STRUCTURE AND GEOMORPHOLOGY AT THE BLUE RIDGE-VALLEY AND RIDGE BOUNDARY NEAR BIG LEVELS, VIRGINIA

Geologic mapping on the Big Levels 7.5-minute quadrangle in central Virginia was completed through the STATEMAP and FEDMAP components of the National Cooperative Geologic Mapping Program. Big Levels, a prominent topographic feature in the western Blue Ridge, is underlain by gently-dipping Antietam Quartzite. Regionally to the southwest and northeast, the Blue Ridge–Valley and Ridge boundary is a major northeast-striking reverse fault. Approximately four km north of Big Levels, there is no evidence for a significant reverse fault. Bedding strikes generally east-west, dips gently north, and has broad north- to northeast-trending folds. The Blue Ridge-Valley and Ridge contact here appears to be situated between two offset segments of the Blue Ridge Fault System. At least four generations of alluvial deposits obscure the nature of the contact as well as the overlying Shady Dolomite and Waynesboro Formation. The youngest alluvial deposits are within or adjacent to active stream valleys. The upstream portions of some deposits probably resulted from debris-flows, based on material size and sorting. One modern debris flow was recognized and a peat deposit was observed beneath a similar deposit, suggesting multiple events over an extended period of time. Intermediate-age deposits include well-developed alluvial fans that extend for several kilometers into the Shenandoah Valley. Some alluvial fan deposits are moderately incised and modified by karst. Older deposits are elevated and dissected remnants. Two samples of deeply weathered fan material exposed in the Acres sand pit were collected at depths of ~15-20 meters below the surface, and dated using the cosmogenic 26Al/10Be burial decay method. The samples yielded ages of 6.9 and 7.9 Ma. It is not yet clear if these dates are from a deeper part of an old but continually active fan deposit, which would suggest an extremely long-lived fan system, or a much older surface that is unconformably overlain by a younger generation of fans.