482 THE ICE AGE IN NORTH AMERICA. 



cies would operate in the ordinary way during the whole 

 period of elevation, the streams carrying down to the sea a 

 large amount of material every season. But when the period 

 of depression had proceeded so far as to bring the surface 

 below the level of the sea, Mr. Croll believes the action of 

 the waves would greatly hasten the operation, and would 

 thoroughly sort out and roll the pebbles, washing the liner 

 particles into deeper water. 



A careful consideration of the forces in operation, how- 

 ever, does not seem wholly to justify this reasoning of Mr. 

 Croll. In the first place, there must have been at various 

 geological epochs, over the area now most studied, extensive 

 land exposures, continuing through a long period of time. 

 The Tertiary deposits contain many vegetable remains as 

 well as animal, showing the existence of land areas of no 

 small extent. The Carboniferous period reveals whole con- 

 tinents maintaining, over a large portion of their extent, an 

 elevation near the sea-level, in which there were continual 

 but slight oscillations, tending, however, on the whole to 

 subsidence. So that land-plants accumulated in sufficient 

 quantity to form the coal-beds — the periods of depression 

 being marked by sedimentary rocks formed by the consolida- 

 tion of the wash that was spread over the whole re'gion dur- 

 ing the times of depression. 



Now, it does not seem possible that a glaciated area so ex- 

 tensive as is that of North America, and so deeply covered 

 with glacial debris, could be so completely removed by or- 

 dinary denuding agencies that no more signs of it should 

 appear than are found of such phenomena in the earlier geo- 

 logical epochs; for the till, or ground-moraine, is not readily 

 removed* by the action of water, even where subjected to the 

 shore-waves of the ocean. The bowlders which are washed 

 out of it form a protecting barricade around the base of the 

 deposit, so that islands like those in Boston Harbor, com- 

 posed wholly of till, are as nearly proof against the waves as 

 are those of ordinary rocks. If there were in progress a sub- 

 sidence of the glaciated area of North America, instead of 



