THE WORK OF ICE-SHEETS 



627 



Formations made by ice-sheets. 1 The drift formations fall 

 chii-lly into three categories, (i) those made directly by the ice (un- 

 stratified), (2) those made by ice and water conjointly (stratified, 

 but stratification more or less disturbed), and (3) those made by 

 water emanating from the ice (stratified; cross-bedding common). 



Ground moraine (p. 1 59) is nearly co-extensive with the ice-sheets 

 themselves, though it failed of deposition in some places, and has 



SCALE 

 



Fig. 519. One phase of ground moraine topography; elongated hills of drift of 

 the type shown, are called drumlins; southeastern Wisconsin. (U. S. Geol. Surv.) 



been removed in others. The ground moraine (till) of the North 

 American ice-sheets is thickest in a broad belt a little within the 

 margin of the drift (Fig. 504), extending from central New York 

 through Ohio, Indiana, Illinois, Iowa, Minnesota, and the Dakotas, 

 and thence northwestward. The topography of the ground moraine 

 varies within wide limits. It is commonly undulatory, involving 

 gentle swells and sags. In some places the swells take on rather defi- 

 nite elongate shapes, with their longer axes in the direction of ice 

 movement. They are then called drumlins (Fig. 519). Drumlins 

 have pronounced development in eastern Wisconsin, where they 

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

 1 Jour. Geol., Vol. II, pp. 517-538, and Internal. Geol. Congr., 1893. 



