Posts Tagged ‘Billy Goat Trail’

In three of my earlier posts I discussed the Billy Goat Trail at Great Falls National Park in Maryland. On Wednesday (quickly becoming Geology Day on my calendar) the team (Alan and I) trekked back to the BGT to further explore the geologic features. This time around the water level of the Potomac was considerably lower than previous, and exposed more outcrops for us to observe.

The bedrock of Great Falls was metamorphosed in multiple events, and evidence for these events is seen throughout the park. One piece of evidence is the presence of porphyroblasts found near the end of Mather Gorge in the heavily metamorphosed metagraywhacke. Porphyroblasts are large mineral crystals that form within a metamorphosed rock during periods of high temperature and pressure; one example is a garnet, the state mineral of Connecticut. When we first went through the BGT in April we came across a small sampling of porphyroblasts in the same vicinity. But, this time with more exposed rock we were in our own smorgasbord of porphyroblasts, boudinage, and kinematic porphyroclasts. Alan and I found five separate areas with the porphyroblasts, and could have found more if it weren’t for the blazing heat melting me to the rocks. One particularly good example is pictured below (pasty-pale hand for scale).

The question now arises: what are the porphyroblasts made of? Well, at this point they are composed of sericite, but only after they experienced retrograde metamorphism. In other words, after the intense pressure and temperature that created the porphyroblasts began to diminish the composition of them changed with the environment. Well then, what were they before they were sericite? Well, I don’t know, and I am not alone. There is some level of debate for this topic as some parties believe it to be kyanite and others sillimanite. The confusion arises out of both being polymorphs of each other along with andalusite. Polymorphs are minerals of the same chemical composition, but differing crystal structures depending on temperature and pressure levels. All three of our polymorphs discussed here have the chemical composition Al2SiO5, in relatively high pressure kyanite is formed while in high temperature sillimanite is formed. According to the USGS paper, “The River and the Rocks: the Geologic Story of Great Falls and the Potomac River Gorge” (1970) the porphyroblasts are sillimanite and suggests that they may have been kyanite and andalusite originally, but changed due to the heat from granite intrusions. George Fisher’s paper “The Piedmont crystalline rocks at Bear Island” from 1971, and discussed by Callan Bentley in his blog entry: “Crystal Ghosts” (April 2010), suggest evidence that all three polymorphs are found within the park, and that this could imply the conditions were near the aluminosillicate triple point in terms of pressure and temperature. I like this idea, and not just because it presents a more interesting scenario, but because there is solid evidence for both sides of the argument. With the triple point hypothesis both scenarios are explained.

I hope to have a better understanding once I complete the Igneous Metamorphic Petrology class, and will possibly explore the topic again then. For now I would have to consider the metamorphic environment of Great Falls during the Taconian Orogeny. Which would have been greater between temperature and pressure, or were both at the precise levels to generate all three polymorphs?


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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

Stop One:

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.

Folding in Metagraywhacke

Great Circle of Stop 1 Joints & Foliation

Poles of Stop 1 Joints & Foliations

Stop Two:

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.

Lamprophyre Dikes

Stop Three:

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.

Stop Four:

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.

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Five hundred million years ago the East Coast of the United States was fairly different than what is generally recognized today, mostly because it didn’t really exist. At this point in time the eastern coast of what is now considered America was somewhere along the border of Ohio and West Virginia. While the mid-Atlantic region was for the most part present, it was buried beneath a warm shallow sea, and not yet the natal home of Willard Scott. This was all about to change though as off the coast a volcanic island arc was approaching on a collision course.

Map of impending Taconic Orogeny by Ron Blakely

As the proto-Atlantic ocean was closing due to subduction, the volcanic island arc was depositing graded-beds of sandstone (graywhacke) onto the seafloor. These will be significant as they end up being the major constituent of the Washington D.C. area’s bedrock. The volcanoes make landfall with the North American plate around 460 million years ago causing housing prices to crash. This tectonic event is what caused the majority of deformation we witnessed along our hike/study, including metamorphosing the graywhacke into METAgraywhacke. It was now ready to take on Godzilla for the rights to torment tiny Japanese cardboard buildings.

Evidence for this tectonic event, known as the Taconic Orogeny, can be found beyond that of just metamorphosed sandstone, but also in the much more picturesque folding of that sandstone, as seen below.

The graded bedding of the original deposition can still be seen in the metagraywhacke in some cases, and is a great indicator of which direction was up in the paleo-environment. As the turbidite flows settle out the larger grain sizes settle first and the smaller grains last. The larger grains will determine the bottom of the deposit.

Hiding amongst the plethora of metagraywhacke was an indicator of even older tectonic forces. Down near the end of Mather Gorge where the Potomac River broadens and turns we found amphibolite, a metamorphosed mafic rock characterized by its rough texture with a alligator-skin-like look to it.

The amphibolite was dated to around 530 ma and can be attributed to ocean floor of the Iapteus Ocean. When the volcanic island arc pushed the graywhacke up onto the continent the amphibolite was sent packing with it. There is a noticeably large boudin hiding amongst the metagraywhacke that was probably broken off during the Taconian or Acadian Orogenies. We know that the Acadian had an impact on the area as located up river from the amphibolite are sets of lampaphire dikes striking their way through the bedrock. (These will be further discussed in Part III in relation to the fault).

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A few weeks ago my Structural Geology class went hiking along the Billy Goat Trail at Great Falls National Park in Maryland. While on the trail we discussed the bedrock, geologic history and any deformation structures witnessed along the trail. For some of us, including myself, this was our initiation to using the lab techniques in the field. Such practices as taking the strike and dip of a joint or foliation became much more real along the hike.

We set out early from the George Mason campus, and got to Great Falls before it got crowded. The weather was going to be great, and I made the mistake of wearing dark jeans. Growing up in the D.C. area I had been to Great Fall many times, but never before armed with knowledge or geological prowess I now possessed. Now every mylonite, tension gash, and ptygmatic fold would jump out at me like a overeager house pet. Well, maybe more like a docile goldfish. Either way the intent of this trip was far beyond just looking at water moving downstream. While there were plenty of lessons to be learned about the bedrock and history of the area we were also tasked with determining whether or not Mather Gorge is controlled by a fault underneath the river.

Looking at the satellite image of Great Falls below it is noticeable that the curvature and broadness of the Potomac River is lost for about a half mile. This area is known as Mather Gorge, and is the subject of debate on what causes the river to straighten so narrowly at this point. Some believe that there is a fault line lying beneath the gorge which is the cause. Faults are weaker areas since the rock is being moved or pulverized by the fault slippage. Rivers and other water flows are drawn to weaker areas since they are easier to cut into. If there is a fault beneath Mather Gorge, it would make sense for the Potomac River to make its path along the fault line since it would be easier to cut into than the surrounding bedrock of metagraywhacke. Once out of the supposed fault zone the river broadens and turns as it approaches the Atlantic Coastal Plain.

satellite image of Mather Gorge from Google Earth

Armed with our Brunton Compasses we were going to strike and dip our way down the Billy Goat Trail collecting randomly determined amounts of data from every joint, foliation, and…well, mostly those two things, joints and foliations. But, with that data we would be able to hypothesize whether or not we thought there was a fault or not.

But before we get there, let me tell you a bit about the geologic past of the Washington D.C. and Great Falls areas:

It all started 6,000 years ago…

(Continued in next post)

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