THE QUATERNARY PERIOD 339 



those of many tons' weight, and are naturally most commonly 

 derived from the harder and more resistant rock formations. 

 Thus erratics from the Adirondack Mountains are very numerous 

 from central to southern New York. Erratics are often found high 

 up on the mountains, where they have sometimes been left stranded 

 in remarkably balanced positions. 



A very extensive glacial deposit, called the ground moraine, is 

 simply the heterogeneous, typically unstratified debris from the 

 bottom of the ice which was deposited, sometimes during the ice 

 advance, but most often during its melting and retreat. When it 



Fig. 213 



Typical drumlins (side view) in western New York. (After H. L. Fairchild, 



N. Y. State Mus., Bui. 111.) 



is mostly very fine material with pebbles or boulders scattered 

 through its mass, it is known as till or boulder clay. The pebbles 

 or boulders of the till are commonly faceted and striated as a result 

 of having been rubbed against underlying rock formations. 



Another type of glacial deposit of unusual interest is the 

 drumlin, which is in reality only a special form of ground moraine 

 material. The typical drumlins of western New York, Wisconsin, 

 and western Massachusetts are low, rounded mounds of till with 

 elliptical bases and steeper slopes on the north sides. Their long 

 axes are parallel to what was the direction of ice movement (see 

 Fig. 213). In height they rarely exceed 200 feet, being most often 

 less than 100 feet. The origin of the drumlins has not yet been 

 satisfactorily determined, though it is known that they formed near 

 the margin of the ice either by the erosion of an earlier drift layer 

 or by accumulation beneath the ice under peculiarly favorable 



