384 FIRST YEAR SCIENCE 



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 

 Scandinavian sword which in later ages carried devasta- 

 tion to these regions. 



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. 



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 

 spreading ice he has left no record from which the time 

 or cause of the Glacial Period can be determined. 



174. Effect of the Glacial Period upon Plants and Animals. 

 All plants and animals were forced either to migrate be- 

 fore the slowly advancing ice or to suffer extermination. 

 Individual plants, of course, could not move, but as the ice 

 spread toward the south with extreme slowness and with 

 many halts, the plants of colder latitudes found conditions 

 suitable for their growth ever opening toward the south. 

 They were thus induced to spread in that direction, so 

 that at the time of the greatest extension of the ice the 

 plants suitable to a cold climate had penetrated far to .the 

 south of their former habitat. 



As the ice receded, these cold-loving plants were forced 

 to follow its retreat or to climb the mountains in order to 

 obtain the climate they needed. They did both, so that 

 in areas covered by the ice, plants similar to those of far 

 northern regions are found on the tops of the mountains 

 in middle latitudes. What was true of the plants was 



