SETTING FOR lliL ICE-AGE DRAMA 



THE SroRY oi THE Grl A I Laki£S is largely a story of the 

 Ice Age in North America, a period geologically recent, 

 which began only about a million years ago. But the 

 events of the preceding eons of geologic time — hundreds of 

 millions of years — when the forces of Nature were warring 

 over the tasks of building, tearing down and rebuilding the 

 rock formations of the continent, all played parts in prepar- 

 ing the stage for the drama of the ice invasion. 



The Rock Formations and Pre-Glacial History 

 OF THE Great Lakes Region 



In the days when the Earth was young and turbulent, the 

 Great Lakes region was the basin-shaped floor of the sea 

 bordering a shield-shaped mass of igneous rock lying to the 

 north — the initial North American continent. Several times 

 in the eons that passed, as the Earth settled down and the 

 continents became stable, the borders of this land were up- 

 lifted to mountain height, only to be worn down almost to sea 

 level during long periods of quiescence. Finally a great con- 

 vulsive, though slow, earth movement lifted to Alpine heights 

 a mountain range extending from Minnesota and Wisconsin, 

 in a northward-bearing arc, to the Laurentian Highland in 

 Canada and the Adirondacks in New York. The several 

 uplifts, whether slow or violent, changed the rocks so that the 

 borders of the continent came to be a region of igneous and 

 metamorphic rocks — granites, gneisses, schists, lavas, slates, 

 and bedded rocks which were crumpled and folded. In the 

 west these mountains have long since been worn away almost 

 to their roots, leaving only rugged uplands in the highlands 

 surrounding Lake Superior. 



When earth forces brought uplifts of this vast area, they 

 were accompanied by local downwarps or flutings of the sur- 



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