Large rivers along the mid-Atlantic coast flow through spectacular bedrock gorges as they exit the Piedmont onto the Coastal Plain and their tidewater reaches. Was this incision driven by tectonic or climatic forcing? When did it occur?

In order to infer the cause and understand the timing, spatial distribution, and rate of the bedrock incision in two such gorges, we collected >70 samples for 10-Be and 26-Al analysis from exposed, fluvially eroded outcrops of quartz-bearing schist along strath terraces of the Potomac and Susquehanna Rivers at the Piedmont/Coastal Plain transition. Here we report primarily on the Potomac gorge.

The first 18 samples, collected from Mather Gorge below Great Falls on the Potomac River, indicate that the most prominent strath terrace, a several km-long bedrock feature 20 to 25 m above the current low water level, was abandoned rapidly as the Potomac River incised about 30 kya (SL, >60 deg, 10-Be PR=5.17 atoms/g). Nine samples, collected from water-polished rock surfaces down a cross-section from this terrace to just above the river, have decreasing nuclide activities consistent with a fluvial, bedrock incision rate of about 70 cm/ky and an effective 6 ky exposure age at water level. Four samples, collected from below the normal low-flow level during the 2000 drought, have activities equivalent to about 5 ky of exposure. Thus, to account for cosmic-ray dosing at and just below the water’s surface, we have subtracted 6 ky of exposure from older terrace surface ages.

The incision of Mather Gorge began about 30 ky coincident with a major drop (>50 m) in eustatic sea level, the result of glacial ice-volume increase. Incision is clearly coincident with cooling climate but we do not know if it was driven by base-level fall or by changes in discharge and sediment loading. However, the similarity of model exposure ages for samples collected from a km-long transect parallel to the river and within 2 km of the present-day knick point at Great Falls, suggests that retreat of the knick point which formed the gorge must have been rapid, lasting only a few thousand years.