Billy Goat Trail

July 2, 2017 General Studies

Billy Goat Trail The Billy Goat trail currently located in Maryland and is part of the Great Falls. After getting familiar with the Billy Goat Trail you can see all the evidence that a great geological phenomenon has occurred at that location. This location went through several geographical changes that I will be mentioning along with some of the evidence from the Billy Goat Trail. The Billy Goat Trail actually started at the bottom of the Iaptus Ocean more than 460 million years ago. At the bottom of the ocean the sand was being deposited on top of the mud making some graded bedding that was caused by turbidity currents.

The rock that was being formed from graded bedding on the bottom of the ocean floor was the sedimentary rock called Greywacke. At about 460 million years ago the Iaptus Ocean started to disappear because the movement of the tectonic plates caused a group of volcanic arcs to collide against North America called the Tocanian Orogeny. This collision made a subduction zone where the massive oceanic crust was pushed below the continental crust causing high pressure and low heat to the rocks being scraped as the oceanic crust was subducted called the accretionary wedge.

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The accretionary wedge was made up of Greywacke turning to Meta-Greywacke cause of the exposure to heat and high pressure. The evidence that we saw at the Billy Goat Trail was that the rock Meta-Greywacke was the dominant rock found. Also the Meta-Greywacke rocks had some folding on them indicating the high pressure it must have been under. Another observation I made was that we saw an igneous rock called Migmatite, which is basically a rock that was left in a partial melting state.

The rock was leaking out granite magma cause of the heat it was under but the heat didn’t last enough for the rock to complete its transformation so it cooled down and was stuck in partial melting. (1) After the Tocanian Orogeny there was another large mass of land that was coming towards the region because of the subduction zone. The Avolanian micro-continent was heading towards North America and at about 360 million years ago they collided and it was called the Acadian Orogeny.

The Acadian Orogeny not only started the mountain building in the region but also brought volcanic activity along and this meant that the rocks in the region now where under a great amount of pressure and extreme amount of heat. Some of the evidence that I observed at the Billy Goat Trail from the Acadian Orogeny was the Gneiss rocks that where hidden underneath the large boulders where we stopped for lunch. The rock Gneiss requires high pressure and high heat because the rock has U-shaped banding due to the pressure and black and white striations from the heat. 2) Another observation from the trip that supports the Acadian Orogeny is the Lamprophyre dikes that cut across the Meta-Greywacke and are found in both sides of the Potomac River. The Lamprophyre dikes are younger than the Meta-Greywacke since they cut across them and since the dikes are made of basalt they can be dated back about 360 million years ago thus linking them to the Acadian Orogeny. Also the dikes weather out faster than the rock around them. (3) After the Acadian Orogeny the continent of North America collided with Africa which was at the time of Pangea.

Then the continents separated and opened up a new ocean called the Atlantic Ocean but there was no evidence found in the region to prove this geological event so the time missing is called an unconformity. During this time the region was just experiencing weathering and erosion. After the time of weathering and erosion the mountain’s sediments started to flow down the large rivers and deposited large amounts of small rocks and gravel in the region. Then the Ice Age affected the region because the glaciers withheld a lot of water and cause the sea level to drop.

When the sea level dropped the Potomac River stopped depositing sediment and instead started to incise its river bay. There were several verifications at the Billy Goat Trail that indicated that the Potomac River cut deep through its bottom. From my observations during the walk through the Billy Goat Trail there were strong facts that proved that the Potomac River had been cutting through its bottom and resulted in the Potomac River to get narrower. The first line of evidence was that rocks not common around the area where inhabiting the region like Quartz Sandstone, Arkose Sandstone, and Green Rock.

These rocks were well rounded and came from the West where you can find the Blue Ridge and the Culpeper Basin. (4) The second line of evidence was the pot holes left on rocks of the Billy Goat Trail that indicates that the river’s bottom used to be there because large boulders rolled at the bottom of the river and some made holes at the bottom of the river but when the river cut down the holes where left right on the surface of rocks. (5) The third line of evidence that I observed was that there were straths that indicate the level where the bottom of the Potomac River used to be.

The Billy Goat trail is one of the recent straths and it’s called Mather Gorge. Also you can find shells in the Billy Goat Trail because of the floods. You can tell that there were past floods because all the tree trunks that are tilting are tilting towards the same direction. (6) In conclusion, the geological process that the region of the Billy Goat Trail has been long but incredible. It was my first experience walking in the Billy Goat Trail and before I took this class I would of never had guessed that the rocks in the trail came from an ancient dead ocean.


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