CAVE AND KARST EVOLUTION IN THE SWAGO CREEK AREA, WEST VIRGINIA (USA)

The Swago Creek area is a hilly region in east-central West Virginia. Cave and karst development is confined to limestones of the Mississippian Greenbrier Series, which is sandwiched between clastic rocks of the Mauch Chunk Group and the underlying Macrady Shale. Karst features evolved together with cave development. Many cave entrances are located in the uppermost massive limestone of the Greenbrier Series. These entrances, commonly vertical shafts, often represent recharge points to the aquifer. Streams sink when they flow off the clastic rocks and onto the limestones. Discharge occurs at springs (many constituting cave en­trances) at the base of the Greenbrier Series, above impermeable rocks of the Macrady Shale. Horizontal cave passages fall into two groups. Passages at elevations of 820-835m MSL are developed along joints at N60E. Passages at 730-850m MSL parallel surface drainage. The passages at 820-835m lie ~200m below an erosional surface (Davis’ Harrisburg Peneplain) and were developed below a stable water table at the time that surface was cut. Younger, lower passages parallel to surface drainage represent trunk conduits developed contemporaneously with surface drainage. A trunk passage in Overholt’s Blowing Cave parallels the axis of Dry Creek but is offset from it ~150 m to the west. This conduit may have developed phreatically beneath Dry Creek when it was a gaining stream. As the creek incised, it migrated to the east, and the water table fell below the level of the phreatic conduit. Incision essentially ceased when Dry Creek was captured by the conduit, which continues to carry the stream today. Elsewhere, subsurface flow patterns are more complex. Dye-tracing experiments conducted in the 1960’s demonstrate that subsurface drainage commonly crosses surface drainage divides. As the landscape has been lowered by regional downcutting, caves have been truncated by erosion and surface streams have been captured and rerouted. The hydrologic pattern of the area is dy­namic and interlinked with its geomorphic evolution.