Virginia's current topography reflects over a billion years of geologic evolution
Source: USGS Earth Resources Observation and Science (EROS) Center, National Elevation Dataset - Shaded Relief Map of Virginia
The short story: Africa collided with North America and pushed up the Appalachian Mountains. The Appalachians we see today are just the roots of older mountains - after 300 million years of erosion, the height of the old Appalachians is low compared to the younger Rockies, Andes or Alps.
But that is only part of the story. After the continents of Africa and North America merged and birthed the Appalachians, they divorced and went their separate ways.
at the start of the Cretaceous Period about 150 million years ago, the Atlantic Ocean was getting larger as the Mid-Atlantic Ridge grew, and North America (in yellow) moved further away from Europe/Africa
Source: US Geological Survey (USGS), Color-coded Continents!
After the Appalachian (or Alleghanian) orogeny, the mountains that had been pushed up by the Africa-America collision eroded. To the west, the sediments filled up the middle of the North American continent in layer after layer.
They permanently raised the bed of the continent's center, after numerous large seas had inundated it previously and deposited limestones, coal, shale, and sandstones, to form the bedrock of Indiana, Illinois, Missouri. The mountains also eroded to the east, depositing sediments on what today is Morocco and Mauritania.
The supercontinent that formed after the African collision, known as Pangea, was unstable. The combined continental plates known as Rodinia had been unable to stay together after the Grenville event; heat trapped under the crust forced the breakup of Pangea.
About 250 million years ago, the tectonic stresses pulled Africa and North America in different directions again. Pangea developed a series of parallel cracks, those cracks became valleys where the crust thinned. The parallel cracks filled with sediments eroding off the Appalachians, and are known today as Triassic Basins. In addition to sediments washing in from above, the thinning crust allowed basalt lava to erupt into the cracks from far below.
One particular crack in the crust of Pangea became the primary release point for the heat to escape. A rift valley with a "spreading center" formed, and the continents split apart along that seam.
lava emerging from the Mid-Atlantic Ridge has pushed Africa and North America apart - or have the tectonic plates moved from other causes, and lava simply rises to fill the gap?
Source: National Oceanic and Atmospheric Administration (NOAA), ETOPO1 Global Relief Model
This major rift continued to grow a few inches per century. Sediments continued to flow into this particular crack from the east and the west, but the rift valley deepened faster than erosion could fill it. The rift valley finally grew so wide that it forced the continents completely apart - and as the salt water flowed into the valley, the Atlantic Ocean was born.
Initially the Atlantic Ocean was only a narrow strip of salt water, but once again Virginia was a coastline state and no longer trapped in the center of a supercontinent. Where topographic relief was lower, such as at the mouths of rivers, mudflats/beaches were created, while some eroded sediments from the Appalachian Mountains settled in the ocean to create a new Continental Shelf.
sediments eroding off Virginia's mountains for 300 million years have created a Coastal Plain and Continental Shelf, east of the Piedmont
Source: National Oceanic and Atmospheric Administration (NOAA), Surface of the Earth (ETOPO2v2) 2 minute color relief images
Over time, several thousand miles of lava built up between the still-erupting rift (known now as the Mid-Atlantic Ridge) and those North American sediments.
The bottom of the Atlantic Ocean continues to grow as the North American plate - carrying Virginia with it - drifts westward away from Europe and Africa at 2-3 centimeters/year, as fast as hair and fingernails grow.
the Atlantic Ocean seafloor has grown, east and west of the Mid-Atlantic Ridge, for 250 million years since Pangea split apart
Source: Library of Congress, Manuscript painting of Heezen-Tharp "World ocean floor" map by Berann
On the eastern edge of the continent, along the new Atlantic Ocean, the sediments have created both a massive new Continental Shelf in the Atlantic Ocean and a Coastal Plain east of Interstate 95. Those 200 million years of sediments sit on top of the ancient, now-metamorphosed rock layers that were once chunks of continental crust and were compressed into the Piedmont in three orogenies, as the Iapetus Ocean closed.
Virginia's rivers have created a wide trail of eroded dirt as the continental plate drifts to the west, like a wedding dress trailing behind a bride going down the aisle. (Of course, not all Virginia rivers drain to the east. Some sediments went down the New River drainage to the Gulf of Mexico, helping to form the birdsfoot delta of the Mississippi River.)
the Coastal Plain is east of the Fall Line, where soft sediments cover the crystalline bedrock
Source: Geological Survey of Virginia, The Clay Deposits of the Virginia Coastal Plain, Map of the Virginia Coastal Plain (1906)
As the last range of mountains from the collision with Africa has eroded away, the old rocks underneath have been exposed after millions of years. Differential erosion has created the modern topography of Virginia. The Blue Ridge has been exhumed by erosion, exposed in its new location as softer rocks around it have eroded away faster than the basement gneiss and hard Catoctin greenstone.
Mount Rogers, with its hard volcanic rock laid down 600-700 million years ago when the supercontinent of the Grenville orogeny split up, has become the highest point in Virginia. The hard sandstones deposited after each orogeny, such as the Clinch Formation deposited at the end of the Taconic orogeny, have become the higher ridges of the Appalachian mountains.
Softer shales and limestones have eroded faster and formed the valleys to the west of the Blue Ridge, while more-resistant sandstones have formed ridges. The Shenandoah Valley formed because the Cambrian and Ordovician limestones have eroded away faster than the volcanic Blue Ridge to the east, or the quartz-rich North Mountain to the west.
In the middle of the Shenandoah Valley, some layers of quartz-rich rocks have not disappeared yet - but Massanutten Mountain is gradually eroding away. Its presence shows the relative resistance of sandstone, compared to the softer carbonate rocks to the west and in Page Valley to the east of Massanutten.
Spectacular mountains have been created and eroded away three times, and maybe more, in Virginia. In the Eocene Epoch, tectonic shifts caused a brief renewal of volcanic activity in what today are Rockingham and Highland counties. Trimble Knob and Mole Hill are remnant volcanic plugs from that time, 35-48 million years ago.
Eocene volcanics west of Harrisonburg and Staunton are evidence of the last volcanic activity in Virginia
Source: Virginia Department of Mines, Mineral, and Energy, Geology Of The Monterey Quadrangle, Virginia (Publication 178)
Trimble Knob is one exposure of volcanic intrusions (green polygons) in the Valley and Ridge physiographic province during the Eocene Epoch
Source: Virginia Department of Mines, Minerals and Energy, Geologic Map of the Virginia Portion of the Staunton 30 X 60-minute Quadrangle (Publication 163)
You can see the roots, and the ruins, of those events if you look at the rocks outside your car window and in your backyard. The modern rivers etching away at the current Virginia landscape are just a gentler, kinder version of the tremendous tectonic forces that have raised and destroyed mountains as large as the Alps in Virginia.
Virginia today is the result of a billion years of rock creation, uplift, erosion, and deposition - and much pushing and pulling
Source: Virginia Department of Mines, Minerals and Energy
Back to: The Orogeny Zones and Virginia Geology