PEORIAN INTERGLACIAL STAGE 507 



the loess, showing that the icefield there was a barrier preventing the 

 loess from being spread over the lower area. 



Immediately after southern Illinois and Indiana were bared by the 

 recession of the ice in late Illinoian and Sangamon time, the early loess 

 there was being laid down, being contemporaneous in part with Illinoian 

 till. It was more abundantly supplied during the Iowan glacial stage, 

 and its later deposition was of the same age as the Altamont moraine. 



Wisconsin Glaciation 



The very diversified drift sheet of the latest grand division of the Ice 

 Age was named in 1894 by Chamberlin for the State of Wisconsin, where 

 in 1877 he mapped and described its interlobate Kettle moraine. In 

 1879 the present writer traced its terminal moraines on Long Island, 

 Martha's Vineyard, Nantucket, and Cape Cod. During the next fifteen 

 years, in the service of the Minnesota and United States Geological 

 Surveys, I mapped the courses of twelve marginal moraines formed by 

 the Wisconsin icefields at their maximum extension and at times of 

 pause or readvance interrupting their general retreat in Minnesota, Iowa, 

 and South and North Dakota. 



The scanty eroding action of the glacial currents while depositing 

 the relatively smooth expanses of the outer and earlier drift, during each 

 of the preceding stages, was strongly contrasted with the vigor of erosion 

 displayed by the planed and striated bedrocks of many districts inclosed 

 by the later moraines. When the ice-sheet heaped these morainic ridges 

 and hills, it had worn into its adjacent bed of rock or of the old drift 

 deposits, accomplishing much erosion on extensive tracts covered by the 

 later and uneven Wisconsin drift, which has plentiful lakes and lakelets, 

 reaching northward from the outermost large and continuous moraine 

 and comprising the far greater part of our drift-bearing area. 



Drainage from the Wisconsin ice-border in its lobate course across the 

 basin of the Mississippi was also more vigorous than when the loess was 

 deposited. The land depression of that time had been succeeded by a 

 moderate reelevation, giving to this interior region of the continent 

 nearly its present height. Wide and deep channels in the loess flood 

 plains were soon eroded by the rivers, which brought down and deposited 

 much sand and gravel from the Wisconsin drift, their finer silt being then 

 mostly borne far southward to the lower Mississippi or even to its delta. 



But the more temperate climate which had been restored, probably 

 by a great subsidence of the continent after the Kansan and Illinoian 

 glaciation, bringing at first much ice-melting and loess deposition, con- 

 XXXIV — Bull. Geol. Soc. Am., Vol. 33, 1921 



