ROCK FORMATIONS AND PRE-GLACIAL HISTORY 



Pennsylvanian sandstones, shales and coal beds. These had 

 little to do with the story of the Great Lakes, except that a bit 

 of their eastern rim was cut into to form the southwestern end 

 of Saginaw Bay. 



After Pennsylvanian time no seas entered the immediate 

 area of the Great Lakes, and for many millions of years Na- 

 ture was busy cutting, carving and wearing away the rocks 

 laid down in the region. The soft shales and salt beds were 

 eroded to form the wide moats, or trenches, between the more 

 resistant formations which made the escarpments, evidences 

 of the channels by which the seas had entered the region were 

 obliterated, and a thick soil cover was developed. However, 

 Nature was very busy building up the rest of the continent 

 of North America — folding the Appalachians to mountain 

 height, wearing them down to almost a plain, then lifting 

 them again in a wide plateau which since has been carved to 

 the Appalachian Mountains as we know them now. The Great 

 Plains were built up, the Sierras and then the Rockies uplifted, 

 and the mountains of the ancient pre-Cambrian area about 

 Lake Superior were worn down. In the entire lake region we 

 have no rock records of this time — known as the Mesozoic 

 and Cenozoic eras, yet we know that with the general uplift 

 of the continent this region was lifted much higher above 

 sea level than it now stands. 



The force that lowered the northern part of the continent 

 and changed the ancient river valleys to lake basins started 

 its work about a million years ago when a climatic change 

 took place. The winters became not so much colder, but 

 longer — so long that the snows of one winter were not melted 

 by the summer sun before another winter set in. This time 

 period we know as the Pleistocene, the age of the great con- 

 tinental glaciers which build the final rock record in the Great 

 Lakes region. It ended when the ice had melted to the small 

 remnants of the Greenland ice cap. Geological formations 

 now in the making arc known as Recent. 



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