THE WORK OF GLACIERS 



173 



what cut up into valleys, the ice sheets filled them with drift, compel- 

 ling the streams upon the retreat of the ice to make new valleys, the 



Fig. 155 



Note the heterogeneous mixture of large and small bowlders and 

 fine clay. (Pennsylvania Geol. Surv.) 



result may be a smoother surface. Many portions of the northern 

 United States have been modified in this way. On the other hand, the 

 irregular dumping and piling up of drift in moraines may roughen the 

 landscape. The northern half of the peninsula of Michigan, which in 

 certain places now 

 rises from 1000 to 

 1 1 00 feet above the 

 surface of the Great 

 Lakes, seems to have 

 no rock standing 

 more than 250 to 

 300 feet above the 

 lakes, there being 

 from 700 to 800 feet 

 of drift on its higher 

 parts. The average 

 thickness of the drift 

 in southern Illinois is 

 somewhat less than 

 30 feet; in north- 

 western Ohio, north- 

 ern Indiana, and 



Fig. 156. — Stratified lake clay resting on till. 



