510 RELATION TO ENVIRONMENT. 



ing plant migration, ist. There are certain physical factors, as 

 wind, water, which float seeds of various plants to great dis- 

 tances. Then the increase of depth of water or the lowering of 

 depth of bodies of water forces to a limited extent migration of 

 plants along other shores. 2d. Tensions in fruits; for example, 

 exploding fruits. 3d. Climatic pressures. These pressures are 

 brought about from variations in the climate, those variations 

 which extend over long periods of time. The most noted of 

 these pressures upon plant distribution occurred in what is 

 known as glacial times. During this epoch of the earth's his- 

 tory a great ice-sheet formed in Canada and British America, 

 flowed down across the border and over a great portion of the 

 northern United States. This great change in the climate, the in- 

 tense cold for so many ages gradually extending southward, forced 

 the plants of northern North America southward. Those which 

 were not able to migrate in advance of the glacier perished. 



978. Action of glaciers. Geologists have learned from a study of the 

 formation and movement of mountain glaciers and of the great ice -sheets 

 covering Greenland and other arctic lands how to determine the former 

 presence of similar glaciers by marks which they have left upon the sur- 

 face of the land. These great masses of ice, possessing enormous weight, 

 grind upon the rock surface, ploughing out basins and scouring the rocks 

 over which they pass. The great scouring action is produced by rocks 

 which become fastened in the lower surface of the glacier, and so shoved 

 along by its movement, ploughing and gouging into the solid rock below. 

 Along the advancing edge of the glacier or ice-sheet, where it is melting, 

 these r6cks, gravels, and clays, mixed together, are deposited, forming 

 what are known as the terminal moraines. 



979. Limits of the great glaciers. According to geologists, the great 

 ice-sheet which flowed down over the northern United States was gathered 

 on the uplands of Canada and British America by a great and long-con- 

 tinued fall of snow. The pressure of great bodies of snow formed wijh 

 the ice-sheet, and this, by its own weight, was forced to flow to the south- 

 ward where there was less snow on account of the warmer temperature. 

 The ice-sheet in its southern movement extended down to the "southern 

 border of New England, across New Jersey, Pennsylvania, and south- 

 western New York, and then followed a crooked course north of the Ohio 

 River. It nearly followed the Missouri River across Missouri, and then 

 northwest through Nebraska and the Dakotas and Montana, Mu^H of 



