2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 147-14
Presentation Time: 5:00 PM


SEVON, W.D., East Lawn Research Center, 30 Meadow Run Place, Harrisburg, PA 17112-3364, wsevon30@comcast.net

Blue Mountain (Mtn.) is continuous NE-SW across Pennyslvnia (PA) from the Delaware River (E) to Maryland (W) and farther south. Blue Mtn. is the SE margin of the Appalachian Mountain and Blue Mountain Sections of the Ridge and Valley Province. The rock forming the mountain is the Tuscarora quartzite, the most erosion-resistant rock in PA. The Tuscarora is well-bedded and has abundant bedding normal parting planes. The rock dips steeply to the W, is either exposed intermittently along the mountain crest or is present at or near the crest on the E side. Because of the mountain orientation, its steep slope, and variation in Tuscarora position, formation of colluvium is very different on opposite sides of the mountain. Colluvium is formed by various near surface rock-disintegration processes and downslope gravity-driven transport mecanisms that are most effective in the winter months when sun angle is low and focused entirely on the SE face of Blue Mtn. Thus, the Tuscarora has been intensely eroded and moved on the SE facing slope, particularly during the Pleistocene when climatic conditions were most conducive to such activity. Erosion was primarily by freeze-thaw activity and transport was partly by free fall and partly by ice movement. Half way down the slope, the underlying bedrock changes to softer, Ordovician-age Martinsburg shale that eroded to a much lower surface slope. Movement of the Tuscarora changes on the lower gradient surface slope: the transport process slows and a lower mountain slope colluvial fan develops, particularly west of the Susquehanna River. In contrast, on the NW facing slope, colluvium is minimal to nonexistent because of (1) lack of direct sunlight and (2) lack of exposure of hard rock, particularly the Tuscarora. Some colluviated Tuscarora occurs locally on the upper slopes when that rock occurs on the mountain crest, but generally the slope is formed directly on softer, younger bedrock that has minimal soil of variable thickness, and little or no downslope movement of surface material.