SURFACE EXPOSURE DATING OF DEBRIS DEPOSITS IN THE VALLEY AND RIDGE PROVINCE, ROCKINGHAM COUNTY, VIRGINIA

Colluvial debris deposits are common on the flanks of quartz sandstone ridges in the Valley and Ridge Province of Virginia. These include talus, boulder field, debris-flow, and debris or alluvial fan deposits. Debris-flow and fan deposits are poorly sorted and chaotic, and contain quartz sandstone boulders, cobbles, and pebbles in a sandy loam matrix. Similar deposits occur on interfluves and are interpreted to be older fans. Absolute dating of debris deposits using cosmogenic nuclides can be challenging due to the potential for reworking. Deposits with very large boulders provide an opportunity to sample material that is less likely to be reworked. In northern Rockingham County on the west flank of Little North Mountain, approximately 4 km north of Brocks Gap, deposits contain very large boulders of quartz sandstone; some of these boulders are estimated to weigh more than 100 tons. Samples were collected from four boulders and analyzed to determine 10Be cosmogenic surface exposure ages. Two samples were collected from boulders in a first-order stream valley. One of these samples was from a colluvial deposit in the upper part of the valley and yielded an exposure age of 96.5 +/- 8.7 ky. A second sample from a possible debris-flow deposit in the lower part of the valley yielded an exposure age of 128.9 +/- 11.9 ky. Two additional samples were collected from fan deposits on interfluves. These samples yielded ages of 183.2 +/- 16.8 ky and 352.3 +/- 34 ky, respectively. The data suggest a significant period of denudation and landscape modification during the late Pleistocene in this area. More sampling is needed to determine if these ages are typical of debris deposits in other parts of the Valley and Ridge Province.