SUBTLE PALEOTOPOGRAPHIC EFFECTS ON MORPHOLOGY OF THE FARMDALE-SANGAMON GEOSOL COMPLEX IN NORTH-CENTRAL ILLINOIS

The Feltes Sand and Gravel pit in Kane County, Illinois contains a well preserved sequence of the Farmdale-Sangamon Geosol complex. The complex consists of the Sangamon Geosol formed in diamicton overlain by the Farmdale Geosol formed in Roxana Silt. The sequence is buried under lacustrine sediment and glacial diamicton of the Wisconsin Episode. We described and sampled four profiles along an exposure containing a 40 m long paleohillslope. The Sangamon geomorphic surface contains 0.75 m of relief and has soil morphological characteristics ranging from well drained to somewhat poorly drained. The Farmdale surface contains 0.55 m of relief and has soil morphological characteristics that range from well drained to poorly drained. In higher landscape positions, the upper solum of the pedocomplex consists of the silty Farmdale Geosol and the loamy former A of the Sangamon Geosol. Morphological characteristics of the upper solum, described as A horizons, include 10 YR colors and platy parting to weak granular structure coated with siltans. Subsoil B horizons have colors with 5YR and 7.5YR hue, strong, fine blocky structure and patchy argillans that become thicker and continuous with depth. Down slope in the lowest landscape position, the Farmdale Geosol is thicker and darker, mucky near the surface, with moderate granular structure throughout. The former A horizon of the Sangamon Geosol is thicker than upslope, with weak granular structure. E horizon characteristics of weak structure, pale color, and low clay content are better expressed in the lower landscape position. Subsoil Bt horizons have colors with 7.5YR hue that overlie a 2.5Y hue Bg horizon near the base of the solum. B horizons have moderate, blocky structure and patchy argillans that become thick and continuous ferriargillans with depth. Subtle topographic relief is related to recognizable differences in soil morphology of the Farmdale and Sangamon Geosols that have persisted following burial.