Historic spark plug dating
As a result, rock formations from the Cambrian through the Silurian are stacked twice in the Shenandoah Valley.Sandstones remained intact, but some shales were metamorphosed to phyllite during the Alleghanian orogeny. In the early Mesozoic, the rifting apart of the supercontinent Pangaea to form the Atlantic Ocean, created large rift basins, which filled with sediments—ultimately forming the Atlantic Coastal Plain.Geologically recent erosion of Paleozoic carbonates forms the many gorges in the Valley and Ridge province, as well as caves.During the late Silurian into the early Devonian, the region was relatively quiet for 40 million years, with little activity besides the deposition of some carbonate banks.The oldest basins in Virginia date to the Triassic and Jurassic.The Cretaceous rocks of the Potomac Group cover these older sedimentary rocks.
The closing of the Iapetus Ocean, to form the supercontinent Pangaea added additional small landmasses, some of which are now hidden beneath thick Atlantic Coastal Plain sediments.The region subsequently experienced the rifting open of the Atlantic Ocean in the Mesozoic, the development of the Coastal Plain, isolated volcanism and a series of marine transgressions that flooded much of the area.Virginia has extensive coal, deposits of oil and natural gas, as well as deposits of other minerals and metals, including vermiculite, kyanite and uranium.Geologists believe that older pre-Grenville rocks were granulite gneiss, interbedded with volcanic and sedimentary rocks, dating back to 1.8 billion years ago.Igneous charnockite, with large amounts of blue quartz, rocks also formed during the Grenville event.
In the Proterozoic, the Blue Ridge region formed between the Roanoke River and Potomac River during the Grenville orogeny 1.2 to one billion years ago.