GLACIAL GEOLOGY OF THE COHOES QUADRANGLE I3 



The boulders are nearly all of rock diifferent from the underlying 

 country rock, showing that they were brought by the ice from 

 regions lying to the north. But the finer parts of the till include 

 many bits of the local rock and there is everywhere a noticeable 

 correspondence between the predominant mineral elements of the 

 till and those of the nearby exposures of the underlying rocks. As 

 the country rocks of the region are largely shales (slates), they 

 give to the soil a predominantly clayey character. It is in general 

 well adapted to agricultural purposes. 



The thickness of the till varies greatly in different parts of the 

 area. Near the summit of the main rock ridges and masses the 

 till sheet is generally thin and, as noted above, there are frequent 

 exposures of bare rock. In places these exposed rocks have under- 

 gone decomposition to a considerable extent, and the residual prod- 

 ucts, having fallen or having been washed from the slopes of the 

 rock surfaces, are added to the materials of the till. 



In the valleys and depressions between the rock hills the till is 

 in general thicker than on the tops and sides of the hills. This 

 may be due in part to the valleys having been occupied by the ice 

 for a longer time than the hilltops or to a relatively greater amount 

 of till having been originally lodged in the valleys, but it is probably 

 also due to removal of till from the hills by washing. The occur- 

 rence of boulders resting on the exposed rock of the hills is evidence 

 of this, the finer materials of the till having been washed away. 



In contrast with rock hills veneered by till are those hills which 

 are apparently made up wholly of till. Many of the low hills, and 

 some of larger proportions, are of this character. The most 

 noticeable one is that southeast of South Easton, marked by a 

 depression contour on its western slope. As far as could be deter- 

 mined by inspection, the materials of this hill are till and the depres- 

 sion is due to irregularity of heaping of the debris derived from 

 the ice. Another hill apparently composed wholly of till is that 

 north of Crandall Corners and crossed by the road. This is of the 

 type of a drumlin. An interesting drumlin is that which occurs on 

 the fioor of the Hudson valley, east of the river and directly 

 opposite Bemis Heights. It stands as a conspicuous oval hill, 

 strewn with cobbles and boulders, rising above the level of the 

 alluvial plain. It is evident that this hill was once covered with 

 lacustrine clays and that when the latter were swept away by 

 flooded stream erosion, the more resistant materials of the hill 

 remained. 



