LIDAR IMAGERY OF THE NORTH CENTRAL PENNSYLVANIA GLACIATED AREA SHOWS SINUOUS SMALL SCALE RIDGES OR BENCHES INTERPRETED TO BE FROM PERIGLACIAL SNOWDUNE EROSION

In areas of thick till in north central Pennsylvania the LiDAR images show a number of somewhat sinuous small scale ridges or abrupt bench-like slope steepenings. Such features are not present in areas of bedrock or thin till on bedrock. The individual ridge-benches (RB) typically run 150 - 600 meters across the hillsides in a southwesterly direction regardless of slope orientation and are usually 2 - 5 meters high. The features are often in near parallel sets of three or more, spaced 30 - 100 meters apart. A few sets are somewhat diverging or converging. On the Ulster 7.5’ map 56 RB features have an orientation average of S23°W, a median of S23°W, and a mode of S25°W. They are 30° to 35° more to the south than nearby hill-top glacial grooves cut in bedrock.

The somewhat sinuous trace of the RB features suggests that they may be small scale eskers but the soil mapped on all of the features is derived from till rather than sand and gravel. In the field a number road cuts through the features expose silty matrix till. At one RB site, a fresh two meter deep road cut showed a reddish stony, sandy-silt matrix till derived from the underlying bedrock. Subglacial till deformation produces small scale ice-flow parallel till ridges and grooves but they are remarkably straight. So the texture and somewhat sinuous form of most of the till ridge-benches makes a subglacial origin for the features questionable.

Similar RB forms are present in Pennsylvania south of the glacial limit and are attributed to periglacial activity. Where the features are slightly oblique or parallel to the hillslope and change orientation as the hillslope orientation changes, they are interpreted to be from gelifluction. On hillsides in central Pennsylvania where the features have a consistent orientation regardless of hillslope orientation, they are interpreted to be from nivation processes associated with parallel transverse to barchinoid snow dunes. Also arguing for a “snow dune erosion” model for such features, are photographs of present day features associated with barchanoid snow dunes that are essentially identical to those features on the LiDAR images. At present, the snow dune model appears to offer the best explanation for the somewhat sinuous RB features.