THE PLEISTOCENE OR GLACIAL PERIOD. 359 



raphy favored more forcible abrasion at some points than at others, 

 while the ice itself was more effective at some times and places than 

 at others. One of the results was the development of rock-basins 

 by the ice-sheets. On the whole, the topographic effect of glacial 

 erosion was probably to soften the surface contours, without notice- 

 ably diminishing the relief. The erosive effect of an ice-sheet of large 

 size is probably greatest toward its edge, but far enough back for the 

 ice to be thick. The position of the area of greatest erosion probably 

 shifted with the decline of the ice-sheet. 



The second great phase of the work of the ice was the deposition 

 of the drift. Some of it was deposited while the ice-sheets were grow- 

 ing, some of it after they had attained their growth and before decay 

 had begun, and some of it while they were declining. Some of it was 

 deposited beneath the body of the ice, and some of it at its edge. In 

 some places, water played an important role in modifying the drift 

 left by the ice, while in others its influence was nil. The deposition of 

 the drift altered the topography notably, especially where the drift was 

 thick and the relief of the underlying rock slight. It is to the inequali- 

 ties in the thickness of the drift that many of the peculiar depressions 

 and elevations of the surface of the drift are chiefly due. Erosion 

 and the deposition of the eroded material are then the two great 

 results of an ice invasion, so far as the solid part of the earth is con- 

 cerned. The effects on life will be considered later. 



The drift formations fall chiefly into three categories, namely 

 (1) those made directly by the ice (unst ratified), (2) those made by 

 ice and water conjointly (stratified, but stratification often irregular), 

 and (3) those made by water emanating from the ice (stratified, often 

 with cross-bedding). To these deposits should perhaps be added, 

 (4) deposits made by floating ice derived from glaciers, and (5) the 

 eolian deposits to which the glacial deposits gave origin. 



Formations made by the Ice-sheets. 1 



The ground moraines, the terminal moraines, and the lateral moraines 

 are the principal types of drift deposited by the ice directly. Of these, 

 the ground moraines are by far the most extensive, while in connec- 



1 Jour, of Geol., Vol. II, pp. 517-53S, and Inst. Geol. Congr. Compt. Rend., 5th 

 Session, 1893; also McGee, idem. 



