26 August 2012

Paw Paw Tunnel GigaPans

Posted by Callan Bentley

My wife and I journeyed out to Paw Paw, West Virginia, yesterday, then crossed the Potomac to enter the southern edge of Maryland.

The purpose of our jaunt? Gigapannery! (That’s what I’ve got my sabbatical to do, of course) We walked along a short stretch of the Chesapeake and Ohio Canal’s tow path, and then entered the Paw Paw Tunnel, a tunnel 3/5 of a mile in length, lined with six million bricks, that takes the canal through a mountain. The tunnel took 14 years to build due to the unstable nature of the rock underlying the mountain (the Devonian Brallier Formation shale), and nearly bankrupted the Canal company. Why would they bother with a tunnel when the other 183.9 miles of the Canal are out in the open air? It has to do with the history of the Potomac River drainage: the Potomac is a rejuvenated river along this stretch, meaning it developed a meandering form, then cut down to lock that sinuous shape into a bedrock “canyon.” When building the Canal, the engineers got to Paw Paw and breathed a sigh of dismay. They were faced with a difficult choice: To get from point A to point B (see map below; contour interval 20 feet), they could either follow the looping shore of the Potomac River (which would mean a longer C&O Canal), or they could take a shortcut by tunneling through the mountain (which would be shorter but maybe a little more work).

So that’s why they decided to build the tunnel: it was shorter to get from point A to point B. Turns out, though, that it wasn’t all that easy.

Why was the tunnel such a hassle? First off, consider how the rock formed, and then deformed. It was originally deposited as layers of mud shed off the second Appalachian mountain-building event, the Acadian Orogeny (~360 Ma). As mountains rose to the east, they shed sediment which was transported west into an epeiric sea, the Kaskaskia Sea. There, they were deposited by turbidity currents. Later, once this mud had been lithified into shale, we would call it the Brallier Formation.

Later, during the ultimate phase of Appalachian mountain-building, the Alleghanian Orogeny (~300 to ~250 Ma), these Acadian deposits got folded and faulted. For the present discussion, consider only the folding. The layers crumpled up, partly by having each layer slip relative to the layers above and below it. The bedding planes became small “faults” in this scenario of flexural slip:

Because a layer could slide bodily relative to its neighboring stratum, slickensides developed on the surface of the beds, sometimes overprinting ripple marks, tool marks, and other primary sedimentary structures. Check out these lovely crystal fiber lineations with upward-facing steps:

So the layers slid relative to one another on slick surfaces, and then came to rest in a variety of orientations. Later incision by a stream (and then augmented by the C&O Canal company) carved the valley called Tunnel Hollow.

That caused a problem – as the strata on the two sides of the hollow now have very different susceptibilities to rock slide. One side of the valley has strata dipping into it, while the other side has the strata dipping away from it.

The side with the strata dipping into Tunnel Hollow is the problem: With nothing to support them from below, slabs can detach and slide downhill into the empty space below. Woe to the C&O Canal worker who was standing in the way!

With rocks like that – big slabs poised to come crashing down any time, no wonder it took the C&O Canal company so long to dig the tunnel. Modern recreationalists using the Canal tow path are somewhat safer, as pins have been drilled into the layers to lock them in place. You can see these giant “safety” pins in some of the images we took yesterday.

And now, without further ado: Here are the results of our trip, four explorable, annotate-able GigaPans…

The downstream end of the Paw Paw Tunnel (see if you can spot the anticline):

The upstream end of the Paw Paw Tunnel (see if you can spot ripple marks):
(…or the torso of a reading instructor from Montgomery College who happened by with her friend as the GigaPan was nearing its conclusion!)

Slickensides on a slab of Brallier Formation shale (see if you can find the lens cap I included for a sense of scale):

Slabs of rock that have slid down the face of tilted beds of Brallier Formation, thankfully before your ankles got in the way (see if you can spot the pins used to hold the big slabs of rock in place, high above you):