A TALE OF TWO BOULDER FIELDS--BLUE ROCKS AND HICKORY RUN, PENNSYLVANIA

Fifty years ago at the first NE GSA meeting an abstract about Blue Rocks boulder field in Berks Co., PA was presented by Potter and Moss (1966, subsequently published in GSA Bull, 1968). This seems a good occasion to consider what we have learned in 50 years and contrast Blue Rocks to the iconic Hickory Run boulder field. Blue Rocks originates in outcrops of Tuscarora sandstone on Blue Mountain, and in its lower part sandstone boulders have moved out over Martinsburg shale. Slopes on the boulder field range from 15^o near the source to a low of 3^oat the terminus. For Blue Rocks imbricate, angular slabby boulders up to 5 m long describe lobes within the boulder field. In the adjacent forest we mapped treads and risers that we interpreted as solifluction lobes and sheets. What’s new? Recently acquired lidar confirms the presence of many solifluction lobes and sheets adjacent to Blue Rocks, and these periglacial features are ubiquitous on the flanks of sandstone ridges in the mountains of central and south-central Pennsylvania indicating a Pleistocene periglacial environment.

In contrast Hickory Run boulder field in Carbon Co., PA has a very low slope of ~1^o. It is difficult for things to move on 1^o slopes. At Hickory Run angular boulders occur, but many are rounded. The boulders are made of the same sandstone as the underlying Duncannon member of the Catskill formation, so boulders need not have moved far from their source. Imbrication is not common, and there is little or no evidence of flow lobes. Lidar there does not show solifluction lobes or sheets near the boulder field, although there are a few to the NE. Low altitude air photos obtained with an Unmanned Aerial Vehicle in 2006 show numerous rectangular sorted polygons within the boulder field that are roughly symmetrical. They would be stretched E-W parallel to the inferred axis of movement if much movement had occurred. Thus the sorting that produced the polygons must post-date any movement, and it may be that Hickory Run boulder field was largely formed by weathering in place.