WIND-POLISHED ROCKS ON APPALACHIAN RIDGE TOPS

Distinctive smooth, shiny rock surfaces on several Appalachian ridge tops in Virginia and West Virginia are believed to have been polished by blowing sand during the last glacial maximum when ridges lacked vegetative cover. Ridge top rock exposures are not polished now, probably due to weathering after polishing ceased. Most important, polish is preserved only where protected from weathering: on the underside of overhanging rocks, on the underside of fallen blocks on talus slopes, or quickly covered by vegetation. This may explain why wind polish has not been recognized before. So far convincing polish has been found only on well lithified quartzose sandstones (e.g., Oriskany, Tuskarora), consistent with published reports that it is best for attaining wind polish. Other lithologies have suggestions of polish but never equal the hard sandstones. Perhaps they do not acquire fine polish or more readily lose it because no other common mineral rivals the chemical and physical durability of quartz. The most convincing evidence is adjacent sand grains cleanly beveled off at the same level on a gently undulating shiny surface. Slip-fractures can be shiny but unlike wind polish have striations and planar rather than undulating surfaces. Joints and bedding planes can be quite smooth but lack grains beveled to the same level or have authigenic crystal faces on individual quartz crystals. Perhaps the greatest difficulty is certainly of polish on surfaces which may have been polished but have experienced some weathering: this becomes quite subjective. No indications of wind direction have been found so far: this would be important because known localities of polish are at least 150km south of the ice margin, probably too far for adiabatic winds draining off the ice hence are more likely related to regional winds, except perhaps closer to the ice margin or beyond as the ice retreated. But where is the sand that was the polishing agent? Was it all removed, concealed under vegetation, or not yet discovered? Less likely, ice/granular snow was the polishing agent and left no trace. Interpretation of climatic and geologic implications must await details of distribution and modes of occurrence. If some ridges have polish it is probably present on many others, so a widespread search for wind polish is strongly encouraged.