Old Rag Mountain
By Dick Hall-Sizemore
It is time to take a break and talk about something entirely different related to Virginia — how the current physical structure of the Commonwealth in the 21st century came to be.
I have been taking an introductory geology course this fall from J. Sargeant Reynolds Community College. (Virtual, of course.) I became fascinated with geology many years ago and determined to take a course when I retired and had the time. My class project was a short paper and presentation on the geology of Shenandoah National Park.
So, to give your mind some relief from constantly checking your computer for any updates on the ongoing ballot counting in other states, here are some of the things I have learned about our mountains:
- For those Western snobs who dismiss the “small” Blue Ridge Mountains in favor of the “majestic” and “awesome” Rocky Mountains, there is this: By the time the Rockies were born or formed, the Blue Ridge had been born twice, weathering and eroding to sea level between “births”, and was well on its way toward its second weathering.
- The oldest rocks in Shenandoah National Park are 1.1 to 1.2 billion years old.
- The Blue Ridge, or more accurately, the Appalachians, of which the Blue Ridge is a part, were formed during the assembling of the ancient supercontinent Rodinia about 1.3 billion years ago. (Some geologists locate Rodinia around the South Pole.)
- The mountains formed during this event are thought to have been as high as the Himalayas.
- Deep in the earth beneath these mountains, was “basement rock” of granite and metamorphic rocks that later became part of the Appalachians, including Shenandoah National Park.
- When Rodinia began breaking up, about 400 million years later, those mountains had weathered and eroded to sea level, but the basement rock was still there.
- The breaking up of Rodinia was accompanied by a great outpouring of lava over the course of several million years. This lava, once cooled, was several hundred feet thick in places and covered Maryland, part of Pennsylvania, and the northern part of what is now Virginia.
- There followed a series of seas and oceans that deposited large amounts of sediment on top of the lava fields.
- Then there came a collision with another continental plate (the northern part of Africa) during the assemblage of another supercontinent (Panagea), about 300 million years ago.
- At the time of this collision, the granite, basalt (cooled lava), and sedimentary rocks formed from the accumulated sediment, were under what is now Richmond and further east. The pressure generated by the collision pushed those accumulations both west and up to their current locations.
- The mountains resulting from this collision (orogeny) are thought to have been as high as the Alps or perhaps the Himalayas. Over the past 200 million years, they have weathered and eroded to their current height.
- Unrelated to the formation of Shenandoah National Park, but interesting nevertheless, the Richmond region sits on a large formation of granite, called Petersburg granite, formed about 350-380 million years ago.
- In comparison, the Rocky Mountains are only about 80 million years old.
So, the next time you are hiking on Old Rag Mountain, realize that the rock around you is 1.1 billion years old. Sort of puts today’s current events into a different perspective.
For anyone, who wants to delve into this deeper, here are a couple of good websites (the information at the JMU site is more detailed):
http://csmgeo.csm.jmu.edu/geollab/vageol/vahist/