April 16, 2018

Cove Mountain looks like a “2,” even to a structural geologist…but why?

Posted by larryohanlon

An iconic southwest Virginia hiking destination only gets more interesting if you zoom out a bit…

By Philip S. Prince, Virginia Division of Geology and Mineral Resources

The interestingly-shaped Cove Mountain. Not something you see everyday, even if you look at a lot of maps.

The Appalachian Trail crosses a number of significant landmarks in the Valley and Ridge of southwest Virginia, but The Dragon’s Tooth rock formation on Cove Mountain is arguably the most popular. A hike to the “Tooth” makes it easy to see why. The views are exceptional, the trail is easily accessible to Roanoke and Blacksburg, the hike is fun and challenging (it includes iron rung ladders), and the rock formations at the summit of Cove Mountain are as dramatic as any in the region. The jagged white Silurian-age sandstone forming the “Tooth” is visible from the valley below, offering a taste of what is to come on the drive to the hike and a chance to have a “we were there” moment on the way home. These features have allowed the Dragon’s Tooth to rival and possibly surpass the popularity of nearby McAfee Knob, an equally photogenic location whose approach hike is not quite so exciting.

How Virginia wants you to see the Dragon’s Tooth feature on Cove Mountain. View is to the southwest, looking towards Newport Road, Catawba Mountain, and Fort Lewis Mountain. (Image source: https://pressroom.virginia.org/2017/06/21-places-that-will-make-you-fall-in-love-with-virginias-outdoors/)

While the rock formations at the summit of Cove Mountain are very interesting, the shape of Cove Mountain itself is equally intriguing to anyone who happens to see it on a digital topographic map of the appropriate scale. Cove Mountain is shaped exactly like a neatly drawn “2.”

Trying to make a point…AR Berkley font seems to match well. The bottom edge of the Cove Mountain “2” is 1.5 miles (2.5 km) across.

While the “2” shape of Cove Mountain initially comes across as a geologic curiosity, it actually has a very direct and rational explanation and provides a great jumping-off point for understanding why the mountains of the Appalachian Valley and Ridge look like they do. Seeing Cove Mountain as an expression of geologic process and not a miles-wide numeral requires a look at structural geology, the different sedimentary rocks of the Valley and Ridge, and how weathering and erosion in the humid-temperate Appalachian climate has affected these rocks differently. It sounds like a lot of stuff to think about at once, but it can all make sense through a combination of field mapping, geometry, and analysis of the beautiful digital maps available in 2018.

The Appalachian Trail follows the outline of the “2,” providing great views and stoking the appetites of northbound thru-hikers headed to Daleville. A spur trail connects the parking area to the AT at Lost Spectacles Gap, just northeast of The Tooth (but still well below it!). The parking area is about 1,500 ft (~450 m) below The Tooth.

This post will stretch out for a few installments to avoid geo-overload, so the first chapter will finish here with a look at an existing large-scale geologic map of the Cove Mountain area. In the southern and central Appalachians, bedrock geology is the main control over large-scale topographic patterns, and understanding why any particular mountain is shaped like it is (or in many cases, as tall as it is!) requires a look at what rocks are exposed on and around the mountain. The geologic map below is a Google Earth overlay produced by the USGS from the Virginia Division of Mineral Resource’s 1993 Geologic Map of Virginia, compiled at 1:500,000 scale and revised and digitized in 2003 (https://mrdata.usgs.gov/geology/state/state.php?state=VA).

The “2” pattern is apparent in Ordovician and Silurian outcrop patterns, but it is not present throughout the Cove Mountain Block. The Cove Mountain Block is bounded by the Cove Mountain Fault and the Catawba Branch Fault. Based on rock ages and the outcrop patterns, a clear folding pattern can be identified…

While not as clear as in digital topography, the “2” pattern is still visible in the outcrop pattern of the Ordovician and Silurian units. Note that this interval of rock is the only one exposed in the “2” shape; older and younger units follow different patterns. Outcrop pattern is indeed the key to the formation of the Cove Mountain “2;” like the rest of the Valley and Ridge, Cove Mountain itself is supported by a specific (and surprisingly thin) sequence of Silurian and lowermost Devonian rocks within a much thicker sedimentary section. Bedrock geology is therefore the ultimate explanation of the pattern, but understanding exactly how the appropriate layers were deformed into the necessary shape prior to being etched into topographic prominence will take a bit more illustration.

The next post will tackle the structure of the Cove Mountain Block, which has been the focus of recent 1:24,000 scale mapping by the Virginia Division of Geology and Mineral Resources. Based on rock ages in the map image above, can you put together a basic idea of what’s going on here?

This post was originally published on The Geo Models blog.