PLANT MIGRATION. 51 1 



our western mountain region where a few remnant glaciers are found was 

 covered with ice, continuing into the great ice-fields of British Columbia." 

 The ice flowed over the north Alleghany Mountains, over the Adirondacks 

 in New York, smoothing and rounding them. All along the southern 

 line of the glacier are the waste materials, forming terminal moraines, 

 which have been carefully studied by geologists. Evidence goes to show 

 that after the first onward movement of the ice -sheet a warmer period 

 came, and the ice-sheet retreated northward, and many ages afterward 

 extended again to the south, nearly to its former limit. Since this last 

 glacial period the ice has receded until the glaciers now occupy only the 

 higher mountains in the northern Rockies, and Alaska and Greenland. 



During the same period a similar ice -sheet covered all northern Europe, 

 extending as far south as the Thames Valley in the British Islands; the 

 North and Baltic Sea basins 5 northern Germany, and northern Russia 

 were occupied. 



980. Effect of the cold wave on plant migration. The pres- 

 ence and movement of these great sheets of ice southward over 

 the northern hemisphere forced the migration of northern species 

 southward. As the ice-sheet reached into the temperate regions 

 it forced in advance of it the species from the temperate regions 

 southward. As the glacier retreated northward, the plants 

 which were able to survive by southward migration again mi- 

 grated northward. Geological evidence goes to show that there 

 were a number of movements back and forth of the ice-sheet. 

 This great climatic pressure, therefore, fluctuated for long 

 periods, forcing the plants southward, then again yielding and 

 allowing the plants to take up their former positions, when again 

 they would be forced southward, and so on. 



981. Evidences of plant migration in glacial times. In stud, 

 ies of the distribution of the plants of North America and Europe 

 and Asia, there are at present evidences of this migration of 

 plants southward. Many arctic plants which at that time moved 

 southward are now left on the higher mountain peaks, or in the 

 cool sphagnum moors formed among some of the terminal 

 moraines. With the proximity of the continents in the arctic 

 circle there is reason to believe that in former times plants mi- 

 grated readily between the continents of North America, Europe, 

 and Asia. During glacial times these were forced southward, 



