Genesis of soils and landscapes in the Ridge and Valley province of central Pennsylvania

Abstract

Abstract The characteristics and properties of the soils on the ridge tops, footslopes, and adjacent limestone valley areas in the Ridge and Valley of central Pennsylvania have been strongly influenced by their parent material and geomorphic history. The ridge top soils have developed in sandstone colluvium which mantles sandstone residuum. The upper part of the original residual ridge top soil was truncated during late Wisconsinan time and then covered with local colluvium or it was cyroturbated. These sandstone parent materials have been stable since the late Wisconsinan and have sandy skeletal Dystrochrepts and Haplorthods developed in them. The Haplorthods are minor soils and are associated with local concentrations of coniferous vegetation. During the late Wisconsinan, the sandstone colluvium also moved downslope and was mixed with bedrock and residual material from shale and limestone and deposited on the footslope over a pre-Wisconsinan soil developed in older colluvium or limestone residuum. The footslope surface colluvial soils vary in texture and drainage because of their parent material, their location in discharge areas, and fragipan development. The age of the brown surface colluvium is considered late Wisconsinan and the age of the pre-Wisconsinan buried soils is not known. The buried soil's bright red (rubified) color and argillic horizon indicate a much greater degree of soil development than noted in the brown surface colluvium, and its age may be correlated with isotope stage 6. The soils developed at the surface in the colluvium are mainly Ultisols although some poorly drained soils, particularly in limestone material, are Alfisols. The Ultisols are parent material Ultisols and the poorly drained Alfisols have a high base status in their parent material or were recharged with bases from the groundwater. The soils of the limestone valleys are developed in residuum. The residuum accumulated from the insoluble residues after the CaCO 3 was leached from the bedrock. If the residuum is thick (2 to 3 m), the soils date back many hundreds of thousands of years, but are still classified as Hapludalfs. In addition to being old, the surface of the Hapludalfs has been modified by periglacial truncation and the addition of eolian materials. These processes have given the upper 50 cm of the Hapludalfs a late Wisconsinan age but the bulk of the B horizon is many thousands of years older. Eolian material has also been added to the ridge top soils as well as the colluvial soils. The mineralogy of all the soils in this landscape, including the buried paleosols, does not show significant clay mineral weathering. This appears anomalous considering the degree of oxidation of the buried paleosols and the limestone soils and their estimated age.