506 W. UPHAM STAGES OF THE ICE AGE 



IOWAN GrLACIATION AND LOESS DEPOSITION 



During the closing part of Illinoian time and continuing into the 

 Iowan stage, a gradual subsidence of the northern ice-laden half of this 

 continent gave low gradients to the Mississippi and its tributaries. A 

 temperate climate then returned upon the previously high and cold region 

 adjoining the ice-border. Rains washed into the valleys much silt from 

 erosion of the drift sheets and from the formerly englacial drift that 

 became exposed on the surface of the melting and receding Iowan ice- 

 fields. Deep and wide noodplains of this alluvial silt, similar to the 

 loess of the Rhine in Germany, whence it is so named, were spread along 

 the Missouri, Mississippi, and Ohio valleys. Its time of most abundant 

 deposition is marked by its northern boundary in northeastern Iowa, 

 found by McGee to coincide with the margin of the ic^-sheet at a stage 

 of glaciation which has been named the Iowan stage. Within the area 

 that was then ice-covered are many boulders, remaining from the en- 

 glacial and at last superglaciai drift, its finer part having been carried 

 away by the streams from the melting ice to be deposited as loess. 



Winds sweeping across the valleys bore far over the uplands a large 

 portion of this fine silt from its noodplains when the rivers were reduced 

 in size, as they doubtless flowed in restricted channels for the greater 

 part of each year. Thus the loess was spread as a general mantle, usually 

 a few feet thick, upon nearly all the extensive area occupied by the 

 Kansan and Illinoian drift. 



The Peorian interglacial Stage 



After the principal deposition of loess and preceding the advance of 

 a lobe of the Wisconsin icefields near Peoria, in Illinois, an interglacial 

 surface of the loess there was exposed to weathering, which has caused 

 the name Peorian to be given to that interval of some glacial" recession 

 and readvance. It was apparently of less duration or areal extent than 

 the Yarmouth and Sangamon stages, though the lobation of the morainic 

 border of the later drift in the Mississippi basin presents a very remark- 

 able change from the more regular previous boundaries of the ice-sheet. 



Loess continued to be plentifully deposited in northwestern Iowa until 

 the time of formation of the outermost Altamont moraine, of Wisconsin 

 age, which in 1880 I observed to be bordered on its west side along a 

 distance of 75 miles, from Guthrie County northwestward to Storm 

 Lake, by an expanse of loess as high as the crests of the morainic hills. 

 The sheet of Wisconsin till east of the moraine is 50 to 75 feet lower than 



