EVIDENCE OF GELIFLUCTION LOBES AT THE GETTYSBURG BATTLEFIELD, PENNSYLVANIA

Previous workers have noted that the diabase boulder-strewn slopes at the Gettysburg Civil War battlefield (Gettysburg National Military Park) in southeastern Pennsylvania are the result of periglacial processes, and focused in particular on freeze-thaw activity as a mechanism for producing boulders up to 8 m in length. Here we use analysis of lidar data to demonstrate that these boulders moved downslope primarily as a result of gelifluction, a form of mass movement within the active layer of permafrost. This layer typically is a half meter or so thick and overlies a frozen inactive layer of permafrost. Gelifluction occurs during seasonal freeze-thaw cycles in periglacial landscapes. During thaw, drainage of excess water is limited by the frozen substrate and the strength of the overlying thawed mass is reduced, leading to elasto-plastic deformation. At the Gettysburg battlefield, the most prominent examples of gelifluction occur on hills formed of Triassic diabase, part of the Gettysburg sill, which is characterized by extensive joints. Elevation of the diabase hills is about 200 m above sea level, and gelifluction lobes are prominent on multiple low tors left behind as boulders moved downslope from outcrop sources. Hills that are well known for playing crucial roles in the Union defense, including Little Roundtop, are rimmed with multiple, stepped gelifluction lobes that extend for tens of meters outward from the hill tops. Some lobes are stacked atop older lobes, and some appear to have flowed around large blocks, possibly intact bedrock (i.e., braking blocks). Permafrost must have existed during at least one cold full glacial period, perhaps as recently as the last glacial maximum (33,300 to 26,500 years BP).