Rates and patterns of late Cenozoic denudation in the northernmost Atlantic Coastal Plain and Piedmont

Topography of a 2800 km2 section of the U.S. Atlantic Coastal Plain and Piedmont is reconstructed at five times from the late Miocene to the Holocene. The reconstructions are based on mapping of fluvial, colluvial, and marginal-marine deposits that are dated by radiocarbon, palynostratigraphy, and correlation to adjacent glacial and marine units. Denudation rates and patterns are calculated by subtracting successive topographies, with 60 m horizontal resolution and 6 m vertical resolution. The overall late Miocene to Holocene denudation rate of 10 m/m.y. is similar to other estimates of long-term denudation in the region. The overall rate resolves spatially into a wide range of local rates, varying from a minimum of ???0.2 m/m.y. on residual uplands to as high as 600 m/m.y. in glacially rerouted stream channels. The distribution of local rates shows that denudation is accomplished by stream incision in response to long-term glacio-eustatic decline, followed by slope retreat into residual uplands from the incised channels. Slope retreat replaces the uplands with pediments and straths. Pediment landforms and sediments, and observation of modern scarps and valley heads, indicate that groundwater seepage from the base of residual uplands is an important mechanism of slope retreat. This process is self-limiting because as the volume of residual upland is reduced, seepage diminishes and slope retreat slows until small residuals undergo little or no further reduction. Thus, relict topography is preserved even as most of the landscape actively erodes.