6 4 8 



HISTORICAL GEOLOGY 



the case, but that the thickness of the ice not far from the margin 

 is practically the same as that of the interior, the surface of which is 

 a comparatively level plain. 



The slope of the sides of the tongues of ice that reached down 

 ravines of the Allegheny River somewhat beyond the margin of the 

 main sheet varied from ioo to 130 feet a mile, and the average slope 

 of the ice lobe of the Hudson valley has been estimated to have been 

 25 to 30 feet a mile. Even the smaller of these figures would make an 

 enormous thickness for the ice sheets at the centers, if the slopes were 

 uniform. Since the ice in Illinois is known to have reached 1500 to 

 1600 miles south of the center of accumulation, an average slope of 

 25 feet a mile would, on this basis, give a thickness of about eight 

 miles at the center. It is probable, however, that the slope was not 

 nearly so steep some distance back from the margin. When this is 

 taken in connection with the fact that the thickness of the ice near 

 the margin was probably approximately the same as that at the center, 

 a much less depth is obtained than on the former estimates. Upon 

 any basis, however, the thickness must have been great. In New 

 England, for example, the ice was so thick that it passed over the 

 Green Mountains where they are 3000 to 5000 feet high, in a course 

 diagonal to their general direction, showing that such a mountain 

 chain made " scarcely a ripple on the surface." (Tarr.) 



Glacial and Interglacial Stages 



The Glacial Period was made up of a number of advances of the 

 ice and corresponding recessions when the ice either entirely, or 

 largely, disappeared from the northern hemisphere. The duration 

 of the various glacial stages was long, and that of the interglacial 

 stages so extended as to permit trees and plants to clothe again the 

 glaciated regions. The proofs of distinct ice sheets (Fig. 566), 

 separated by long intervals, when the land was free from glaciers and 





FlO. 566. — Diagram showing the proof of successive ice sheets. Resting upon the 

 ill sheet and underlying the upper are ancient peat bogs (solid black). The 

 erosion of the older flower) drift where exposed is much further advanced than that 

 "' thi upper). The composition of the drift sheets differs also. 



