POTENTIAL PALSA RELICTS IN EASTERN PENNSYLVANIA

Possible periglacially-derived depressions were found along the floodplain of a small, entrenched, first order stream on the Piedmont of Pennsylvania. If the structures were produced by permafrost during the last glaciation, their preservation over 10 ka implies an anomalously low rate of sediment deposition and surface erosion. To characterize the features, we took ten soil cores across three oblong depressions (5-10 m) on the floodplain and constructed two cross sections, which show undulating sediment layers following the curvature of the depressions. A GPR survey of the site corroborated the continuous nature of the subsurface layers.

Palsas are periglacial features formed when a layer of peat or water-logged clay an ice nucleus. This ball of ice expands due to capillary action, compressing underlying layers and expanding overlying layers to form an ovular mound (1-20 m), which frequently occurs with several other palsas. As the ice melts, the structure collapses and forms a shallow depression with a surrounding berm. This morphology closely conforms to the studied depressions. Additionally, the high clay content, nonplanar water table, and periglacial history of the area would render the formation of permafrost structures possible. However, palsa collapse would have occurred over 10 ka ago. For such a small feature to retain its current level of preservation, it would require that virtually no sediment be deposited for more than 10 ka.

The possibility that this area is subject to a low rate of sediment transport has been posed in a previous study of the creek. It revealed that the stream is incapable of transporting the sediment in its banks and bed, pointing to probable glacial legacy deposits and indicating near sedimentary stasis since the last glacial maximum. Additionally, relict palsas have been identified in Belgium, Germany, Wales, and Ireland, lending further credibility to the interpretation of the features as periglacial. The subtlety of these features may have led them to be largely overlooked, making it possible that more palsa remnants can be found elsewhere along floodplains in former periglacial regions. If undescribed palsas are indeed prevalent, their presence can be used as a proxy to identify past periglaciation and low post-glacial sedimentation rates.