360 GEOLOGY. 



tion with the ice-sheets, lateral moraines (Vol. I, p. 302) have little 

 development. 



The ground moraine (Vol. I, p. 301) is the most familiar and wide- 

 spread phase of drift, and its features are those usually given as charac- 

 teristic of drift in general. The ground moraine (or till) is nearly co-ex- 

 tensive with the ice-sheets themselves, though it failed to be deposited 

 in some places, and it has been removed, or buried by stream deposits, 

 in others. The ground moraines of the North American ice-sheets 

 are thickest far from the centers of the ice-fields, in a broad infra- 

 marginal belt extending from central New York through central and 

 northern Ohio, Indiana, Illinois, Iowa, Minnesota, and Dakota, and 

 northward to an unknown limit in Canada. 1 Towards the centers 

 of the ice-fields, and often near their outer borders, the drift is thin, 

 because in the former place it was never left, and in the latter often 

 because it has been removed by erosion. 



The topography of the ground moraine varies within wide limits. 

 It may be nearly plane, but is more commonly gently undulatory, 

 the undulations involving gentle sags and swells. The former are often 

 the sites of marshes, ponds, and lakes (right-hand part of Fig. 498). 

 The sags and swells frequently show a tendency to elongation in the 

 direction of ice movement. The hills of ground moraine sometimes 

 take on rather definite elongate shapes, with their longer axes in the 

 direction of ice movement and two to ten times the shorter. Such 

 hills of till are drumlins (Figs. 495 and 496). They are the most dis- 

 tinctly defined aggregations of ground moraine. Many hills and 

 swells of the ground moraine, however, are not drumlins. Drumlins 

 find their most pronounced development in the United States in east- 

 ern Wisconsin, where their number has been estimated at 10,000 (Buell), 

 and in central and western New York, 2 though they are not confined 

 to these localities. The drumlins of New York (Fig. 496) are, in gen- 

 eral, much longer than those of Wisconsin. 



The origin of drumlins has been much discussed, but there is, 

 as yet, no generally accepted conclusion, and the subject is still 

 under active inquiry. Opinion is chiefly divided between the views, 



1 For descriptions of the ground moraine in various regions, see State Reports. 



2 For the topography of the drumlins, see the following topographic sheets U. S. 

 Geol. Surv. : Wisconsin: Sun Prairie, Watertown, and Waterloo; New York: Oswego, 

 Palmyra, Clyde, Brockport, and Weedsport; Massachusetts: Boston. 



