worked 

 its way up the broad, low Black River Valley and finally into the 

 Mohawk Valley. At the same time the Champlain ice lobe found 

 its way into the upper Hudson Valley, and sent a branch lobe 

 westward up the broad, low Mohawk Valley. The two Mohawk 

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

 the midst of the Mohawk Valley. As the ice sheet continued to 

 push southward, all the lowlands of northern New York were 

 filled; a tongue or lobe was sent down the Hudson Valley; and 

 finally the whole state, except slight portions of the southern 

 border, was buried under the ice. The general direction of flow at 

 this time of maximum glaciation was southward to southwestward, 

 with perhaps some undercurrents determined by the larger topo- 

 graphic features. Thus we learn that the major relief features of 

 the state very largely determined the direction of ice currents, ex- 

 cept at the time of maximum glaciation, when only the under- 

 currents were controlled. These ideas are abundantly borne out by 

 the distribution of glacial striae and boulders over the state. 



Evidences of glaciation, such as striae, boulders, lakes, etc., 

 occur high up in the Adirondacks, the Catskills, the Green and the 

 White Mountains, and the Berkshire Hills, so that the greatest 

 depth of ice over New York and New England could not have been 

 less than some thousands of feet. In fact we have every reason to 

 believe that all of the mountains named, except possibly the 

 Catskills, were completely buried. The reader may wonder how 

 an ice sheet a mile thick in northern New York could have thinned 

 out to disappearance at or near the southern border of the state, 

 but observations on existing glaciers show that it is quite the habit 

 of extensive ice bodies to thin out very rapidly near the margins, 

 thus producing steep slopes along the ice fronts. 



There is little reason to doubt that the vast ice sheet over the 

 upper Mississippi Valley was also thousands of feet thick. The 



