626 THE PLEISTOCENE PERIOD 



The southern limit of drift in Illinois is not less than 1,500 or 

 i, 600 miles from the center of movement. An average slope of 

 even 25 feet per mile for 1,600 miles would give the ice a thickness 

 of 40,000 feet at the center, the slope of the surface on which the ice 

 rested being disregarded. This thickness seems incredible. Even 

 an average slope of 10 feet per mile would give a thickness of about 

 three miles at the center. If by reason of relatively great precipita- 

 tion near its margins, the only part of the ice-cap which had con- 

 siderable slope was its outer border (Fig. 518), a lesser maximum 

 thickness would suffice. 



Stages in the history of an ice-sheet. The history of an ice- 

 sheet which no longer exists involves at least two distinct stages. 

 These are (i) the period of growth, and (2) the period of decadence. 

 If the latter did not begin as soon as the former was completed, an 

 intervening stage, representing the period of maximum ice ex- 

 tension, is to be recognized. In the ice-sheets of the glacial period, 

 each of these stages was probably more or less complex. The 

 general period of growth was doubtless interrupted by short inter- 

 vals of decadence, and the general period of decadence by brief 

 intervals of growth. In the study of the work accomplished by an 

 ice-sheet, it is of importance to distinguish between these main 

 stages. 



Work of Ice-sheets 



Erosion and deposition were the two great phases of ice work 

 (p. 147 et seq.}. The surface over which the ice-sheets moved prob- 

 ably had an erosion topography, and was covered by a layer of man- 

 tle rock. The ice removed the mantle of decayed material, and cut 

 deeply into the undecayed rock beneath. By its erosion, the ice 

 modified the topography to some extent, for weaker formations 

 were eroded more than resistant ones, and topography favored more 

 forcible abrasion at some points than at others. On the whole, the 

 topographic effect of glacial erosion was probably to soften the sur- 

 face contours, without diminishing the relief. 



The second great result of the ice-sheets was the deposition of 

 the drift. Some of it was deposited while the ice-sheets were grow- 

 ing, some of it after they had attained their growth, and some of 

 it while they were declining. Some of it was deposited beneath 

 the body of the ice, and some at its edge. Where it was thick, the 

 drift altered the topography notably, especially where the relief 

 of the underlying rock was slight. 



