LIDAR, ORTHOIMAGERY, AND FIELD ANALYSIS OF PERIGLACIAL LANDFORMS AND THEIR COLD CLIMATE SIGNATURE, UNGLACIATED PENNSYLVANIA AND MARYLAND

High-resolution orthoimagery, topographic datasets (e.g., LiDAR), and GPS surveying offer opportunities to map fine-scale landforms, even where forested, over broad areas. Using these technologies, field mapping, backhoe trenching, geophysical techniques, and coring, we are compiling a GIS database of relict periglacial landforms south of Pleistocene full glacial ice margins in Pennsylvania and Maryland. In particular, we search for Pleistocene periglacial landforms diagnostic of the former existence of permafrost (ground that remains at or below freezing ≥2 years). Continuous permafrost can occur today in regions with mean annual air temperatures (MAAT) less than −6° to −8° C, and discontinuous permafrost can occur in regions with MAAT less than ~ −0.5° C to −2° C. The boundary of continuous-discontinuous permafrost latitudinally shifted with cold glacial–warm interglacial climate cycles during the Quaternary Period, with the Last Glacial Maximum from ~26.5 to 19 ka.

We identify the following evidence of permafrost and/or its thaw: 1) extensive networks of thermal-contraction polygons (TCPs) on crests and side slopes of shale hills; 2) thick, ubiquitous stacks of gelifluction sheets and lobes on quartzite, sandstone, schist, and diabase ridges and side slopes; and 3) potential pingos in synclinal valleys with high relief. We document two other landforms—retrogressive thaw slumps and thermokarst gullies—that are common in regions of permafrost thaw today. All landforms become less pronounced to the south. We only find well-developed TCPs—diagnostic of continuous permafrost—on Paleozoic shale bedrock in Pennsylvania. Roadcut and roadbed exposures reveal vertical to sub-vertical wedge-shaped structures along TCP boundaries (with sand and gravel infill to a depth of ~1.5 m), which often parallel structural joints in bedrock. We only find rimmed circular pingo-like features on valley floors within tightly folded rocks in central Pennsylvania. Paleotemperatures are within the range of the "frost-cracking window", explaining the ubiquitous presence of brecciated bedrock and shattered bedrock fragments on slopes and valley bottoms throughout the region.