Shaler.] 300 



we assume the least time which can be reasonably assigned to the 

 present epoch, and suppose the glacial condition to have passed from 

 this hemisphere at least 100,000 years ago, and admit an average rate 

 of deposition of sedimentary materials of only one-tenth of an inch 

 per annum, we would have the depth of the cavities reduced over 800 

 feet. AVhen Ave consider the extreme activity of all the forces tend- 

 ing to fill up the basins in the diluvial condition which must have en- 

 sued from the melting of the glacial sheet, it would seem that this 

 estimate of one-tenth of an inch per annum is not excessively high, 

 and when we call to mind the fact that one hundred thousand years 

 is about the minimum of time which could have elapsed since the ces- 

 sation of the glacial period, and that the true time is probably much 

 greater, it will be evident that many of our still deep basins have had 

 their depth diminished at least one thousand feet. To satisfy the 

 facts it is necessary that theory should explain how, through the oper- 

 ation of forces brought Into action by the Imposition of the glacial 

 sheet, these exceedingly deep excavations could have been formed. 



It Is evident that the origin of these depressions is to be sought In 

 some cause producing local irregularities In the erosive action of the 

 glacial sheet. We have already given reasons for concluding that, in 

 most cases, this local Increase in the wearing action could not be 

 ascribed to the impinging of Ice moving down steep slopes against the 

 level surface at the base. It may be further objected that this action 

 would not be competent to produce basins, but could only develop 

 them still further, except where steep declivities already existed. 

 Nor is it conceivable how such great depth, with a moderate length, 

 could be attained through the operation of such a cause. The easiest 

 method of comprehending the nature of the forces operating to 

 abrade the surface at the base of the glacial sheet. Is to conceive a 

 gradual return to the conditions existing on the land surface within 

 the drift area during that period. At first, as In our winter sndws, 

 the surface of the glacial sheet would correspond in a general way to 

 the outline of the area on Avhich It rested, only the smaller Irregulari- 

 ties would be leveled over. As the thickness increased, the outline 

 of the land would be less and less represented by the irregularities of 

 the crust of the glacier, and finally, when the mass had come to have 

 a depth of thousands of feet, only the most Important reliefs would 

 have any effect on the contour of the upper surface of the Ice. The 

 vertical thickness of the Ice would be greatest over the valleys and 

 less over the hills. In proportion as they rose above the general sur- 

 face. 



The importance of these differences In the depth of the glacial 

 mass becomes apparent when we consider some other effects of the 

 accumulation. Acting; as a nonconductor, the o;lacIal mass would 



