NEW PHYSICAL EVIDENCE FOR STREAM CAPTURE AS THE DRIVER OF ACTIVE RETREAT OF THE BLUE RIDGE ESCARPMENT

The striking and enigmatic Blue Ridge Escarpment (BRE) of the southern Appalachians has attracted nearly constant study for more than a century. Abruptly rising 300-500 m above the Piedmont lowlands, the crest of the escarpment generally coincides with the Eastern Continental Divide (ECD) and separates steep, eastward-flowing streams of the Atlantic basin from low-gradient streams of the Blue Ridge Upland draining west-northwest, ultimately to the Gulf of Mexico. Recent studies of exhumation patterns and erosion rates have confirmed inland erosional retreat of the BRE, but the mechanism, timing, and magnitude of retreat remain poorly understood. Numerous fluvial terraces preserved atop the ECD at the escarpment crest indicate the role of large stream capture events in producing inland divide migration and associated retreat of the BRE. Terraces are preserved in gaps and sags along the ECD/BRE crest and are frequently drained by clearly underfed Upland streams. Along-strike variation in weathering of the deposits suggests retreat is an ongoing but episodic process ultimately dependent upon the occasional capture of high-order Upland basins. Roundness of vein quartz or quartzite clasts indicates terrace material experienced 10’s of kilometers of transport from sources seaward of, and below, the present BRE. Many well-rounded clasts show little sign of weathering, implying rapid dissection of a recently captured basin led to their preservation atop the present ECD/BRE. Further investigation will attempt to constrain terrace sediment provenance and erosion rates in captured basins to better understand the contribution of local retreat events to the evolution of the BRE as a whole. The abundance of stranded fluvial debris atop such a mature feature as the BRE has implications for the use of upland surficial deposits in the study of younger and potentially better preserved passive margin escarpments.