UPTILT OF LAND AT CLOSE OF ICE AGE 347 



Lake Warren was one of comparative stability of the land, 

 and, lastly, that the draining of Lake Warren was followed by a 

 second period of rapid uplift and earthquake disturbance. 

 The strongly marked hinge lines, additional to the initial one 

 indicated for the Algonquin beaches in the profiles by Gold- 

 thwait from the west shore of Lake Michigan, when considered in 

 the light of this northeasterly migration of the still earlier hinge 

 line in the southern district, are best explained through the as- 

 sumption of a succession of quick recoveries of the crust by up- 

 lift, separated by periods of relative stability, and brought on by 

 the removal in turn of the ice burden from successive blocks of 

 the shell which are separated by the several hinge lines (Fig. 375). 



The elaborate study of erosion in the outlet of Lake Agassiz 

 had indicated identical interruptions in the up-canting process 

 for that basin. 



Future consequences of the continued uptilt within the lake 

 region. One of the most distinguished of American geologists, 

 Dr. G. K. Gilbert, in order to determine whether the uptilt revealed 

 by canted beach lines is still in progress, carried out an elaborate 

 study upon the gauge records preserved at the various gauging 

 stations about the Great Lakes. Upon the basis of these studies, 

 he concluded that the uplift continues, that the axes of equal 

 uplift (isobases) take their course about fifteen degrees north of 

 west, so that the lines of greatest uptilt should be perpendicular to 

 this direction, or fifteen degrees east of north. He further believed 

 that the basin was undergoing an up-cant in the simple manner of 

 a trap door, the hinge of which lay to the southward of Chicago, 

 and the study of the gauge records led him to believe that " the 

 rate of change is such that the two ends of a line one hundred miles 

 long and lying in a south-southwest direction are relatively dis- 

 placed four tenths of a foot in one hundred years." 



Gilbert's prophecy of a future outlet of the Great Lakes to 

 the Mississippi. The natural rock sill, over which the waters 

 of Lake Chicago once flowed to the Mississippi, is to-day but 

 eight feet above the common mean level of Lakes Michigan and 

 Huron, and if the tilting of the lake region were to continue upon 

 Gilbert's assumption of a canting plane with the hinge of the 

 movement to the south of Chicago, a time must come when the 

 " Chicago outlet" will again come into use and the lakes once 



