THE QUATERNARY PERIOD 465 



Some he has relegated to remote regions ; thus the wild turkey 

 was formerly common throughout eastern United States, but 

 is now to be seen only in certain mountainous portions of the 

 southern states. Still others he has transported all over the 

 world; for example, the brown rat, originally a native of 

 northwestern Europe, is now found in every continent and 

 island in the habitable zones. In more recent times man 

 has even been instrumental in producing entirely new varieties 

 of animals, and especially of plants, by the method of con- 

 trolled breeding, which is now so successfully practiced. 



These are but a few examples of the many changes of which 

 the human races have been the cause ; but they are enough 

 to show how very important the geologic and biologic influ- 

 ence of this highest of the mammals has become. 



QUESTIONS 



1. In the shape of the edge of the last ice sheet, what evidence 

 is there of the former existence of a large valley where Lake Michigan 

 now lies ? 



2. Part of this valley is now below sea level. To what extent 

 do rivers erode their valleys below sea level ? What other factors 

 may have been important here ? 



3. In Indiana and Illinois the large bowlders in the drift are 

 chiefly igneous and metamorphic rocks, such as granite, gneiss, 

 gabbro, and quartzite, while bowlders of the sandstone and lime- 

 stone which underlie the drift are less common. Why should this 

 be true? 



4. Have the uplands of Figure 477 been glaciated ? The evidence ? 



5. At a point on the edge of the Wasatch Mountains in Utah 

 a glacial moraine has been found dislocated, as shown in Figure 

 481. What events are indicated ? 



6. In northeastern California trees have been found associated 

 with a bed of fine volcanic 



ash in the relations shown in 

 Figure 482. What inferences 

 may be drawn from this ? 



7. Why should the skele- 

 tons of mastodons and other 

 large animals of the Glacial 



epoch be found in peat bogs ? FIG. 481. Dislocated terminal moraine. 



