THE CHANGING FACE OF NATURE n 



to the caverns of southern Indiana and of Kentucky so that their 

 fame is familiar. 



In frigid regions the rivers, which in our latitude rise in lakes 

 and pursue their lively way down the outletting valleys, may be 

 replaced by streams of ice (Fig. 10) which take their origin in 

 the great snow fields of the mountain basins and plow their 

 way down the valleys slowly but with irresistible force. The 

 loose soil is pushed before them; the solid rock is worn away by 

 the terrific grinding force of the ice, hundreds of feet deep, hold- 

 ing in its grip the fragments of rock that abrade like giant's 

 sandpaper in the hands of Hercules. Once upon a time, as will 

 be detailed later, the Chicago region was covered by the glacial 

 ice cap that overspread nearly all of northern North America. 

 The surface of the bed rock here is planed off, covered with 

 scratches and parallel grooves made by the moving ice sheet 

 (see Fig. 41), and the earth that overlays the rock is full of 

 bowlders of granite, greenstone, diorite, and other foreign rock 

 transported from the ledges far to our north, worn smooth and 

 scored with striations in their progress (see Fig. 40). 



The pelting rain is no insignificant agent of erosion. Each 

 drop seems a puny thing, but multiply them endlessly and let 

 them act age after age and they do wear away the land. The 

 wash on steep hillsides is very apparent. The water runnels 

 during the heavy rains cut steep-sided valleys, between which 

 there stand up sharp-edged clay ridges and pinnacles (Fig. u). 

 These serve to call attention strikingly to an agency that works 

 so unobstrusively it might easily go without notice. Such 

 pinnacles in clay, the result of rains and spring freshets, are seen 

 along the lake shore as, for instance, near Lakeside where the 

 clay moraine comes to the lake. In a similar way the rock itself 

 is worn away by the pelting rain, and pinnacles are left standing 

 mute testimony of the former height of all the land, now largely 

 gone through erosion. 



The heave of water in the rock crevices when it transforms 

 to ice is another destructive action of water. Man realizes its 



