The characteristics and timing of multiphase major landslides along the Blue Ridge Escarpment of the southern Appalachians revealed by combined cosmogenic nuclide dating and Schmidt hammer rebound measurements

The Blue Ridge Escarpment (BRE) in the southern Appalachians of the USA is one of Earth's great passive margin escarpments. The escarpment is characterized by steep slopes and high relief separating the eastern USA's Piedmont and Blue Ridge physiographic provinces. Major rock avalanches, rockslides, and debris slides have led to impressive mass movement deposits along the escarpment's base, including boulders up to ~20 m in diameter. The characteristics and timing of the formation of these landslides have not been previously determined. A combination of mapping landslides and 10 Be terrestrial cosmogenic nuclide surface exposure dating of landslide boulders within the Pacolet River and Hickory Nut Gorge regions along the southern Blue Ridge Escarpment (BRE) in North Carolina provides insights into their emplacement processes and timing, ranging from 4.7 ± 0.5 to 89 ± 2.2 ka. Schmidt hammer sampling was used to identify the difference in boulder weathering as a function of the boulders' strength, as measured by their rebound values (R-values). The spatial distribution of R-values was used to detect rockfall clusters within individual deposits, revealing their episodic nature of accumulation. A weak correlation between age and elevation of the boulders further suggests that numerous catastrophic events occurred at intervals of thousands of years, at ~4.7–10.2, 19.2, 35.9–39.4, 64.2–68.8, and 83.7–89.0 ka. The development of Schmidt hammer calibration curves for quantitative dating was inhibited by the weak correlation between exposure ages and R-values, possibly due to variability in gneiss composition. Several mechanisms, such as periglacial conditions with alpine permafrost (including freeze-thaw effects and frost cracking), rainfall, or earthquakes, may have triggered mass-movement activity during the Late Quaternary. Understanding the timing and nature of these geomorphic processes enables the estimation of landscape development in passive margin escarpments. • Mass movement deposits (60,000 to >200,000 m 3 ) were investigated at four sites along the Appalachian Blue Ridge Escarpment • Collectively, they record five phases of landslide activity at approximately 4.7–10.2, 19.2, 35.9–39.4, 64.2–68.8, and 83.7–89.0 ka • Schmidt hammer methods detect rockfall clusters within individual deposits and reveal episodic accumulation • During the Late Pleistocene, periglacial processes, rainfall, and earthquakes could have triggered mass-movement activity • The timing and nature of these geomorphic processes provide quantitative data on passive margin escarpments