Geology of Clover Hollow and surrounding area, Giles and Craig Counties, Virginia

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Virginia Polytechnic Institute and State University

Structures in the study area, from southeast to northwest, are the Blacksburg synclinorium, Poplar Hill anticline, Saltville fault, Spruce Run syncline, Clover Hollow anticline, and Johns Creek syncline. The folds are believed to have had their inception by early Middle Ordovician time. The Knox-Middle Ordovician unconformity is well expressed on the Poplar Hill and Clover Hollow anticlines. The contact between the Knox Group and Middle Ordovician is less well defined in the Spruce Run syncline. The character of the unconformity and depositional thickening of the Middle Ordovician limestones in the Spruce Run syncline were caused by compressional downwarping.

Comparison of the Middle Ordovician limestones of the Giles synclinorium with those of the Blacksburg and Salem synclinoria to the southeast suggests that the Giles synclinorium subsided before or faster than the southeastern synclinoria during early Middle Ordovician time.

Colluvium, which covers about 10 percent of the area, is the result of great mass-wasting which may have begun as early as Late Cretaceous time. Dominant contributors to the colluvium are the Silurian Tuscarora, Rose Hill, and Keefer sandstones. Northwest ridge slopes are extensively covered with colluvium as compared to southeast ridge slopes. Moisture content and temperature, as well as topography, appears to have a strong influence on colluvial development.