20 A NATURALIST IN THE GREAT LAKES REGION 



Evidently they must have formed below sea-level, and the 

 mountains have been reared since then. 



Volcanoes are another agency of land formation. The lava 

 welling from their craters or from cracks in the earth pours out 

 in great sheets (Fig. 18) while molten, only to harden as it 

 cools into beds of rock. Fortunately for our peace of mind no 

 volcanoes exist in the Chicago area, though we do have lavas, 

 like the granite and greenstone glacial bowlders that have been 

 transported by the ice from the old lava beds to the north of us 

 into our region. 



The glacier builds as well as destroys. The ice mass incorpor- 

 ates into itself the rock fragments torn from its bed and sides, so 

 the onward moving river of ice is loaded with debris and abraded 

 material. At some place it encounters a temperature sufficiently 

 high to melt it as rapidly as it pushes forward. Here it must 

 drop its load of rock powder and rock fragments and so it 

 forms a great pile along its front, a terminal moraine (Fig. 19). 

 Streams of water made by the melting ice pour out of gaps in the 

 moraine on to the country beyond, carrying gravel and sand that 

 are deposited in great fans or along the valley, if the outwashing 

 stream follows such, in elongated heaps or valley trains. These 

 and other forms of deposit characteristic of the glacier are found 

 about Chicago as will appear in a later chapter. 



