i8 



A NATURALIST IN THE GREAT LAKES REGION 



portion being thrown into folds, some of which thrust the rock 

 layers up, some down (Fig. 17). These changes in level of the 

 earth's crust sometimes are so sudden as to cause an earthquake, 

 but usually they are so gradual as to be imperceptible except as 

 comparisons are made at long intervals. The northern shores of 

 Lake Michigan are now rising have risen on the west side of 

 the lake two feet or so in a generation. We know that the lime- 

 stone that makes up the bedrock of the Chicago region, now 



II 



FIG. 17. Up-arched rock strata 



dry land, was formed under the surface of the ocean, for 

 it contains fossil remains of animals and plants, shellfish, and 

 seaweed that live only in the sea. We shall see evidence of 

 many changes in level in our region. 



Along the Atlantic Coast there is evidence of a sinking shore. 

 Stump lands along the coast are now completely under water. 

 The Hudson River Valley is shown by soundings to continue 

 well out to sea, its lower end having been "drowned" as the 

 land sank there. Along the western coast of South America 

 coral reefs are found well up on the sides of the mountains. 



