=The hinge lines of uptilt.=—Still later in the study of the region, it was learned that the axis or fulcrum about which the region has been uptilted, instead of lying to the southward of the lake district, as had been assumed by Gilbert, lay within the region and about halfway up the basin of Lake Michigan (Fig. 373, _D_, and Fig. 374). Similarly, in the uptilt which followed the ice retreat in northern Europe a definite hinge line of movement has been discovered. Lastly, it has been shown, as a result of the use of precise leveling methods, that not one but several hinge lines of movement lie within the region, and that the separate sections into which they divide the area are each in turn characterized by increased up-cant as we proceed to the northward (Fig. 373, _E_ and Fig. 374). [Illustration: FIG. 375.—Series of idealistic diagrams to indicate the nature of the quick recovery of the crust by uplift in blocks unloaded of the ice in succession. A further and slower uptilt, added after the completion of the first movement, is brought out in the last diagram (_b_´).] The beaches of Lake Maumee, the earliest of the series of lakes within the Huron-Erie lobe and within the extreme southern portion of the Great Lakes area, show only the slightest possible northerly uptilt, and the well-marked hinge line disclosed in the Whittlesey beach is evidence that the elastic recoil, as it were, from the weight of the mantling glacier did not begin until after the draining of Lake Whittlesey. The determination by Taylor that there is a similar initial hinge line in the Warren beach—that this strand begins its uptilt some fifteen miles farther northeast than does the Whittlesey beach—is one of the greatest importance in obtaining a correct idea of the recent uplift; for it shows that the draining of Lake Whittlesey was followed by a period of quick uplift and seismic activity, that the stage of 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 Goldthwait 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 assumption of a succession of quick recoveries of the crust by uplift, 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.