THE PLEISTOCENE OR GLACIAL PERIOD 



397 



Somewhat later, the Saginaw ice-lobe retired so that a crescentic 

 lake (Lake Saginaw) gathered about its extremity, and discharged 

 through the Grand River outlet into Lake Chicago, and thence by 

 the Illinois route to the Mississippi. For a time, Lake Maumee con- 

 tinued to discharge by the Imlay outlet into Lake Saginaw, and thence 

 to the Mississippi; but in the course of the retreat, a lower outlet across 



Fig. 517. — A later stage in the development of Lakes Chicago and Maumee. The ice 

 has retreated farther, and the outlet of Lake Maumee has been shifted. (Leverett 

 and Taylor, U. S. Geol. Surv.) 



the " thumb " of eastern Michigan was discovered, and the Imlay 

 outlet was abandoned. 



Later, the whole Erie basin, and a portion of that of Ontario, became 

 free from ice, and a lake twice the area of the present Lake Erie developed 

 (Lake Arkona), and was marked by its own set of beaches. According 

 to the recent determinations of Taylor, an advance of the ice followed, 

 closing the lower outlet across the Thumb of Michigan, and forcing 

 the water to occupy a higher one at Ubly. This stage was attended 

 by the formation of a beach (the Belmore) at a higher level than the 

 Arkona beaches, which were submerged but not wholly obliterated. 



