Soil formation at Millstone Bluff and Johnson Ridge in southern Illinois

Silty soil materials were deposited as loess on the surfaces of two sandstone ridgetops during the Wisconsin Glacial period (10,000 to 20,000 years B.P.). Between A.D. 900 and 1450, Mississippian Indians occupied the Millstone Bluff summit site near Dixon Springs, Illinois, and built huts with floors dug into a silty soil. Huts and a cemetery occupied most of the 1 ha summit. From A.D. 1450 to the present, the Millstone Bluff site would have been vegetated by grass and/or trees and was never used as a European settlement site. The gently sloping summit of Johnson Ridge was cleared in the 1850s for a European settlement but had not been occupied since the 1940s. The objectives of this study were to determine 1) the time and extent of diagnostic soil-horizon formation and classification of soils on stable loess covered Southern Illinois ridgetops, 2) effects of 550-year-old Indian settlement on diagnostic soil-horizon formation and classification of soils on the loess covered Millstone Bluff ridgetop, 3) the effect of direction and distance from fly-ash source on ash deposition on elevated ridgetops, and 4) whether or not tillage occurred during the of 90 years of European settlement on Johnson Ridge. The soils on the stable summits developed thick argillic horizons, and the soils on the shoulders developed cambic horizons during the 10,000 years B.P. since last loess deposition. The thickness of the silty materials to the underlying sandstone bedrock varied with landscape position. A buried topsoil (Ab), developed in silt loam or silty clay loam materials, was present in a few profiles, most likely as a result of previous hut locations. It appears that a mollic epipedon developed in the hut depression as a result of decomposing hut materials, grasses, leaves, and retention of rain during the last 550 years. The depression reduced the depth to redoximorphic features and affected the soil drainage class (somewhat poorly drained versus moderately well drained). The summit probably was not cultivated during the last 550 years due to being completely surrounded by a steep sandstone escarpment. The A.D. 1450 date of Mississippian Indiana settlement abandonment established the time required for the observed diagnostic soil horizon formation in the hut depression. Fly-ash spheres were produced by adjacent railroad steam engines. The fly-ash distribution in the upper 20 cm of Millstone Bluff and Johnson Ridge profiles suggest no tillage had occurred on the ridgetops since at least the 1850s, the time of initial European settlement in the area and on Johnson Ridge.