BOULDER COLLUVIUM ON THE SOUTHEAST-FACING SLOPE OF BLUE MOUNTAIN IN CUMBERLAND COUNTY, PENNSYLVANIA

Blue Mountain (BM) extends southwestward across PA from the Delaware River in the north-east to the MD-PA state line and beyond in the southwest. The mountain ridge is broken only by several water gaps. The crest and upper southeast-facing slope of BM is composed of quartzite of the Silurian Tuscarora Fm. This upper slope is considerably steeper than the lower slope that is underlain by shale and siltstone of the Ordovician Martinsburg Fm. All of the BM slope is covered with boulder colluvium (BC) derived from the Tuscarora quartzite (TQ). Where exposed, naturally or artificially, such as at Wagoner’s Gap, the TQ shows bedding planes that are separated by beds several to many cm thick. In addition, the TQ has numerous structurally generated fracture planes that are generally approximately normal to bedding and occurring at intervals up to a meter or more apart. The underlying Martinsburg is almost never exposed on the slope.

During the Pleistocene, freeze and thaw action in the periglacial climate that affected all of the non-glaciated parts of PA utilized the numerous bedrock parting planes to break down the TQ into a multitude of boulders and blocks. This material gradually descended the slope of BM to create a covering of BC. The upper, steep-slope BC comprises, basically, broken rock that fell, slid, and rolled down slope assisted by snow and ice and additional freeze-thaw breakdown action. There is no surface pattern in the upper slope BC.

Once the downward moving BC achieved the lower, gentler, Martinsburg slope, the movement process changed to one controlled by solifluction in an often water-saturated environment. This BC moved in masses generally having the form of low, rounded, elongate, curved mounds with steep fronts, rounded tops, and less steep backs. These masses are called solifluction lobes. This BC generally has more, smaller pieces of rock and some dirt that contributed to its lobe-forming tendency. Solifluction lobes are very common in PA on lower, gentler slopes of mountains. Multiple soil profiles on the BM mountain slope indicate that at least 3 periglacial episodes occurred during which BC movement took place.