A few days ago I discussed the possibility of a fault underlying the Potomac River along Mather Gorge. Today I will go over the data that was collected by my classmates and myself and give my impressions on what it concludes.
Our trip led us down the Billy Goat Trail starting at the north western end and moving southerly in a downstream path. Along the way we took measurements of joint faces and foliations found in the metagraywhacke. The thinking here is that if there is not a fault, and the river is just cutting through a more cohesive area then all the joints would share a similar orientation. The data was collected at four stops along the trail which I then plotted onto stereonets to better visualize their relationships.
Below I will summarize each stop and include the stereonets from that stop and a picture in applicable. I will wait until the end to give my conclusion
This was just near the beginning of the trail, but still within about 30 meters of the river. We found plenty of nice structural features such as folds and plumose structures. While I wasn’t completely comfortable with taking measurements at this point the quantity of data was less than that of the future stops. Luckily, I have intelligent and capable classmates I could lean on to help with the collection amount.
At the second stop we encountered (and were told about) some sets of lamprophyre dikes. The dikes were located on the Virginia and Maryland sides of the river. The dikes most likely formed during the Acadian Orogeny when a larger “Northern Europe” terrane collided with the North American terrane. Age dating of the dikes places them at around 360 million years old. Since the lamprophyre dikes are less stable material than the metagraywhacke they are weathering out leaving just the cavities.
I was more comfortable and familiar with my Brunton compass and was able to compile much more data this time around.
At this stop we encountered more bees, and while the left us alone they limited where some of us were willing to take measurements. Not me though, I am fine with bees. Snakes are another story. Just south of the stop we were privy to a good example of migmatite which forms from the partial melt of the parent sedimentary rocks and forms into a granite.
After stopping for lunch over a beautiful overlook, we continued on. Unfortunately for me, at this point my iPhone (which was acting as my camera for the day) decided to turn off and not come back on. So you will have to use your imagination from this point on, and take me on my word when I say we hung out with Sasquatch.
At this stop we took our usual measurements and also took note of the unusually placed boulders along the trail. It’s not that they were odd shaped or sized, but more the composition that was unusual. We found boulders of red sandstone from the Culpeper Basin and pieces of the Catoctin and Weverton formations from the Blue Ridge Mountains. These boulders were transported during high energy floods of the river that deposited them up on top of the trail.
Imagine how many times I walked over these rocks without considering where they came from.
Here are the stereonets of all four stops combined into one:
By now we had collected a substantial amount of measurements of the bedrock jointing. By looking at the location of the data on stereonets I do not see enough of a correlation to convince me otherwise of the location of a fault underneath the gorge. That is not to say there is one, but more that I would need to find evidence elsewhere.