Significant Multiple Pre-Illinoian Till Exposure in Southeastern Indiana

Abstract

Five tills are exposed in a stream cut south of the Wisconsinan glacial boundary in Decatur County, southeastern Indiana. All tills are mineralogically or texturally distinct. The uppermost till is leached 7 ft and is regarded as Illinoian. Because there is a thick paleosol (7 ft of leaching and 11 ft of oxidation) in the surface of the till immediately below the Illinoian till, four of the tills in this section are considered pre-Illinoian in age. This is the largest number of pre-Illinoian tills known east of the state of Illinois. On the basis of lithologic characteristics and fabric, all tills appear to have been deposited by ice moving from a northerly or westerly direction. The chert-rich basal till of this section is very distinctive and, where described from other localities in southeastern Indiana, always is the lowermost till of the section. For this reason, and because the till contains a high percentage of weathered chert pebbles and non-calcareous “red” clay material, it is regarded as representing the first ice advance into southeastern Indiana. Although no definite age can be assigned to the pre-Illinoian tills, it seems that because of their textural and mineralogical differences they represent (1) deposition by nearly contemporary ice sheets advancing from slightly different directions and/or (2) deposition by non-contemporaneous ice sheets which advanced from a common direction after erosion had modified the pre-existing surface enough to expose new source materials.