THE WORK OF GLACIERS 



175 



being reserved for the moraine formed at the time of the greatest ex- 

 tension of the ice. In this discussion the term " terminal " will be used 

 to include both. " The surface of these moraines (Fig. 145, p. 160) 

 is a jumble of elevations and depressions, which vary from low, gentle 

 swells and shallow sags to sharp hills a hundred feet or so in height, 

 and deep, steep-sided hollows. Such tumultuous hills and hum- 

 mocks, set with depressions of all shapes, which are usually without 

 outlet and are often occupied by marshes, ponds, and lakes, surely 

 cannot be the work of running water. The hills are heaps of drift, 

 lodged beneath the ice edge or piled along its front. The basins were 

 left among the tangle of morainic knolls and ridges as the margin 

 of the ice moved back and forth. Some bowl-shaped basins were 

 made by the melting of a mass of ice left behind by the retreating 

 glacier and buried in its debris." (Norton.) 



Moraines of the Last Great Ice Sheet in North America. — These 

 moraines usually occur in belts three to ten or fifteen miles in width, 

 their position being marked in Minnesota, Wisconsin, and other states 

 by thousands of lakes. In regions of little topographic relief moraines 

 may be the most conspicuous features of the landscape. Some of 

 the moraines have been traced several hundreds of miles and if the 

 correlations are correct have been identified over a distance of a 

 thousand miles or more (Fig. 160). 



A B 



Fig. 159. — A shows the drainage of a portion of the "driftless area" indicating that it 

 had a normal development. It is in decided contrast to the adjoining glaciated area B. 

 CLELAND GEOL. — 12 



