CONSTRAINTS ON GORGE INCISION AT HOLTWOOD AND MATHER GORGES, SUSQUEHANNA AND POTOMAC RIVERS

The Fall Line runs north-northeastward though (at least) Virginia to New Jersey, separating Piedmont from Coastal Plain. Recent studies of cosmic ray-induced isotopes in the quartz component of bedrock shows that the Susquehanna and Potomac Rivers cut gorges at the Fall Line between about 35,000 and 14,000 years ago (University of Vermont investigators and colleagues). Strath elevations and incision ages at the two gorges are similar. Other hydraulic factors being equal, if the Piedmont moves up 10m over a zone 2km wide on a pre-existing slope of 0.0005, the new slope at the zone is now eleven times the slope before movement. This increases the velocity by a factor of 3.3 (Manning equation and my interpretation of Zen, 1997). Using an overturning Froude Number as a measure of the movability of rock blocks in the channel, the thickness of overturnable blocks increases by the same factor as velocity (by 3.3). Continued incision leads to three constraints on gorge development: (1) thalweg slope decreases as incision works upstream, (2) boulders clogging the channel become more immovable as slope-controlled velocity decreases, and (3) (most importantly) incision rate slows because successive unit increments of incision require removing ever larger rock volumes from the gorge side slopes. This third constraint assumes self-similar gorge cross-sections as the gorge incision propagates vertically downward. This third constraint also implies continued resetting of the cosmic ray clock along the walls of the gorge, so that the walls between the pre-incision strath and the final thalweg should have approximately the same cosmic ray-induced age. Published elevations and ages (26 pairs from Reusser et al., 2004) agree fairly well with this implication: the three terraces from Holtwood Gorge (Susquehanna River) have average elevations of 35.0, 39.3 and 43.4m, but their average ages are, respectively, 14,400, 18,700 and 30,100 years BP.