MODIFICATION OF SCALLOPS BY CONDENSATION CORROSION IN SNEDEGAR'S CAVE, WEST VIRGINIA, USA

Scallops are erosional markings dissolved in soluble rocks and are common in caves. Scallops typically have low-angle surfaces that ascend to crests and steeper descending surfaces in the direction of flow, thus indicating flow direction. Models of flow dynamics posit a “jet” flow separation at crests, a transition to turbulence, re-attachment at the next downstream crest, and re-circulating lee eddies. Dissolutional rates are highest on the ascending surfaces, resulting in a downstream propagation of scallops into the bedrock. In Snedegar's Cave (West Virginia, USA) scallop morphology in hydrologically active canyon passages is modified by abrasion from clastic sediments and by features produced by condensation corrosion. Condensation results from cooling of warm, moist, surface air, which enters along ceilings during the summer. Condensation rates are sufficient to promote a variety of dissolutional and depositional features including drop dents, rill trails, splash patches, and drip holes. When condensate evaporates, drip-hole rings form on mud floors and crusts form between drop dents and rill trails as calcite is precipitated. The pattern of drop dents and rill trails on scallops in the active stream passages indicates that there is often a balance between rates of formation of condensation features and rates of dissolutional corrosion. Scallops low on bedrock walls lack condensation corrosion/deposition features, due to more time exposed to vadose flows, while scallops higher on walls support drop dents and rill trails. In several areas, many scallops have drop dents and rill trails evident only on the descending parts of scallops within the lee eddies, where dissolution rates are lower than on the ascending surfaces.