The Orogeny Zones and Virginia Geology

In that Iapetus Ocean, zillions of tiny sea creatures with tiny shells of limestone lived and died. After they died, their shells drifted to the ocean bottom and formed an ooze of limey mud. More and more sediments of shells accumulated, covered by an occasional layer of mud and sand after major storms onshore created spectacular erosion. The sedimentary layers compacted and the tiny individual shells merged into thick beds of limestone (calcium carbonate) and dolomite (with magnesium). If you go to the Bahamas and the Carribean today, such as Andros Island, you can the same process happening today, forming a "carbonate bank." Or you can keep adding spoon after spoon of sugar to your Cheerios, and see the grains accumulate on the bottom of the bowl.

The shoreline of Virginia sank, and all of the state went underwater. As much as 25,000 feet of limestone sediments accumulated on top of Virginia in the early days of the Paleozoic Era (the Cambrian and Ordivician periods). Virginia - and most of the United States west to California - was covered by a shallow ocean sea. The basalts and Grenville rocks were pressed down and metamorphosed, creating at this time the mineral "epidote" that gives the Catoctin basalts a particular greenish color.

So far, so good? You've seen the Cheerios float together, mingle with Rice Krispies, and drift apart, leaving the Blue Ridge mountains as the Virginia shoreline way-y-y-y back when. You've seen sugar accumulate on the bottom of the bowl, piling up the limestone on the bottom. And you've seen an eruption of basalt/milk coat the cereal/Blue Ridge, and then the while clump of Cheerios submerged completely under the milk as layers of sugar accumulate on them.

In other words, by 600 million years ago, the basement rocks of Virginia (what you find if you keep digging down, down, down...) had been formed in a continental collision (or collisions) known as the Grenville Event. When that continent had rifted apart, volcanic rocks had covered much of the surface that would become the Blue Ridge, and then everything was buried in lime-rich sediments underneath the Iapetus Ocean.

Now comes the good part, when the milk stirs again.

For whatever reason, the continents reversed direction. Africa and North America started to move together again, and the Iapetus Ocean began to close. The bottom of the ocean buckled, and one continental plate slid underneath the other. (That's how the Cheerios and Rice Krispies actually merge together.)

Before the Iapetus Ocean closes completely and Africa slides up on top of Virginia and the rest of eastern North America, there will be three collisions that form mountains: the Taconic, Acadian, and Appalachian orogenies.

There were volcanic island chains in the Iapetus Ocean, equivalent to Japan or the Aleutian Islands today off the coast of Alaska. These islands were "rooted" to the ocean floor. When the Iapetus Ocean began to close and the ocean crust began to move towards North America, the islands were carried towards the continent too. On the ocean floor between the islands and the continent, new sediments fell along with the limestone skeletons - volcanic ash, blown in from the approaching islands on the tradewinds. Then mud, sand, and volcanic debris from the islands took the place of the lime-rich sediments. As the ocean closed, the depth and chemistry of the water changed.

Finally, the first part of the ocean seafloor closed completely when an island arc collided or "docked" with the continent. The scrunching together of the island arc with the continental margin formed mountains as high as the Himalayas, in a mountain-building event known as the Taconic orogeny. Try squeezing a balloon between your hands, and you can see it rise up higher and higher as the distance shrinks between the hands. That's what happened to the earth's crust - it rose higher into mountains, as the width occupied by the crust got smaller and smaller.

The mountains, roughly where Chincoteague and Virginia Beach are located today, may have been 25,000 feet high and covered with glaciers. In the vegetation-free Ordovician period 450 million years ago, the erosion would have been immense. Sediments poured from the mountains towards the west, until the mountains were flattened. Thick layers of sedimentary rock stretched to Chicago, as 25,000 feet of rock eroded to near sea level. As the mountains washed away and became lower and lower, the energy to move the rock lessened. The width of the sediments being deposited thinned, and the rock layers stretched less distance from the mountain range itself.

The sediments were capped by a final layer of sand, when most of the relief was gone and just the hard-to-dissolve quartz remained. The layer concluding the Taconic orogeny is known today (depending upon your location) as the Tuscarora, Clinch, or Massanutten sandstone. After being buried by other sediments, it is exposed on the surface again and is visible today as the slow-to-erode rock that forms many of the Appalachians ridges, including the eastern and western ridges of Massanutten Mountain.

