SEDIMENTARY CHARACTERISTICS OF LATE PLEISTOCENE PERIGLACIAL STRATIFIED-SLOPE DEPOSITS IN THE BLUE RIDGE OF CENTRAL VIRGINIA

Stratified-slope deposits were examined at three sites on the eastern side of the Blue Ridge in Shenandoah National Park. The exposures were located on the same drainage at (site 1) an elevation of about 3500 feet (1065 m), (site 2) at about 2300 feet (700 m), and (site 3) at about 1200 feet (365 m). There is no age control for site 1, but radiocarbon dates for site 2 (45 and 22 Ka) and site 3 (25 and 19 Ka) indicate some overlap in age. The sedimentary features in these outcrops indicate the dominance of mass-wasting due to ground ice (solifluction) during the last glacial period. The data indicate that stratified-slope deposits include more detailed climatic information than previously recognized.

At site 1, pebble- to boulder-sized clasts are mostly oriented horizontally and float in a muddy matrix. The largest clasts form clusters with vertical orientation. This style of deposit resembles published cross-sections of modern solifluction lobes. At site 3, the deposits are better stratified with rhythmic alternations of matrix-supported cobbles and pebbles and sand layers. These deposits are similar to the classic "greze litee" that is also attributed to solifluction. Unlike the classic deposits, the site 3 deposits also have 30-40-cm thick sandy foresets with thin continuous clay drapes.

The site 2 deposits are dominated by wedges of matrix-supported pebbles and cobbles with thin sandy partings. This produces a crude cross-stratification 20-50 cm thick. Pebbles and cobbles are oriented parallel to wedge margins, and coarsen to vertically oriented boulders towards wedge toes. Some layers are sand rich with remnant clusters of wedge fronts that have laminated clay lenses 2-30 cm thick on the upslope side. The variations in fabric suggest that solifluction was interrupted by periods of sheet flooding that produced shallow ponds.

Variations in sedimentary features suggest changes in temperature. Colder conditions produce a predominance of ice creep, whereas warmer conditions are characterized by alternate freez-thaw conditions and sheet wash. The spatial variations of different fabrics may reflect both differences in altitude and temporal variations in temperature. More data are needed to distinguish the influence of other variables such as provenance, and to define the random spatial and vertical variability.