80 IOWA ACADEMY OF SCIENCES. 



Valley Excavation during the Sangamon Interglacial Stage — The 

 large streams in western Illinois and southeastern Iowa are 

 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 lowan loess. The excavations 

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

 They are broad and shallow. On Skunk river, along the bor- 

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

 thirty to forty feet below the level of the uplands, but the val- 

 ley 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 

 indicate that during the Sangamon interglacial stage the stream 

 had a lower gradient than at subsequent stages. On the neigh- 

 boring 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 ero- 

 sion 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 sel- 

 dom that sufficient deepening of streams has occurred to pro- 

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

 mine the amount of work accomplished during the Sangamon 

 interglaciaj stage. On the borders of these lowlands the lowan 

 loess rises above the level of the modern streams, and at such 

 places occasional exposures were found in which the junction 

 of lowan loess and Illinoian till is marked by a thin bed of 

 material more pebbly than the typical till; a feature which is 

 thought to indicate moderate stream action prior to the deposi- 

 tion of the loess. A similar feature has been noted on the bor- 

 ders of many of the small valleys in western Illinois and south- 

 eastern Iowa. 



EXPLANATION OF PLATE IV. 



lowaa loess and S tngamon soil exposed In a cutting on the Santa Fe railway west 

 of Williamsfield. Knox county, III Tnickness of loess twelve feet. The shaded band 

 below the loess Is the Sangamon soil. It h-s a depth of one and one-half to two feet. 

 The Illinoian till beneafi is leached and deeply oxidized for about four feet. View 

 taken by Prank Leverett, May, 1892. (Above.) 



lowan loess and Sangamon koII near Wllliamsfield, III., near preceding but at much 

 closer range. View obtained by Frank Leverett, May, 1893. (Below ) 



