CHAPTEE VII. 



THE SANGAMON SOIL AND WEATHERED ZONE. 



After the lUinoian stage of glaciation the surface of its drift sheet 

 became weathered, and a black soil was formed on the level or podrly 

 drained portions. This weathering and the forming of a soil continued for 

 a considerable period, as is apparent from the state of decay of the pebbles 

 and the leaching of the till, there having been usually a complete leaching 

 of the lime from the finer part of the till to a depth of several feet, accom- 

 panied by a nearly complete dissolution of limestone pebbles. 



This weathering has not continued to the present day, for the surface 

 of the lUinoian has received a coating of silt several feet in depth, which 

 now prevents further weathering. This silt deposition and interruption of 

 the weathering, as indicated below, are thought to have occurred in con- 

 nection with the lowan stage of glaciation, in which case the soil and 

 weathered zone stand for an interglacial stage between the lUinoian and 

 lowan glacial stages. 



The name Sangamon has been suggested for this interglacial stage by 

 the writer because of the excellent development of the soil in the Sangamon 

 River drainage basin, in central Illinois, and because the exposures there 

 were among the earliest recognized in this country.^ 



The Sangamon soil and weathered zone may be seen beneath the sur- 

 face silt in thousands of exposures in southeastern Indiana and southwestern 

 Ohio, for the general thickness of the silt is onlj^ 4 or 5 feet. Farther north 

 there are, in addition to the silt, the heavy deposits of Wisconsin drift, which 

 have buried the soil and weathered zone to such a depth that it is rarely 

 exposed. However, a few exposures have been found in the deeper valleys, 

 and wells not infrequently penetrate both the silt and the soil under the 

 Wisconsin drift. 



'The Sangamon soil and -weathered zone, by Frank Leverett: Proc. Iowa Acad. Sci., Vol. V, 1898, 

 pp. 71-80; also Jour. GeoL, Vol. VI, 1898, pp. 171-181. Numerous instances of occurrence within the 

 limits of the Illinois glacial lobe are cited in Monograph XXXVIII of this Survey. 



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