Geologists Map Ancient Rocks in Blue Ridge Tunnel

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Sandstone interlayered with greenstone formed the boudins, named after the French word for sausage. Photo: Malcolm Andrews.

We know a lot about the social and economic history of the Blue Ridge Tunnel and its part in the larger history of the Valley, but there’s a hidden story, too. The tunnel’s ancient rocks have kept their own secrets for many centuries, and one of the state’s top geologists and his students are now sharing them. In fact, Chuck Bailey of the William and Mary Geology Department had crawled through the narrow pipe that formed the tunnel’s only access a number of times before the tunnel was opened up for restoration. Bailey has returned several times and, with a number of geology students and graduates, led a hike through the tunnel recently, pointing out the features that explain the history of some of the oldest rocks on earth. 

It’s not an orderly chronological story, though, Bailey said. Rocks have moved and changed over the centuries. “There are different aspects. One is the rocks themselves—yes, they’re very old, maybe the oldest in Eastern North America. Then there are the structures, which have changed over the years.” He said the ancient mountains have actually rejuvenated themselves as recently as a mere 30 million years ago or so. That’s fairly recent in terms of the earth’s history: These rocks were once basaltic lava flows formed about 570 million years ago. The crash of the two giant tectonic plates that pushed up the Appalachian mountain chain caused the basalt to metamorphose into greenstone. The greenstone is a large part of the catoctin formation, an important geologic unit in our mountains. 

At the first stop at the eastern entrance to the tunnel, Chuck Bailey talks about the outcropping of greenstone, a rock metamorphosed from basaltic rock. Photo: Malcom Andrews.

Other fascinating geological features are visible during the mile-long walk through the tunnel: faults in the rock that enable a steady water flow, distortion of the rocks into sausage-like shapes, and manmade arches and brickwork. In the east, Crozet had his workers blast the tunnel inch by inch out of the hard catoctin greenstone, but once they got to the western end, they needed several courses of brick to stabilize the sandstone there. The brickwork isn’t uniform, Bailey said: “It’s kind of like filling a cavity. You pack a lot of brick into the deeper holes, slowly building it up to join a thinner layer where the rock is relatively smooth.” Visitors can clearly see the differences in color between the work of Crozet’s masons and the more recent brickwork done before the tunnel was reopened. 

The students found much to interest them in the tunnel and identified in a film some of the major ones at eight stops. Eventually, they hope to post a QR code at each stop so visitors can find the explanation on their phones. The video shows the geologists explaining what’s interesting about each stop. The film is an important part of the geology project, Bailey said, as it was the intention of the students to share their excitement. Until the QR codes are active, Crozet Gazette readers are invited to see the video on YouTube, available in the online edition of this article.

Shifting rocks exposed crevices with distinct space between them, a process called daylighting. Photo: Malcom Andrews.

Stop #1

The geologists chose an outcropping outside the east entrance as a point of interest. The outcropping is representative of much of the rock inside the tunnel, but its position before the entrance makes its features more visible than similar rocks and structures in the dark, and you can see its bluish-green tint. Bailey described it as catoctin greenstone, a rock that originated as basaltic but metamorphosed into its current state, a structure featuring planes of aligned minerals that geologists call foliation. The planar alignment produced a distinctive grain in the rock, which happened when the solid rock became hot enough to flow during the massive collision of continents millions of years ago.

Stop #2

Closer to the entrance you’ll see another outcropping of greenstone. The metamorphose created new minerals within the basalt, visible to the eye: chlorite, quartz and epidote. Another feature of this stop is the steep downward slant of the two distinct rocks, resting upon each other with a visible space between, a structural feature called “daylighting.” In the film, one of the students jokes that this is not the place to be if the rocks begin to slide.

Once inside the eastern portal of the tunnel, visitors appreciate the magnitude of the engineering challenge. Photo: Malcolm Andrews.

Stop #3

This stop is right inside the eastern portal, where Crozet began the tunnel. The railroad jogged a little here, before the tunnel descends 4400 feet straight underneath Rockfish Gap, cutting primarily through the same kind of greenstone formation seen before the entrance, with rocks that are also well foliated. Bailey acknowledged that cutting through these tough, durable rocks, blasting inch by inch, was a significant engineering challenge.

Stop #4

A few hundred feet from the east portal, groundwater pours through cracks in the rocks. The young geologists wondered about the age of the water and where it might have been stored before it found its way to the fault. Bailey said he suspects the time between the water first percolating into the earth and discharging into the tunnel is probably quite short: days to weeks, rather than centuries.

Water pours through a narrow fault in the rock. Photo: Malcolm Andrews.

Stop #5

Perhaps out of respect for Crozet’s ancestral home, this stop—which features sausage-like odd shapes in the walls—is called “the hall of boudin,” boudin being the French word for sausage. Here, layers of greenstone and sandstone have been stretched and segmented to form these wobbly shapes. The blobs tend to be green because they contain a lot of epidote, a metamorphic mineral frequently found in areas of low-grade metamorphism under wet conditions. 

Stop #6

The northwestern third of the tunnel is shored up with bricks. At the start of the brickwork, there is a metaconglomerate, another type of metamorphic rock, with pink potassium feldspar and sandstone fragments.

At Stop #7, visitors can see manmade structures of brick and limestone in addition to phyllite. Photo: Malcom Andrews.

Stop #7

There’s a graceful arch of limestone quarried from the Shenandoah Valley and built by Claudius Crozet’s talented masons to frame the west portal. To the north, there is an outcropping in the rail cut that exposes phyllite, a metamorphic rock. The rocks here are about 20 million years younger than the rocks at the tunnel’s eastern end. 

Stop #8

We’re outside the tunnel now, and the trail makes a sharp bend here. You’ll see some large boulders of diabase. This dark igneous rock is the youngest unit in the Blue Ridge, formed 200 million years ago when magma was injected into the earth’s crust, forming what geologists call igneous dikes.

This rock detail near stop #6 shows the foliation caused by the tectonic shift. Photo: Malcolm Andrews.

Find a video description of each stop, explained by Bailey and his students, at https://youtu.be/L9HIgqlIPQA. 

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