482 ELEMENTARY GEOLOGY 
modified by the action of water furnished by the ice 
melting. Its resultant structure therefore is complex, 
in some places being a jumble of pebbles and clay, 
known as bowlder clay, while perhaps even in the 
same hill, are found layers of sand and gravel 
assorted and stratified. The surface outline of the 
Fic. 266. 
Very rocky terminal moraine, Cape Ann, Mass. 
moraine is also irregular, and the typical morainal 
topography is that of hummocky and tumultuous hills 
(Fig. 267). It is one of the most irregular surfaces, 
in small details, that is known in this country. 
Lakes Formed. As the ice withdrew, in the course of time the 
valley of the Great Lakes became uncovered; but before the 
