A periglacial stratified slope deposit in the valley and ridge province of central Pennsylvania, USA: Sedimentology, stratigraphy, and geomorphic evolution

The sedimentology and stratigraphy of a stratified slope deposit in central Pennsylvania together with existing pollen data and global climate simulations are used to develop a model for stratified slope deposition within the context of a waxing and waning Late Wisconsinan periglacial climate. The deposit consists of well‐stratified beds of platy, subangular shale chips ranging in size from very fine to very coarse gravel and is characterized by two facies: a matrix‐supported facies and a clast‐supported facies. Mechanical breakdown of bare, highly fractured shale bedrock by frost shattering and freeze‐thaw activity in a periglacial environment in front of the Late Wisconsinan glacial boundary provided an abundant and continuous sediment supply. Gelifracts were transported from bedrock source areas to depositional sites along valley side‐slopes by sheetwash and debris flow processes, creating an inclined (10º), aggradational surface of low relief. Surface runoff was derived from snow melt and rainfall onto impermeable frozen ground and bedrock surfaces. The cyclic nature and timing of deposition indicate that sediment was produced and transported immediately prior to and after the Late Wisconsinan glacial maximum. The Late Wisconsinan glacial maximum was characterized by permafrost conditions with reduced sediment supply and erosional truncation of the stratified slope deposit. Frost cracks, ice wedges and soil wedges developed on an unconformity within the deposit during the glacial maximum.