126 THE ILLINOIS GLACIAL LOBE. 



that the fossiliferous clay of the section is undoubtedly loess. He calls 

 attention also to the prevalence of conditions suitable for the growth of 

 an arboreal vegetation prior to the deposition of the loess. 



In the section just given, the black soil appears to be the representative 

 of the Sangamon interglacial stage. But this interglacial stage, like the 

 Yarmouth stage, is often represented by a leached and slightly reddened 

 till surface, unaccompanied by black muck. These two phases seem to be 

 mutually exclusive, there being but slight reddening of the till surface 

 where the black muck is present, or but slight accumulations of black muck 

 where the reddening of the till is pronounced. The black muck is 

 developed in portions of the uplands where the surface is very level and 

 conditions for drainage are defective, while the reddened soil is developed 

 on the more undulatory tracts, where a fair condition of drainage probably 

 existed. The black muck phase is common in the Sangamon Basin, and 

 also in the northern part of the white clay district of southern Illinois, and 

 southwestern Indiana just outside the limits of the Shelby ville drift sheet. 

 In western Illinois and southeastern Iowa it has a more restricted develop- 

 ment, for the surface there is generally more elevated and better situated 

 for the development of drainage lines than in the districts first mentioned. 

 In southern Illinois and southwestern Indiana and throughout much of 

 western and northwestern Illinois, the reddened till surface is a conspicuous 

 feature. 



In certain areas an alternation of sand with peaty beds has been found 

 between the Iowan loess and the Illmoian till. This phase is apparently 

 restricted to the borders of valleys where stream action has probably been 

 influential but with intermittent activity, the sand being deposited by the 

 streams, while the peat was accumulated at times when the streams failed 

 to cover the land. 



The accompanying views, PL X, A and 2?, taken in cuttings along the 

 Santa Fe Railway in eastern Knox County, Illinois, show a dark soil (b) at 

 the junction of the loess (a) and Illinoian till (c). In the exposures seen in 

 these views, acid tests show the till to have been leached to a depth of 

 about 4 feet below the dark-colored soil. The overlying loess is calcareous 

 at base. The leaching, therefore, took place prior to the loess deposition in 

 connection with the development of the soil. The depth of leaching noted 



