518 PLEISTOCENE OF INDIANA AND MICHIGAN. 



the chain of lakes along the Paleozoic margin, and extending from the Arctic Ocean through 

 Alaska to Bering Sea, has been elevated 300 to 400 feet out of the sea since the disappearance 

 of the last ice sheet. It is perhaps somewhat significant that in Alaska this belt includes a 

 large region which has not been glaciated, and hence where direct depression by ice weight 

 did not occur. No great ice sheet was near to that region. 



If the crustal creeping movement of Tertiary time had not entirely ceased at the time of the 

 glacial period it is easy to see that a slight crustal movement toward the southwest might be the 

 cause of the obseiwed uplifting and tilting. It would not require much horizontal movement of 

 a crustal sheet 5 to 10 miles thick to produce the relatively slight arching recorded in the beaches. 

 It may well be that the ice sheet influenced this movement to some extent and contributed in 

 some degree to the determination of its place and limits, although it is at present hard to see just 

 what effect it would have and how it would act. The general course of the margin of the up- 

 lifted belt is broadly parallel with the limits of the ciystalline rocks for a great distance but 

 appears to turn away westward across Alaska to the Bering Sea. Even if the uplifted belt is 

 some sort of fold or upswelling produced by recent crustal creep from the northeast, it is not 

 possible to say at present to what extent, if any, ice weight and geologic structure may have 

 influenced the result. 



Orogenic movements of no small amount have occurred in the region of the Cordilleran 

 mountain belt within the Quaternary period, and these seem to imply a continuance, at least in 

 some slight degree, of the creeping movement of the continental crustal sheet. 



CONCLUSIONS. 



Although the uplifts were spasmodic in character, the time relations are so intimate that it 

 is hard to resist the impi ession that the land rose simply because the ice and its weight disappeared. 

 And yet there is no reason why tectonic movements in the earth's crust may not have occurred 

 with just these time and place relations to the retreating ice sheet. Perhaps the truth lies in a 

 middle course and both causes may have contributed to the result. If the uplifting movements 

 were to any large extent tectonic or epeirogenic and independent of resilience after depression by 

 ice weight, then the undeviating course of the hinge line across large ice lobes and unglaciated 

 reentrants is not a matter of surprise nor of special difficulty. 



However, it is not intended here to attempt to reach final conclusions as to the cause of the 

 uplifts but merely to point out some of the limitations established by the known facts. Ice 

 weight may have been an important contributory factor in the great Quaternary regional uplifts 

 but probably did not have the values nor act in the simple and complete way supposed by many. 

 The limitations seem to carry the problem suggestively near to the realm of another class of earth 

 movements due to entirely different causes. 



There appears to be no reason to question the validity of Woodward's result in his deter- 

 mination of the elastic compressibility of the earth, nor of Lane's quantitative application of the 

 same to the concrete case assumed by him. But the facts fall so far short of agreeing with these 

 theories that the whole matter becomes uncertain. 



In conclusion it may be said that while the preponderance of evidence may appear at the 

 present time to be slightly in favor of resilience following depression by ice weight as the main 

 cause of the uplifting of the land and the deformation of the shore lines in the region of the 

 Great Lakes, the difficulties in the way of a complete demonstration of this as the main cause 

 seem well-nigh insuperable. It seems certain that some new basis of physical principles better 

 adapted to a quantitative analysis of the problem will have to be found before the ice-weight 

 hypothesis can be firmly established. 



Standing as a close second to the hypothesis of deformation by ice weight is that of deforma- 

 tion by uplifts of the land incidental to crustal creeping movements, which are simply the latest 

 or most recent impulses in a process of continental growth reaching back into Tertiary time. 

 Both causes may have acted, but if certain evidences which are now supposed to indicate rela- 

 tively recent crustal creep toward the southwest are substantiated by future investigations the 

 hypothesis of resilience following depression by ice weight seems likely to become of secondary 

 importance. 



