OPOSSUM CREEK DRAINAGE BASIN, CUMBERLAND COUNTY, PA: REMNANTS OF A PENEPLAIN, PEDIPLAIN, OR SIMPLE EROSION?

The Opossum Creek drainage basin (OCdb) occurs on a belt of Ordovician-age Martinsburg shale, is bounded on the north by Blue Mtn. (BM) that is capped by Silurian-age Tuscarora quartzite and is bounded on the south by Conodoquinet Creek (CC) that is the general north boundary of older carbonate rocks. The basin is 2.5-2.7 km in width and 5 km in length. Opossum Creek heads on the lower slopes of BM and it and its tributaries have created within the OCdb a gently sloping topography between the two divides that separate it from adjacent drainage basins. In Cumberland Co. there are 27 topographically similar drainage basins with varying dimensions. Similar topography on the belt of Martinsburg shale extends continuously from the Delaware River in NE PA southwestward across PA and on southward across MD, WV, and VA.

OCdb is somewhat unique in that the lowest divide elevations occur at the base of BM and the highest elevations are adjacent to the creek junction with CC. Of considerable importance are Tuscarora boulders that occur on the upper elevations throughout the OCdb. Such boulders occur similarly elsewhere within this topographic belt in PA and may occur outside PA.

Campbell (1903) called this belt of shale upland the Harrisburg peneplain. Because the probable original topographic form was a relatively flat, sloping surface and because of its position between lower carbonate rocks to the S and the higher BM to the N, it is more probable that the original surface was a pediment. When that original surface was formed has been considerably debated and times suggested range mainly from Eocene to Late Miocene. When and how the boulders were transported across the surface is speculative, but periglacial transport during the early Pleistocene is a reasonable mechanism in PA when northern PA was glaciated and OCdb was tundra. Transport and deposition of boulders across this surface was followed by in situ boulder let-down during subsequent surface erosion.

The upland remnants of what was once a continuous, uniform surface creates more questions than there are currently solid answers.