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. The metamorphism of pre-existing rock obscured evidence of the earlier geology of the region. 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. The unusual crystallization suggests that the rocks formed in a low-water crustal environment. In Roseland, between Charlottesville and Lynchburg, is a nine-mile-long intrusion of the 90 percent
plagioclase feldspar rock
anorthosite, the formation of which remains poorly understood by geologists. The Roseland area was a major source of titanium for many years. For 500 million years until the beginning of the
Phanerozoic terrestrial erosion wore down the Grenville rocks. Igneous activity resumed in the
Eocambrian 600 million years ago. Magma ascended through the crust, but stopped due to a high-density zone and through a process of fractional crystallization, cooled to form
basalt. Overlying rocks melted to form granitic magma and erupted as
rhyolite on the northern end of the Blue Ridge, intruding overlying granulite and charnockite. The Robertson River granite, with a 70-mile-long outcrop is a remnant of this
felsic melting. To the west of the Blue Ridge, the Chilhowee Group sandstones formed thick sequences, recording a beach environment in
Iapetus Ocean.
Paleozoic For 100 million years in the
Cambrian and
Ordovician, eastern North America was covered in a shallow tropical sea. In parallel with the proliferation of multi-cellular life, large carbonate deposits, including sequences of
limestone and
dolomite formed, together with sandstone deposits. The closing of the
Iapetus Ocean attached the Inner Piedmont belt and the Smith River
allochthon to the Virginia Piedmont. Sediments shed during the
Taconic orogeny covered the new terranes, as the trough filled during the
Silurian. The Tuscarora and Massanutten sandstones formed as a massive shoal during this period. 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. However, the Iapetus Ocean was still closing, adding additional small terranes to the region, including one which is likely concealed by Coastal Plain sediments. Mountains, uplifted during the
Acadian orogeny shed clays, which formed shale deposits during the Devonian in the trough. In the
Mississippian and
Pennsylvanian periods of the
Carboniferous, large tropical swamps formed in the west and were periodically buried with clay, to form coal deposits. By the time of the
Alleghanian orogeny 320 million years ago, the Iapetus Ocean was completely sealed. Tectonic forces formed the
Massanutten synclinorium and other small synclines and anticlines in the Valley and Ridge province. A thrust fault, forced the rocks of the Blue Ridge on top of the Shenandoah Valley. 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. Veins filled with quartz, calcite and pyrite.
Mesozoic (251-66 million years ago) 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. The oldest basins in Virginia date to the
Triassic and
Jurassic. The
Cretaceous rocks of the
Potomac Group cover these older sedimentary rocks. The Potomac Group is subdivided into the
Patapsco Formation and
Patuxent Formation, which comprise gravel
conglomerate, sandstones and sandy clay that often outcrops along rivers. In the Jurassic, volcanic activity in Blue Ridge formed isolated volcanoes, as well as dikes and sills in the Valley and Ridge province. Examples include Mole Hill, to the west of Harrisonburg, and Trimble Knob in Monterey. Natural Chimneys Regional Park preserves a thin basalt sill, near the base of pinnacles of limestone.
Cenozoic (66 million years ago-present) In the
Paleocene period of the
Cenozoic, high sea levels from the Cretaceous dropped, bringing an increase in terrestrial erosion. The sedimentary rocks of the
Pamunkey Group formed during a
marine transgression in the
Eocene. The component Aquia and Nanjemoy formations contains sand, clay, small
gypsum crystals, green
glauconite grains as well as fossils. 35.5 million years ago in the
Eocene, a
bolide (meteor) that impacted what is now
Eastern Shore of Virginia created the
Chesapeake Bay impact crater. Depositing a layer of
breccia, the crater had long term effects on later sediment deposits, shifting the location where the Chesapeake Bay would eventually form. Sea levels dropped again during the
Oligocene, only to return to widespread inundation during the
Miocene and
Pliocene, depositing the two cycles of the
Chesapeake Group. The Calvert, Choptank and Saint Marys formations are the lower group, deposited in the Miocene, while the Eastover and Yorktown formations deposited from the late Miocene into the Pliocene. The two phases of deposition are marked by sand, clay and
coquina. Cornwallis Cave is eroded into the cross-bedded coquinas of the Yorktown formation, along with the sea cliffs at Westmoreland State Park (which also exposes underlying Eastover and Choptank rocks). During the 1.8 million years of the
Pleistocene, sea levels rose and fell six times, dropping 400 feet during the last glacial maximum. The Chesapeake Bay was not present for the first half of the Pleistocene due to low sea levels. The Windsor and Charles City formations formed along an open coastline during this time. The surficial Shirley formation, beneath what is now Newport News and Hampton, deposited during the Middle Pleistocene, reaching the York and James River estuaries. The Tabb formation formed during the late Pleistocene. Estuary, beach and swamp deposits have become much more numerous in the last 11,000 years of the
Holocene. ==Hydrogeology==