Subsurface Geology


 
 
Subsurface Geology
 
While the surface geology described for each distinct geophysical province is quite unique, the underlying rocks reveal much more detail about the history of the area.The rocks of Virginia have been compressed, folded, thrust faulted, and telescoped so that most been moved from their site of origin and stacked like a shuffled and bent deck of cards. The exceptions are the Allegheny Plateau and the Coastal Plain. To understand the geology of Virginia, one must understand that the rocks of the area were different when first deposited when compared with how they look today. 
     
For example, the Grenville igneous and metamorphic rocks now exposed in the Blue Ridge and underlying most of eastern North America, buried under the sedimentary rocks, were originally found somewhere to the east, perhaps as far as Richmond. They were transported to their present location during the Alleghenian Orogeny along a major thrust fault (Cross Section). Indeed most of the rocks in the Blue Ridge and Piedmont have been transported far from their original locations. Furthermore, most of the Piedmont rocks were not originally part of North America but were added in separate pieces during the Paleozoic, and each of these pieces has its own tectonic history that may or may not correspond with the timing of events in the rest of Virginia.  The lower and middle Paleozoic sedimentary rocks of the Valley and Ridge do not stop at the Blue Ridge; they extend far to the east under the Blue Ridge and Piedmont (Cross Section). To put it another way, the Piedmont and Blue Ridge have been thrust faulted over the Cambrian-Ordovician DCM (divergent continental margin) sediments.
 
Contributed by Lynn Fichter 
Friday, July 18, 2014
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