Billy Goat Trail- Mather Gorge

These are the lamprophyre dikes on the Virginia side of the Potomac River.  There are dikes located on both sides of Mather Gorge.  But while Mather Gorge is straight, the dikes on the Virginia side and the Maryland side are not aligned.  It is unknown why they do not line up.

From  http://www.nvcc.edu/home/cbentley/gol_135/billy_goat/readings.htm   

There are two hypotheses that were discussed in class.  The first and most popular hypothesis is that the dikes were originally straight but they were cut by a fault.  The reason this hypothesis is so popular is because of how straight Mather Gorge is.  It is thought that the river followed the fault, which would be the weakest and easiest path for it to take.  The picture to the left shows "A" as the first theory and "B" as the second theory. The second hypothesis is that the dikes were never straight and were originally jagged.

The structure students took the strike and dip of foliation, bedding, joints, fold limb, and lamprophyre dikes and the trend and plunge of fold hinges.  If Mather Gorge is indeed caused by faulting, the strike and dip of the joints would give us some clues.  When the class data of the joints was plotted on a stereonet, this is what it looked like.

There appear to be about three trends that the joints follow.  However there is still not enough data to conclude anything.  Then I plotted the stereonet of all the foliation, bedding, joints, fold limbs, lamprophyre dikes, and fold hinges.  The result is this.

The purple represents the foliation, the red is the bedding, blue is joints, pink is fold hinge, orange is fold limbs, and the green is the lamprophyre dikes.  However, looking at this the only trend is the green of the dikes.  I believe that the data is scattered because it was a majority of the classes first time measuring strike and dip, therefore there much of the data received is not correct.  And by looking at the data collected it is obvious to see that it is not conclusive enough to state that Mather Gorge is formed from a fault.

History of Billy Goat Trail

It all started around 510 ma, when an island arc was moving towards ancestral North America.  Between the two was an ancient ocean, the Iapetus Ocean.  At the bottom of the ocean, there were graywacke sediments.  The graywacke seen at the Billy Goat Trail exhibited graded bedding which are associated with turbity currents.  Turbidity currents form in deep ocean environments when sediment flow along the bottom, depositing the coarse grains first and the finer grains on top.

Here's some graded bedding found on the trail.














The Taconian Orogeny occurred 450 ma when the island arc collided with North America.

Map by Ron Blakely of Taconian Orogeny

 The Iapetis Ocean basin crust was subducting under the volcanic arc, which caused metamorphic activity within the rocks.  The three we see at Billy Goat is meta-graywacke, gneiss, and schist.  The meta-graywacke formed when the temperature and pressure aligned the minerals within the graywacke.  Schist was formed from the oceanic mud.  The schist here is abundant in muscovite mica, which has also oriented in the same direction.

Extreme metamorphism can be seen in the gneiss.  The gneiss is quartz, feldspar, and biotite rich.  The gneiss has been folded which indicates two separate events.  One for the foliation and one for the folding.

Another occurrence in metamorphic activity is partial melting.  When partial melting occurred in the graywacke, the felsic minerals, which had a low melting temperature, seperated from the mafic materials, which had a higher melting temperature.  When this happened, a rock called migmatite formed.  We saw an abundant amount of migmatite on "Bear Island."


Ancestral North American experienced another orogeny 360 ma called the Acadian Orogeny.  A microcontinent by the name of Avalonia, collided with the eastern side of ancestral North America.  This orogeny formed the lamprophyre dikes at Billy Goat Trail.  This is a view of the dikes on the Virginia side.

Again, the purpose of this trip is to explain why Mather Gorge is so straight.  The lamprophyre dikes seen above are said to be evidence that it is due to a fault.  This will discussed in the next post.

Billy Goat Trail Introduction

Two weeks ago, on February 24th, I went on a field trip to Billy Goat Trail with my structural geology class.   Billy Goat Trail follows the path of the C&O Canal and the Potomac River.The C&O Canal stands for Chesapeake and Ohio Canal which was built parallel to the Potomac River and divided by Mary's Wall.  Led by our professor, Callan Bentley, we learned about the geologic history and formation of the area. This was also my first time using a Brunton compass out in the field.  After much confusion and asking for the TA, Aaron's, help I finally managed to get a hang of what I was doing.  Hopefully this information sticks, otherwise field camp will be rough.  Another thing this trip taught me was to invest in a good rain jacket.  The day started off sunny but Mother Nature decided to be mean and give us rain.  My rain jacket was soaked within 30 minutes.  Since I'm going to Ireland for field camp, I absolutely need a good rain jacket.

At our first stop, Callan described the formation of the area and sent us on our way to measure strike and dip of outcrops with the profound advice, "Friends don't let friends map float."  This is a picture of the first fold that I had used a Brunton compass on.  Silly thing to be proud of, I know.

The purpose of gathering the strike and dip of outcrops was so we could hypothesize whether or not the formation of Mather Gorge was due to a fault.  In order to do so, the class compiled all their data and we must make a conclusion from that.  But before I do that, I want to talk about the geologic history of the area.