X MAN AND THE GLACIAL PERIOD. 



would have been rapidly effected by the reversed drainage 

 consequent upon the arrival of the ice-front at the southern 

 shore of the Lake Erie basin. During all the time elapsing 

 thereafter, until the ice had reached its southern limit, the 

 stream was also augmented by the annual partial meltiug 

 of the advancing glacier which was constantly bringing 

 into the valley the frozen precipitation of the far north. 

 The distance is from thirty to seventy miles, so that a 

 moderately slow advance of the ice at that stage would 

 afford time for a great amount of erosion before sufficient 

 northern gravel had reached the region to begin the filling 

 of the gorge.* 



Mr. Leverett also presented an important paper before 

 the Geological Society of America at its meeting at Madi- 

 son, Wis., in August, 1893, adducing evidence which, he 

 thinks, goes to prove that the postglacial erosion in the 

 earlier drift in the region of Rock River, 111., was seven or 

 eight times as much as that in the later drift farther 

 north ; while Mr. Oscar H. Hershey arrives at nearly the 

 same conclusions from a study of the buried channels in 

 northwestern Illinois. f But even if these estimates are 

 approximately correct — which is by no means certain — they 

 only prove the length of the Glacial period, and not 

 necessarily its discontinuity. 



At the same time it should be said that these investi- 

 gations in western Pennsylvania somewhat modify a por- 

 tion of the discussion in the present volume concerning 

 the effects of the Cincinnati ice-dam. It now appears that 

 the full extent of the gravel terraces of glacial origin in 



* See an elaborate discussion of the subject in its new phases by 

 Chamberlin and Leverett, in the American Journal of Science, vol* 

 xlvii, pp. 247-283. 



f American Geologist, vol. xii, p. 314f. Other important evi- 

 dence to a similar effect is given by Mr. Leverett, in an article on 

 The Glacial Succession in Ohio, Journal of Geology, vol. i, pp. 129- 

 146. 



