58 



A NATURALIST IN THE GREAT LAKES REGION 



WTiile, as suggested in the last chapter, there is no evidence 

 that rock strata were laid down in the Chicago region after the 

 Devonian period of geological time, in the long stretch of ages 

 that elapsed between then and the onset of the last glacial period 

 many changes were going on in the area. For many centuries 

 the forces of erosion were active. The Devonian rocks and 



Fig. 39. — An attempted reconstruction of the preglacial drainage of the 

 Great Lakes region. Present lakes, streams, and boundaries in dotted lines. 



possibly later ones on top of them were w^orn away until only 

 vestiges of them wxre left and the removed material was carried 

 off into the oceans. The surface of the hard Niagara limestone 

 and of the softer Richmond shale was dissected by the rivers 

 into steep-sided valleys that later broadened and became less 

 abrupt w^hile rounded hills of goodly height stood between. The 

 rocks gradually disintegrating through hundreds of thousands 

 of years formed deep soil. Vegetation flourished and animal 

 life found abundant shelter and food. Probably where Lake 

 Michigan now stands was a broad and fertile valley. 



