Posted by: Alan Pitts | December 10, 2011

Not Necessarily Geology Mountain Bike Guide: Part I

Welcome all, to the first installment of the Not Necessarily Geology Mountain Bike Guide.

The NNG Mountain Bike Guide is an idea that has been bouncing around in my head for a few years now. Basically, the concept unifies two aspects of my life that I’m really passionate about; geology and bicycles. I started thinking about this back when I lived in San Francisco, California. I was enjoying an extended summer break, employed as a professional (a term that is certainly debatable) bicycle tour guide. On my days off from “work” I liked to pedal across the Golden Gate Bridge into Marin County to explore some amazing geology and enjoy some world-class mountain biking. As a tool for exploring the area, I purchased a mountain bike guide, which contained a wealth of useful information leading me to outcrops and points of view inaccessible from roadways. During this time I also tried to read whatever I could find on Bay Area geology to familiarize myself with the complex regional picture. After spending a lot of time reading field guides in both geology and mountain biking, I thought I would like to combine the two concepts. So here it is.

The first in this hopefully long series of posts is from Conway Robinson State Park in Gainesville, Virginia.

This hidden gem of a park is literally a stone’s throw from Rt 66 in Northern Virginia and just over 30 miles from Washington DC. It is one of the few parks that allow mountain biking in the area. For me, this place has long been a wooded refuge from the sprawling suburban colossus that is Northern Virginia. Taking a ride at Conway Robinson is an easy way to escape all the traffic, McMansion town-homes and chain restaurants, which have become all too common around here. Or, if you’re the type who would like to get away from the present reality in space and time, if only for a few hours, you can do that too.

Take a visit to Virginia’s Jurassic Park.

It is pretty unlikely you will find any dinosaurs here (living or non), nothing like the depiction of the Jurassic Period made famous in the Classic 1990s film. Yes, there were dinosaurs on Earth at that time, and there is even good fossil record of dinosaurs nearby, just not in this park (that I know of). This post has nothing to do with dinosaurs because… well, personally I find the dinosaur inhabitation of Earth to be far less interesting than the tectonic history during that period. There, I said it.

Before we get into the mountain biking, here is some background on the tectonic history and regional geology of the area.

The following 3 images are from The Paleomap Project by Christopher R. Scotese. This is a great resource for visually exploring reconstructions of global Plate Tectonic history.

This first image is of the Earth during the early Triassic period, Pangea is seemingly still a happy marriage of the continents. The North American Plate is still in contact with the African Plate. However, you can see some shaded areas in the middle, which likely represent the beginnings of the end of this continental assemblage.

The next image, also from Scotese, is a reconstruction of the early Jurassic period. This looks pretty similar to the first image but actually a lot has changed, especially on the North American/African boundary.

In this image the spreading is now very visible. The northern end of the rifting zone is opening up like a zipper and has admitted seawater where the new Atlantic Ocean is forming. Africa and North America, now mostly separate from each other, are spreading away which will eventualy result in their present placement on the globe.

The last picture in the 3 photo sub series (also from Scotese) is the from the late Jurassic, 152 million years ago.

At this stage the Atlantic Ocean is established and on the way to becoming a major oceanic basin. The view at this stage in Earth’s history is beginning to look more like the Earth we know today. The continents are starting to take on their current shapes and there are more recognizable features, like the Atlantic and Pacific Oceans.

Now back to Virginia.

Conway Robinson is located in Virginia’s Culpeper Basin, a tectonically induced rift basin, which was activated during the early stages of the break up of Pangaea in the Triassic. It is basically a giant tear in the crust that failed to become the new Atlantic Ocean, and remained attached to the North American Plate. This basin, along with several others of similar age along the east coast of North America, filled with sediments and flood basalts.

I have previously written about a conglomerate from the Culpeper Basin, in which I described some local geology. Also, last year I wrote a rather cheesy post using pieces of my sandwich as a rudimentary analogue for the extensional strain which formed the Culpeper Basin. Though, one of these posts may (or may not) be good background reading for this, here are a few recycled images for those who did not read those posts.

This graphic is just a simple Google Map image of the area, which I annotated to mark the approximate location of the Culpeper Basin.

Here is a general cross-section of the Culpeper Basin by C.M Bailey of The College of William and Mary. The two units that are featured inside Conway are Jurassic Diabase, shown in red, and Triassic/Jurassic sediments, shown in green.

There is a lot of useful information in the above diagram. Illustrated are the structural boundaries of the Culpeper Basin, both of which are normal faults, creating a down-slumping block of crust. The action along these faults created space for sediments to be deposited (TJs). As spreading continued, eventually mafic feeder dikes (Jd) cut through the older sedimentary units delivering flood basalts to the surface.

Now here is a closer view of the park itself. The main trail is marked in red and follows the perimeter of the park.

