Surficial geology of the Delta quadrangle, Harford County, Maryland and York County, Pennsylvania
1998, Pazzaglia, F.J. and Cleaves, E.T.
Report of Investigations 67
Detailed surficial mapping of the Delta quadrangle has established the stratigraphy, sedimentology, soil stratigraphy and morphology, and major surficial processes for an area of central Appalachian Piedmont of Pennsylvania and Maryland . The fundamental, coarsest-scale mapping unit is a surficial terrane defined by the integration of a characteristic landform, a dominant or characteristic surficial deposit , and a characteristic distribution of deposits across previously defined landscape units (Cleaves and Reger, 1991). The identification of seven surficial terranes illustrates the influence of lithology and the relationship attained between the distribution of surficial deposits and topography . In contrast to more conventional views that hold that the Piedmont is an old landscape dominated by saprolite and residuum, mapping has demonstrated that colluvial diamictons, are the primary surficial unit. Other surficial deposits include structured saprolite, residual soil, alluvium, and minor upland gravels . Deposits are defined primarily on a textural basis which also generally mirrors their relative age . A soil chronosequence estimates deposit age by distinguishing late Wisconsinan (late Pleistocene), Illinoian (late middle Pleistocene), and pre-Illinoian (middle Pleistocene) pedons. Piedmont surficial deposits are superimposed on a landscape that reflects the interaction between long-term, post-middle Miocene base level fall and climatic fluctuations . Piedmont physiography within the quadrangle is generally characterized by valleys with a lower, steep, fluvially incised V-shaped component that opens both upslope and upstream to an upper concavo-convex hillslope component separated by relatively flat , broad interfluves. Accelerated fluvial incision, initiated in the late Tertiary and continuing through the Quaternary in response to base-level lowering , increased relief of the late Tertiary piedmont landscape . Various geomorphic processes responding to Quaternary climatic fluctuations have worked to reduce that relief through several erosional episodes that have removed large quantities of former saprolite and weathered rock from the uplands and incorporated this material into multiple deposits of colluvium.