THE WEATHERED ZONE l8l 



characterized by high-level terraces. The valleys of which these 

 terraces are the bottoms have been formed in the Illinoian till 

 sheet, and are covered by the Iowan loess. The excavation 

 may, therefore, be referred to the Sangamon interglacial stage. 

 They are broad and very shallow. On Skunk River, along the 

 borders of Lee and Des Moines counties, Iowa, the terrace is only 

 30 to 40 feet below the level of the uplands, but the valley is 

 nearly two miles in average breadth. The valley cut below 

 the level of the terrace is more than 100 feet in depth, but 

 is only one-half mile in average breadth. These features indi- 

 cate that during the Sangamon interglacial stage the stream 

 had a lower gradient than at subsequent stages. On the 

 neighboring portion of the Mississippi the valley formed at the 

 Sangamon stage was shallow, as on Skunk River, but was 

 not much wider than the inner valley. The large volume of 

 water flowing through the valley at the time when it constituted 

 an outlet for the glacial Lake Agassiz and the glacial lake in 

 the Superior basin is perhaps the cause for the relatively great 

 erosion subsequent to the Sangamon interglacial stage. 



In southern Illinois and southwestern Indiana the main 

 streams usually flow in broad shallow valleys, in some cases 

 several miles in width, which were apparently built up by the 

 glacial and fluvio-glacial deposits of Illinoian age. It is seldom 

 that sufficient deepening of streams has occurred to produce 

 well-defined terraces ; and it is not an easy matter to determine 

 the amount of work accomplished during the Sangamon inter- 

 glacial stage. On the borders of these lowlands the Iowan loess 

 rises above the level of the modern streams and at such places 

 occasional exposures were found in which the junction of Iowan 

 loess and the Illinoian till is marked by a thin bed of material more 

 pebbly than the typical till ; a feature which is thought to indi- 

 cate moderate stream action prior to the deposition of the loess. 

 A similar feature "has been noted on the borders of many of the 

 small valleys in western Illinois and southeastern Iowa. 



Frank Leverett. 



