GLACIAL FORMATIONS 297 



across the basins of the Great Lakes and the upper 

 Mississippi valley and across a part of the Missouri 

 valley. It wrapped in its icy mantle almost the entire 

 region between the Ohio and Missouri rivers and the 

 Atlantic Ocean. 



Another great ice invasion spread out from the high- 

 lands of Scandinavia. As in later days the Norsemen, so 

 at that time the glacial ice, overspread northern Europe, 

 carrying Scandinavian bowlders across the Baltic and what 

 is now the basin of the North Sea, forerunners of the Scan- 

 dinavian sword which in later ages carried devastation to 

 these regions. 



Prehistoric man probably saw the great ice mantle; he 

 may even have been driven from his hunting grounds by 

 its slow encroachment. His rude stone implements are 

 found mingled with the glacial gravels. But like the spread- 

 ing ice he has left no record from which the time or cause of 

 the Glacial Period can be determined. 



The thickness of the ice over these central areas was very 

 great, probably approaching a mile. The pressure on the 

 ground below must have been tremendous and the scouring 

 and erosive effect vast indeed. The soil which previously 

 covered the surface was swept away and borne toward the 

 ice margin, leaving the rocks smoothed and bare. 



Glacial Formations. The traces left by these ancient 

 glaciers are unmistakable. When a glacier melts, all the 

 material which it has moved along under it as well as that 

 which it has carried on its surface or frozen in its mass is 

 deposited, forming what is called ground moraine. This 

 is the formation which constitutes the soil of many of our 

 northern states. The soil throughout the glaciated region 



