STRATIGRAPHY AND STRUCTURE OF THE SINKS OF THE COWPASTURE RIVER, A MAJOR KARST FEATURE IN HIGHLAND COUNTY, VIRGINIA

In drier months, the entire flow of the Cowpasture River sinks in its bed along an ~500 m reach about 8 km upstream from its confluence with the Bullpasture River near Williamsville. The only water in this 8 km reach is then in slack pools of varying size and depth, or in short stretches of inflow from tributaries. In 2007 the riverbed was “refilled” between December 2 and December 13, after riparian vegetation had gone dormant at the end of the growing season, and subsequent to a late autumn snowfall.

Mapping of the Devonian exposures along and in the riverbed shows how stratigraphy and structure have controlled the location and development of this significant karst feature. Structurally, the Cowpasture flows along the northwestern limb of the Shenandoah Mountain syncline; this is also the southeastern limb of the Bullpasture Mountain anticline. Smaller folds are indicated by local changes in dip, from 75° SE to vertical to 45° NW. Stratigraphically, the river flows on shale and siltstone (Needmore, Millboro, and Brallier Formations) upriver from the sinks, and is a gaining stream all year. It becomes a losing stream just below an Oriskany Sandstone exposure where the river flows onto – and into – the Licking Creek (New Scotland) Limestone, the outcrop of which is buried by the river gravel. Below the sinks the riverbed swings south and east, again crossing the Devonian clastics, but with no primary streamflow. About 3 km below the sinks, a meander has cut into Bullpasture Mountain, and the Licking Creek Limestone, which has been deformed by a small kink fold, crops out along the bank. This is a secondary sink point, but in many months all primary flow is underground and any flowing water in this reach is only from tributaries. Below the meander 4 km to the Bullpasture River, bedrock is the Devonian clastics, and any streamflow is again only from tributaries.

The most likely resurgence of the Cowpasture is Coursey (Meadow) Spring, which supplies water to a state fish hatchery. If true, then the underground Cowpasture River must flow beneath the Bullpasture River, an intriguing hydrogeological oddity. A definitive trace has not yet been made to test this hypothesis by investigating the probable connection between the insurgence at the sinks and Coursey Spring.