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