THE PLEISTOCENE OR GLACIAL PERIOD 863 



eye to be nearly plane; yet even so moderate a slope may lead to 

 very extraordinary conclusions. 



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

 1,600 miles from the center of movement. An average slope of 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, nearly eight miles, 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 precipitation near its margins, the only part 

 of the ice-cap which had any considerable slope was its outer 

 border (Fig. 574), so great a maximum thickness would not be 

 demanded. 



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

 sheet which no longer exists involves two distinct stages. These 

 are (1) 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 case of the ice-sheets of the 

 glacial period, each of these stages was probably more or less com- 

 plex, the general period of growth being marked by temporary 

 intervals of decadence, while during the general period of decadence 

 the ice was subject to temporary intervals of growth. In the study 

 of the work accomplished by an ice-sheet, it is of importance to 

 distinguish between these main stages. 



THE WORK OF AN ICE-SHEET 



Erosion and deposition were the two great phases of ice work, 

 and both have been discussed briefly (pp. 251-68). It may be 

 noted here that the surface over which the ice-sheets moved proba- 

 bly had an erosion topography, and was covered by a layer of mantle 

 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, the topography favored more 

 forcible abrasion at some points than at others, and the ice itself 



