DEFORMATION OF SHORE LINES. 511 



lobe which covered the area between the two hinge lines was affected by depression and resilience, 

 and further, that the movements affecting it were all of the first period and had ceased before 

 the beginning of Lake Algonquin. It may therefore be said that the whole lobe lies within the 

 area of horizontality when considered with reference to the Algonquin period of elevation. 



From the front of this great lobe back to its base fine the surface of the ice sheet must 

 have risen in the usual way. The data for determining the rate, however, are meager. The 

 present land surface at both ends of the base line is something above 1,500 feet above sea level. 

 Both of these points, however, were about 100 feet lower than now before the beginning of the 

 first period of uplift. The rim of the Lake Erie basin rises gradually eastward from 760 feet 

 above sea level at Fort Wayne, Ind., to above 1,000 feet across most of northern Ohio and to 

 above 1,500 feet at Salamanca, N. Y. Northward from Fort Wayne the rim rises to over 1,000 

 feet above sea level in northeastern Indiana. The rim of the Lake Michigan basin rises from 

 590 feet at Chicago to over 1,000 feet above sea level in northeastern Indiana and gradually to 

 1,000 feet in southern Wisconsin and to over 1,500 feet at the northeast corner of the Driftless 

 Area. The present surface of Lake Erie is 573 feet and of Lakes Huron and Michigan 581 feet 

 above sea level. 



The ice surface must also have risen along the base fine from the two ends toward the 

 middle as well as backward along the central axis from the apex. At an average of 10 feet per 

 mile for 400 mdes the surface would rise 4,000 feet from the apex to the base. This would 

 allow for a rise of 2,000 to 2,500 feet in 300 miles from the ends of the base line to the middle. 

 This average rate seems low enough. If the rise be. divided into sections with a higher rate at 

 the front diminishing toward the base any estimate that seems tenable leads to a rise of at 

 least 4,000 feet. For example, if the rate averaged 50 feet per mile for the first 10 miles, 20 

 feet per mile for the next 90 miles, 10 feet per mile for 100 miles, and then 5 feet per mile for 

 200 miles, the total rise to the base line would be 4,300 feet. A rise of 4,000 feet in 400 mdes 

 on the central axis and of 2,000 to 2,500 feet in 300 miles on the transverse axis at the base 

 line does not seem excessive — certainly not in comparison with the Greenland and Antarctic 

 ice sheets as now known. Indeed, it is almost certainly an underestimate, rather than the 

 reverse. This would mean a depth of ice of something near 4,000 feet over the head of Saginaw 

 Bay and the south end of Lake Huron. In the Huron-Erie basin the Whittlesey hinge line 

 lies about 75 miles south of the Algonquin hinge line, and the surface of the ice over Lake St. 

 Clair and for 100 miles or more to the west along the former hinge line may be supposed to 

 have been between 500 and 700 feet lower than at the base line. Even if the thickness of the 

 ice on the middle of the base fine was as little as 3,000 feet, this is equal to the thickness found 

 by the revision of the estimate for Keweenaw Bay in Lake Superior and is more than twice the 

 thickness which Lane found so closely accordant with his results for elastic resilience and hydro- 

 static readjustments in that region. He found that 1,250 feet of the ice there produced depres- 

 sion followed by 650 feet of resilience (combining both causes), but on the Whittlesey hinge 

 line, 75 miles south of the base line of the Great Lakes lobe, there was probably fully 3,000 

 feet of ice, possibly even 4,000 feet, and yet there is no evidence whatever of either depression 

 during ice occupancy or of resilience following its removal. 



RELATION OF ICE WEIGHT AND HINGE LINES TO ICE MARGIN. 



EFFECT OF DRIFTLESS AREAS. 



In Wisconsin the first hinge line passes through the central part of the Driftless Area, 

 which was at the time of the Wisconsin ice sheet simply a large reentrant angle, and it passes 

 also across the central part of the driftless reentrant south of Salamanca. Neither of these 

 areas was covered by the Wisconsin ice sheet at its maximum, nor were they covered by 

 any of the earlier ice sheets. Yet the hinge fines keep their courses with scarcely any devia- 

 tion across the Great Lakes ice lobe, the Driftless Area, and the Salamanca reentrant angle. 

 Indeed, looking at the details a little more closely, it appears that the only place where the 

 first hinge fine is concave, even very slightly, toward the south is in the central part of the 