Virtually the last erosion from the island arc was iron-rich sediments deposited in the Silurian period. These sediments did not form thick layers; they were created when there was little relief and the high-energy, powerful rivers of the Ordivician period had been replaced by mild, meandering streams. The Silurian sediments deposited in patches by these streams were the sources for much of the Virginia iron industry in the 1800's.

At the same time the Silurian iron deposits were eroding from the east, most of the area now west of the Blue Ridge was a shallow sea. In it, more carbonate (limestone) sediments were deposited. In addition, there were a few zones where the water evaporated and salt was also deposited. In the colonial era, before transportation of bulk commodities became relatively easy, these salt deposits were highly valued by pioneers and settlers in the mountains - and by the Confederacy in the Civil War, for that matter. Before the railroad came to town, Roanoke was known as "Big Lick." Saltville relied upon its salt and gypsum deposits as the primary source of employment for over a century, until Tennessee's complaint about salt pollution affecting the drinking water of the Holston River finally brought a factory closure.

The Silurian was the calm before the storm. A second island arc docked in what is called the Acadian orogeny. The rocks of Virginia record a second transition, repeating the sequence. The oldest rocks in the sequence are low-energy carbonates deposited in shallow seas (or on a continental shelf). On top of them, as the island arc approaches, are deposited black shales and volcanics. Finally, thick wedges of sediments erode from the mountain, with a final layer of relatively-pure sandstone at the end after everything else has washed away and the mountain is becoming a flatland. The Tuscarora/Clinch/Massanutten sandstone ended the Taconic orogeny, and the Oriskany sandstone ended the Acadian orogeny.

The Acadian, like the Taconic, widened the North American continent. The "terranes" or island arcs that docked onto the shores of Virginia extended North America east of the Grenville-age Blue Ridge, as well as increased the depth of the younger sediments that eroded to the west.

But the last collision was different. When Africa finally slammed into North America, it rode up on top of the continental edge. It's possible the African plate slid inland and reached as far as the Allegheny Front - nearly the western boundary of Virginia. In the process, it squeezed Virginia from the side. The pressure from Africa caused the rocks on the eastern side of Virginia to break free from their roots. The eastern rocks were folded and thrust, like a stack of slick Sunday newspaper inserts sliding off a table, 40 miles or so to the west. Sediments that were once east of Richmond ended up at Charlottesville.

Picture a spectacular car crash in a Hollywood movie. A car bangs into another, and they both stop. But then a second car with a high bumper rams them and slides up onto the trunk. Other cars and trucks come by, and ride over top of the smashed cars. In the movie, as everyone scatters and the gas tanks begin exploding, a tractor trailer might roll in and climb halfway up the pile before pushing over the cars at the top. Forget the gasoline explosions, speed up the film so millions of years are compressed into a minute, and replace the layers of cars with layers of rocks.... and you have the Appalachian orogeny, when Africa came calling.

In addition to breaking and thrusting the rocks on the coastline, the energy of the collision with Africa wrinkled the layers of sediment to the west as well. Those limestones and shales and sandstones from the Taconic and Acadian erosion cycles were folded, crumpled, and (close to the shoreline) broken. In some cases the flat-lying layers of sediments cracked and lifted slightly, and then the eastern portion slid overtop the western part of the same layers. This doubled the thickness of the sediment layers and shrank the width of Virginia, while absorbing the energy of the collision.

And of course the African collision produced its own mountain range on top of all the Virginia rocks left behind by the Grenville event, the Taconic orogeny, and the Acadian orogeny.. [Surely you have figured out by now that the solid earth is anything but static, and that "native Virginia soil" came from rocks that are constantly moving around the continent...] Once again, all the drainages were rearranged by a new series of mountains. Only one river, known ironically as the New River, seems to have had the ability to continue to continue in its old channel, cutting across the rising Appalachian Mountains as the African continent wrinkled the surface into new ridges and valleys.

On to: The Rest of the Story About Virginia Geology
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