512 PLEISTOCENE OF INDIANA AND MICHIGAN. 



Great Lakes lobe, 100 miles or more west of Lake St. Clair (see fig. 14), and that the only places, 

 within the area here discussed, where the first hinge line is convex toward the south are in the 

 Driftless Area of Wisconsin and the Salamanca reentrant angle. In the Great Lakes lobe the 

 southward concavity of the fine looks as though the uplifting forces had met some resistance 

 there which had prevented them from reaching as far south as they otherwise would. On the 

 contrary, the southward convexities in the Driftless Area and the Salamanca reentrant look as 

 though the uplifting forces had found less resistance there and had reached a little farther 

 south than the mean. The relations of these details certainly present problems of great diffi- 

 culty for those who believe the uplifts to be due solely to depression by ice weight and resilience 

 following its removal. 



These facts and relations raise a number of important questions. What was the order of 

 magnitude of the uplifting force ? How much pressure and how large an area were required 

 to produce the initial effect of depression 1 On the one hand, it was manifestly of such an 

 order of magnitude that the weight of 3,000 or 4,000 feet of ice fell a trifle short of being 

 critical; that is to say, it did not produce appreciable depression, for the removal of ice of 

 that thickness did not produce resilience at the first hinge line, although at points a little 

 farther north, where the ice was thicker, there was a shght uplift following removal of the ice. 

 Yet the removal of less than this thickness of ice over the central part of the ice field north 

 of the hinge fine was apparently followed by relatively rapid uplift of 100 to 200 feet through- 

 out a large part if not all of the region still covered by ice. In this movement the ice load 

 then remaining was uplifted along with the land. On the best interpretation now available it 

 is assumed that the removal of 3,000 or 4,000 feet of ice at the hinge line by melting and the 

 retreat of the ice front, including the marginal belt with its relatively steep surface gradient, 

 would be accompanied by a lowering of the central area of the ice field, not in like amount 

 but in a considerably less amount, perhaps half as much or less. The surface gradient near 

 the central area would be low, even if gravity were the only cause of the radial movement 

 of the ice. If radial movement of snow by the action of powerful and permanent anticyclonic 

 winds upon it before its consolidation into ice is an important factor, the surface gradient 

 would be hkely to be still lower. 



On the assumption that the removal of the weight of the ice was the cause of the uplift, 

 these considerations lead to the conclusion that at the hinge fine the superposition of 3,000 or 

 4,000 feet of ice on the land did not add weight enough to start a movement of depression, not 

 even by elastic compression, and consequently its removal was not followed by resilience, but 

 that in the central part of the ice field the accumulation of 10,000 to 15,000 feet of ice did pro- 

 duce depression, and the subsequent removal of say 1,500 to 2,000 feet (considerably less than 

 the maximum thickness at the lunge line) did cause resilience with resultant uplift. Whether 

 such a conclusion is tenable or not in the hght of known physical and dynamic laws is a problem 

 for the geophysicist. 



On the other hand, the first hinge line takes a direct course across driftless reentrants as well 

 as across great ice lobes, as if the uplifting movement had had nothing whatever to do with 

 such ice weight as affected the land under the basal part of the Great Lakes lobe. Apparently, 

 a variation of surface load ranging from zero to the weight of 3,000 or 4,000 feet of ice was not 

 enough to cause marked deviation in the hinge line. Indeed, the very shght deviation observed 

 is in the wrong direction to have any significance with regard to ice weight. If ice weight 

 produced depression it seems certain that it was confined chiefly to an area much farther 

 north, where the ice was much thicker than at the base line of the Great Lakes lobe. In 

 order to fit the facts, it seems necessary to suppose that marked depression by ice weight 

 occurred only where the ice was two or three times as thick as at the base line. If, as seems prob- 

 able, there was a large area of ice of great thickness farther north and northeast, strong depres- 

 sion may be supposed to have taken place there. It seems probable that the area of depression 

 would not be limited by a sharp boundary with vertical sides or with a very steep gradient, for 

 since the surface of the ice rose gradually from the edge back to the highest point, the amount of 

 depression would diminish gradually from the central area out toward the ice margin. But it 



