LANDSCAPE EVOLUTION AND MODERN SOIL GEOMORPHIC RELATIONSHIPS ASSOCIATED WITH PATTERNED GROUND IN SOUTH CENTRAL ILLINOIS

Recent mapping of high pH, sodium affected soils (SAS) in south central Illinois has revealed the occurrence of extensive networks of polygonal patterned ground (PPG) associated with SAS. SAS occupy approximately 383,512 ha of the Springfield Plain and occur on extremely low-relief, planar landscapes where 1-2 m of Wisconsin Episode loess overlies a lower permeability Sangamon Geosol developed in Illinois Episode glacigenic sediments. SAS associated polygons are typically 10 to 80 m in diameter with 4 to 6 m borders. Polygon interiors are darker colored with lighter colored borders and referred to as “reverse patterned ground”. PPG occurring on the Bloomington Ridged Plain in northern and central Illinois in Wisconsin Episode glacigenic diamictons and Peoria Silt have the same dimensions but have darker colored borders surrounding lighter colored interiors. The SAS related PPG on the Springfield Plain is interpreted to have formed as a result of Wisconsin Episode permafrost formation and degradation. We will focus our discussion on the geomorphic significance of permafrost related processes and how they have led to a unique microtopography that has in turn led to the post-glacial formation of SAS in south central Illinois. We also propose that the southern limit of Wisconsin Episode permafrost is farther south than previously mapped.