POSSIBLE PRE-WISCONSINAN GLACIAL INFLUENCE ON BOULDER FIELDS IN NORTHERN BUCKS COUNTY, PENNSYLVANIA

Massive boulders (~0.3 to >5 m) covering large tracts of Jurassic diabase exposures north of Lake Nockamixon, Bucks County, Pennsylvania, have been attributed to weathering along several sets of joints and downslope transport. Often rounded, the boulders extend westward for >10 kilometers (South Quakertown-Milford), consistent with the general trends of diabase exposures. Many clasts are covered with several generations of crustose lichens, suggesting minimum exposure for many centuries. Although exotic lithologies have not been discovered among the larger sizes, a limited glacial transport cannot be excluded, particularly due to relatively low resistance of Triassic sedimentary and metasedimentary rocks enclosing the plutons. In addition, the distance of large clasts (>2 m) from nearest outcrops (10s to 100s of m), wide spacing (> 5m) on relatively level surfaces, and perched mega-clasts are indicative of many terrains with glacial erratics. The county’s highest point, Haycock (Ghost) Mountain (294 m), is steepest on its southeast slope and a number of 3-4-m-long outcrops have distinct near vertical slopes in an easterly direction. This morphology would be consistent with a roche moutonnée terrain sculpted by a southerly advance of a glacial lobe. Preliminary field inspection yielded several boulders with grooves resembling striations that trend in a N80W-S80E direction. Thick soil between and below the boulders and the location of the proposed terminus of the Laurentide Ice Sheet 40-50 km to the north have been used to refute the Wisconsin age of the boulder tracts. However, earlier glacial phases, which extended beyond the Wisconsinan limit (till-based pre-Illinoian terminus ~20 km to the north) cannot be discounted as possible mechanisms for bedrock abrasion and short-distance boulder transport.