Graywacke

By William T. Hathaway

Graywacke

Earth science students on some of our field trips have been particularly amazed at the geological information concerning graywacke (also spelled “greywacke”). It seems that clay minerals are the primary cement that binds sand-sized pebbles of feldspar and quartz along with fragments (indicated by yellow arrows in the photo above) of igneous, metamorphic, and sedimentary rock.

Geologists believe that graywacke formed when underwater landslides occurred, releasing sediment that haphazardly settles before sorting. Since our specimen was photographed at a quarry site northeast of Renan, my guess is that the formation was at the edge of the Triassic basin, the hardened sediments from which now cut across Pittsylvania County from southwest to northeast.


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