624 



THE PLEISTOCENE PERIOD 



Fig. 516. Small protuberances of rock showing the effect of ice wear. Glacial 

 knobs and trails. Movement of ice from left to right. The projections consist of 

 chert in limestone. Near Darlington, Tnd. (U. S. Geol. Surv.) 



temporarily they played an important role. It does not preclude 

 the idea that, contemporaneously with the production of the great 

 body of the drift by glacier ice, the sea may have been working on 

 some parts of the present land area, modifying the deposits made by 

 ice and ice drainage. The glacial theory does not deny that rivers 

 produced by the melting of the ice were an important factor in trans- 

 porting and depositing drift, both within and without the ice- 

 covered territory. It does not deny that lakes, formed in one way 

 and another through the influence of ice, were locally important in 

 determining the character and disposition of the drift. Not only 

 does the glacier theory deny none of these things, but it distinctly 

 affirms that rivers, lakes, the sea, and icebergs must have co-operat- 

 ed with glacier ice in the production of the drift, each in its appro- 



Fig. 517. Diagram to show the effect of ice wear on slight depressions in the 

 surface of rock. 



priate way and measure, and that after the disappearance of the 

 ice and the ice-water, the wind had some effect on the drift before 

 it was clothed with vegetation. 



Development and Thickness of the Ice-sheets 

 The development of glaciers from snow-fields has been dis- 

 cussed (pp. 124-7). If the expansion of the ice-sheets was due prin- 