I created this trail map using MyTracks, a wonderful and really useful app for my Droid smart phone. I just started the app, which tracks GPS location, tossed my phone in my bag and rode the trail. The app does the hard work calculating the moving statistics, all I had to do was enjoy myself and look out for rocks!

The trail marked above is actually the longest possible loop around the edge, which is probably several trails and there are several more trails which cross the middle of the park. I plan on improving this map to include more specific trial info and updating this post. Just as soon as I get more time to ride and record more trail data. There are nearly 7 miles of riding within the park. So there is a good amount of area to explore, but not enough to constitute any real danger of getting lost. As you can see the park is basically encased by highways, on ramps, off ramps, neighborhoods and golf courses. You can basically experience the joy of getting lost (and found) several times in one afternoon.

As far as the difficulty of the mountain biking, it’s mostly moderate with a few difficult single track sections near the backside of the park. One of the things I’ve come to learn about the Culpeper Basin (also visible in the “outline of the Culpeper Basin” graphic) is that it is pretty darn flat, not a lot vertical relief. So there are not many sustained climbs or long miles of technical downhill.  Dont worry though, there are lots of smaller hills to make the ride fun and challenging enough for those looking for good workout.

As far as the geology goes, it’s pretty great.

Here is the same trail map image from above, which I imported into Google Earth, paired with the USGS Virginia State geologic map overlay (available here) and added some additional annotations to indicate the bedrock geology.

You can see in the above image, the park is underlain by two general rock types. The red is the Jurassic Diabase and the light green is Triassic shale and siltstone. Most of this is covered by vegetation and soil, but there is one really good outcrop of Jurassic Diabase near the “backside” of the park marked on the above map.

Here are a few pictures of the diabase outcrops.

Some of these exposures appear very columnar to me, like the one above and the first picture featured in the post. If they are in fact columns, then they owe their existence to the cooling of this once molten rock. Columns form (mainly) in igneous rocks (usually basalts) as contraction fractures. As the rock cools it shrinks and fractures in order to accommodate the volumetric change.

Here is a link to some other pretty world-famous columns I saw last summer in Ireland at the Giant’s Causeway.  Also here is a post by my former Structural Geology professor Callan Bentley, about some great columns in meta basalt in Shenandoah National Park.

This next picture is an up close look at the vertical faces of these “columns”. Notice the horizontal fractures perpendicular to the vertical joint faces. Or maybe these are arrest lines, I’m not sure really.

Here is an annotated version of the first diabase photo illustrating the relationship between the vertical joint faces of the column and the horizontal fractures.

Here is a look at a one of the fractures, notice the zig-zag shape of the fracture and the overwhelming lichen coverage.

And here is a look at a fresh surface. You can see the “salt and pepper” look to the rock. The pepper being black clinopyroxene and the salt, white plagioclase feldspars.

At several points the trail crosses obstacles which represent the meeting between geologic consideration and mountain bike trial description. A contact, if you will in both a metaphorical and geologic sense.

Here is what I’m talking about. While most of the riding is pretty tame, at points the trail crosses what local mountain bikers call “rock gardens” ranging from 10-50 feet in width.

These rock gardens are made of rounded and weathered pieces of the Jurassic Diabase and outcrop as linear features cutting across sedimentary units. However, I imagine they have a more tabular nature in the subsurface. I think these could be the base of feeder dike systems that supplied the basalt flows on the surface. Or another idea is that they indicate the geometry of the igneous body and how its shape influences the exposure. I guess I’d have to get off my bike and map these features in greater detail to get closer to the answer.

On the bike this can actually be a pretty treacherous crossing,  plenty of opportunities to scrape your chainring or bash your foot into rocks. The best way to handle these is; try to pick a line on the approach, maintain decent speed, and keep your weight centered on the bike, you should cruise right through. Or you can just get off and walk for a bit.

I took the two photographs above a few years ago with my long-lost favorite bike. Don’t let the drop handle bars and “painter’s blue” bar tap fool you, that is no road bike and certainly not a work in progress. That, my friends, is a Surly Cross Check, and pretty much an indestructible swiss army axe of bikes. The Cross Check is perfectly suited for both road and trail, although not ideal for serious single track. Unfortunately this fine machine was stolen from me two summers ago. The culprit is still at large.

One more picture to finish this up. I was lucky enough to meet this little fellow on one of my trips who I thought was worth featuring here.

Additional Links:

The Virginia Department of Forestry page on Conway Robinson

Mountain Bike Trail Reviews for Conway Robinson


Responses

  1. Whoa! Great pics and maps. Let’s go riding!

  2. Dude, we need to fashion a rock hammer ‘holster’ out of a water bottle cage and market it for biking geologists…

    • Haha. Yes, I like it! And perhaps a handle bar mount for a Brunton.


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