WHAT‘S IN THESE WOODS WE DO NOT KNOW: LIDAR REVEALS WIDESPREAD LOBATE PERIGLACIAL FEATURES ON FORESTED MOUNTAINS IN SOUTHCENTRAL PENNSYLVANIA

Lidar elevation data, recently available for all of Pennsylvania from PAMAP, reveals widespread lobate features in the forests on both Blue Mountain and South Mountain adjacent to the Great Valley in south central PA. Nearly every forested slope that consists of colluvial sandstone rubble displays these lobate features. The lobes typically have broad treads parallel to the down slope direction and end in steeper risers dropping to the next tread. Lidar-derived shaded slope maps accentuate the steeper risers and their lobate form. Relief on risers on Blue Mountain is commonly 4-6 m. Some risers on South Mountain reach heights of 6-10 m. Field observation of some lobes shows that they are real but subtle, whereas others are very obvious. On the north-facing lower slopes of South Mountain, lobes extend out onto gently sloping (<2^o) aprons of alluvium and colluvium overlying carbonates. Here there appear to be lobes of more than one generation. The youngest, and closest to the mountain appear fresh and un-modified, whereas older lobes are modified by karst so that they have numerous closed depressions and vernal ponds.

These lobate features are almost perfect replicas of periglacial gelifluction lobes ubiquitous in arctic and alpine regions today, and we infer that these are relict from Late Wisconsinan and earlier cold intervals when glaciers approached as close as 100 km to the north. Interpretation as periglacial features is reinforced by the presence of macrofossils of tundra vegetation between 16,000 and 14,000 years B.P. in a core taken from Kings Gap Pond at the base of South Mountain (Delano, H. L., Miller, N. G., and Potter, N., Jr., 2002) and tors on South Mountain.