THE PLEISTOCENE OR GLACIAL PERIOD 865 





by ice and water conjointly (stratified, but stratification often 

 irregular), and (3) those made by water emanating from the ice 

 (stratified, often with cross-bedding). 



Ground moraines, terminal moraines, and lateral moraines are 

 the principal types of drift deposited by glaciers directly. So 

 far as ice-sheets are concerned, ground moraines are the most 

 extensive by far, and lateral moraines have little develop- 

 ment. 



The characteristics of the ground moraine are those usually 

 given for drift in general. Ground moraine (or till) is nearly co- 

 extensive with the ice-sheets themselves, though it failed of deposi- 



Fig. 576. A Wisconsin drumlin seen from the side; two miles north of 

 Sullivan. (Alden, U. S. Geol. Surv.) 



tion in some places, and has been removed in others. The ground 

 moraine of the North American ice-sheets is thickest in a broad 

 belt a little within the margin of the drift (Fig. 560), extending 

 from central New York through Ohio, Indiana, Illinois, Iowa, 

 Minnesota, and Dakota, and thence northward to an unknown 

 limit in Canada. 



The topography of the ground moraine varies within wide limits. 

 It is commonly undulatory, involving gentle swells and sags (north- 

 east half of Fig. 228). In some cases the swells take on rather 

 definite elongate shapes, with their longer axis in the direction of 

 ice movement. They are then called drumlins (Fig. 575). Drum- 

 have pronounced development in eastern Wisconsin, where 

 they are numbered by the thousand, in central and western New 



