Geology of the Harrisonburg and Bridgewater quadrangles, Virginia
The Harrisonburg and Bridgewater quadrangles comprise an area of approximately 117 square miles in Rockingham County in northwestern Virginia.This area is in the western part of the Shenandoah Valley, a portion of the Valley and Ridge physiographic province, and contains parts of three major geologic structures-the Staunton and North Mountain thrust faults and the Massanutten synclinorium.The roeks within the two quadrangles are folded and faulted limestones, dolomites, shales, and sandstones of Paleozoic age. These rocks have been divided into groups, formations, and members that include the Elbrook and Conococheague formations (Cambrian), the Stonehenge Formation, the Beekmantown Group, the New Market Limestone, and the Lineolnshire, Edinburg, and Martinsburg formations (Ordovieian), the Massanutten Sandstone and the Bloomsburg Formation (Silurian), and probably Upper Silurian and Iower and Middle Devonian limestones and shales that are not exposed but are judged to be on Massanutten Mountain. Triassic diabase dikes and Eocene volcanic pipes intrude the Paleozoic sequence locally. Much of the sequenee is eovered by alluvium or talus of Quaternary age. Selected strata of the Cambrian and Ordovician carbonate roek sequence are sources for road aggregate, agricultural lime, concrete aggregate, dimension stone, tettazzo, portland cement, flux stone, and pellet lime. Some of the carbonate rocks may be potential sources of lead and zinc minerals. Ordovician shale formations have potential for structural clay products.