EROSION OF THE DEEP VALLEYS SECTION, APPALACHIAN PLATEAUS PROVINCE, PENNSYLVANIA: DESTRUCTION OF A FORMER UPLAND SURFACE BY INCISION

The Deep Valleys Section (DVS) is a topographically unique area wthin PA that comprises deep, narrow, steep-sloped valleys that were incised into a former higher, minimally dissected, plateau surface developed on Pennsylvanian-Devonian rocks occurring in a series of low-magnitude SW-striking anticlines and synclines.

The DVS was created by incision of the West Branch Susquehanna River (WBSR) and its primary tributary systems, Pine and Sinnamahoning Creeks. Stream orientation and overall pattern are strongly controlled by structure, both obviously and subtly. The DVS incised valleys are up to 305 m (1,000 ft) deep. Their bottoms are generally less than 150 m (500 ft) wide with narrow floodplains, but in places are more than 600 m (2,000 ft) wide. Upper margins of the incised valleys are 850 m (2,800 ft) or more wide. Slopes are steep, up to 33°. The topographic boundary between valley slope and the upland is generally very sharply defined and slices through the upland surface with no regard to pre-incision topographic form. Views in Clinton County from the valley bottom at Hyner and an upland margin at nearby Hyner View State Park provide excellent vistas of the overall topographic character.

Topographic configuration of the valleys shown on 1:50,000-scale topographic maps indicates that valley incision cut into a low-relief upland whose surface was minimally eroded by the WBSR drainage system. The post-incision upland remnants, particularly the very sharply defined valley-upland margins, have suffered minimal subsequent downward erosion.

The DVS defining incision started where Pine Creek and WBSR had earlier cut through the Allegheny Front at Torbert and Lock Haven, respectively. Incision was presumably rapid, followed the pre-existing drainage system, and created the deep valleys. Valley incision apparently had minimal affect on the unincised upland. Specific sites demonstrating ongoing headward erosion and drainage reversal exist today.

This erosional event probably commenced about 16 mya in the middle Miocene and was a contributor to increased sediment input to the Baltimore trough at that time. A very reasonable erosion rate of about 20 m/my is calculated for the incised valleys near the Allegheny Front margin of the DVS.