THE PLEISTOCENE OR GLACIAL PERIOD. 357 



Baraboo, Wis., 1 the edge of the ice at the time of its maximum exten- 

 sion in that region lay along the side of a bold ridge, the axis of which 

 was nearly parallel to the direction of ice movement. The position 

 of the upper edge of the ice against the slope of the ridge is sharply 

 defined. For the last one and three-fourths miles, its average slope 

 was about 320 feet per mile. This, it is to be noted, was at the extreme 

 edge of the ice, where the slope was at a maximum. In Montana, 

 the slope of the upper surface of the ice for the 25 miles back from 

 its edge has been estimated at 50 feet per mile. 2 Calculations based 

 on data from New Jersey and adjacent parts of New York, indicate 

 for this region a slope of about 30 feet per mile 3 for the upper sur- 

 face of the ice when it was there thickest. It is to be noted that the 

 data for this calculation were drawn from localities which, while 

 relatively near the edge of the ice-sheet, were still some miles within 

 it. At first thought, a surface slope of 30 feet per mile does not seem 

 excessive, for the surface of such a slope would seem to the eye to 

 be nearly plane; yet even so moderate a slope may lead to very extra- 

 ordinary conclusions. 



The southern limit of drift in Illinois is not less than 1500 or 1600 

 miles from the center of movement. An average slope of 30 feet per 

 mile for 1500 miles would give. the ice a thickness of 45,000 feet at 

 a point 1500 miles from its margin, if the slope of the surface on which 

 the ice rested be disregarded, and this slope was so little as to be of 

 no great consequence in this connection. This thickness, more than 

 eight miles, seems incredible. Even an average slope of 10 feet per 

 mile would give a thickness of nearly three miles at the center of the 

 ice-sheet. If by reason of relatively great infra-marginal accumula- 

 tion, the only part of the ice-cap which had any considerable slope 

 was its marginal part, the surface of the central portion being nearly 

 flat, so great a maximum thickness would not be demanded. 



Nansen 4 found that the surface of the ice-sheet of Greenland rose 

 abruptly at either margin, and less and less rapidly as its summit was 

 approached. He crossed the ice where it was about 250 miles wide. 

 On the east side he found a slope of about 220 feet per mile, and on 



1 Jour, of Geol., Vol. Ill, p. 655. 



2 Calhoun, Jour, of Geol., Vol. IX, p. 718. 



3 Smock, Am. Jour. Sci., Vol. XXV, 3d Series, p. 339. 



4 Nansen, The First Crossing of Greenland, Vol. II, p. 465. 



