Strath terraces, perched above spectacular gorges found at and above the fall line separating the piedmont from the low-relief coastal plain, typify major rivers draining the mid-Atlantic region of eastern North America. Here, as elsewhere in the world, the age distribution of rock surfaces exposed on such terraces is not known. Reliable terrace dating would place the formation of the gorges and process of downcutting and nickpoint retreat in a temporal context, constraining the rate and pattern of bedrock erosion and perhaps allowing such features to be understood in terms of climate and sea level changes.

Using measurements of in situ produced ¹⁰Be and ²⁶Al, we propose to constrain the age distribution of terrace surfaces bordering two of the region’s largest rivers, the Potomac and Susquehanna. Such age estimates will allow us to calculate rates of bedrock removal and nickpoint retreat as well as understand the pattern by which nickpoints retreat and terraces are formed over time. Preliminary isotopic data, included in this proposal, demonstrate the feasibility of our approach.

The Potomac and Susquehanna Rivers drain adjacent, lithologically and topographically similar basins that have dissimilar Quaternary climate histories. Both rivers drain humid-temperate regions and both are repeatedly affected by large magnitude run-off events triggered by hurricanes and tropical storms. The Susquehanna Basin was in part glaciated and the river carried significant amounts of glacial sediment and meltwater. The Potomac Basin remained unglaciated. Thus, by estimating the age of terraces and the rate of nickpoint retreat in these two specific river systems, we should be able to detect whether or not glaciation, and the concomitant increase in sediment load and runoff, was a major control on the age and rate of terrace formation.

Potomac River bedrock terraces have been mapped extensively by Zen, a collaborator on this proposal. Seven bedrock terraces flank the river from the cataract at Great Falls where the lower 4 terraces converge, through Mather Gorge just downstream, to the boundary with the Coastal Plain below the gorge, The terraces are cut into schist and accordant bedrock surfaces are generally well preserved as demonstrated by the presence of water-polished quartz and spectacular potholes. The Susquehanna terraces, gorge, and cataract at Holtwood has been studied by Sevon, Thompson, and Pazzaglia.

This proposal seeks funds to support focused mapping to identify specific sample sites, sample collection, isotope analysis, and data reduction necessary to estimate the age distribution of terrace surfaces exposed along the Potomac and Susquehanna Rivers. We will measure the abundances of ¹⁰Be and ²⁶Al in 90 samples. Most samples will be collected from fluvially eroded bedrock; however, we plan use a few samples to estimate the age of a boulder bed on the highest Potomac River terrace and the deposition age of a flight of well-characterized, sediment-mantled strath terraces along the Susquehanna River at Marietta, PA. On bedrock straths, we will sample perpendicular to each river and along the length of several terraces in order to estimate both the rate of nickpoint retreat and the time intervals between incision events. Together, these age estimates will be used to determine limiting rates of mass removal from the river gorges. Our goal is to estimate when and how quickly major channel incision events occurred, if they were coincident on the two rivers, and if they were related to large-scale climate forcing such as glaciation.

The project has a strong educational and mentoring component including funding for two students, one at the MS level, the other an undergraduate. The students will work closely with Bierman and his laboratory technician at the University of Vermont, in the field with USGS collaborators Pavich and Zen, and at Livermore with collaborator Caffee making AMS isotopic measurements.