Last week the George Mason structural geology class, in which I am enrolled, went on one of its first hiking trips of the semester to the Billy Goat Trail. The trail is located on the Maryland side of Great Falls National Park and runs parallel to the Potomac River for 184.5 miles. The predominant rock type exposed along the trail was graywacke or metamorphosed graywacke (metagraywacke). The trail is of great significance to the class because it cuts across many large geologic provinces in the mid - Atlantic and exposes otherwise buried strata to passersby. The exposed strata can then be interpreted by studying the rock type, orientation, size and primary and secondary structures which is an integral part of structural geology.
While on the trail students were encouraged to measure and record the strike and dip of beds, trend and plunge of folds, and discuss the different deformations in order to interpret the geological history of the area and discrete mountain building events. This was my first time taking such measurements in the field and proved to be a lesson better understood outside of the classroom. Although the first measurements were just that- measurements -as the day progressed they became more than just recorded data. They became a story that consisted of the birth and death of oceans, mountain ranges and super continents.
It all started . . . . a long, long, geologically long time ago. The time was roughly 510 million years ago during the late Cambrian and there lived an ancient ocean called Iapetus (named after Atlas's father) that surrounded Ancestral North America.
After the break up of Rodinia, another super-continent in Earth's history, Ancestral North America was traveling in a north west direction and was oriented much differently than it is today. In order to give you a better idea as to how different, you would need to rotate modern North America 90 degrees to the south, that is, today's east coast was situated along the south coast.
While on the trail students were encouraged to measure and record the strike and dip of beds, trend and plunge of folds, and discuss the different deformations in order to interpret the geological history of the area and discrete mountain building events. This was my first time taking such measurements in the field and proved to be a lesson better understood outside of the classroom. Although the first measurements were just that- measurements -as the day progressed they became more than just recorded data. They became a story that consisted of the birth and death of oceans, mountain ranges and super continents.
The class began the trip at ten o'clock in the morning on a Saturday and because of the dreary weather expected many needed at least two cups of coffee for that extra kick of brain power. Luckily before the class set out on our journey into geologic past there was a stop at the restrooms which after two cups of java was much, much needed. It was roughly fifty degrees and partly sunny at first but within thirty minutes the clouds started to accumulate in the sky and they were accompanied by a drop in temperatures. At first I was dressed appropriately for the weather with a proper rain coat compliments of Callan Bentley and synthetic pants but when it started to rain there was a sudden drop in temperatures that I was not prepared for.
At the start of the voyage we were briefly educated about the unique history of the Billy Goat Trail and the Chesapeake and Ohio Canal. This was extremely interesting to me because I was unfamiliar with the history of the area. After the lesson it was time to venture into geologic past and embark on our true purpose: discerning whether or not Mather Gorge was the product of a fault or series of faults beneath the Potomac River. Faults tend to weaken the rock around them and cause them to break apart due to large amounts of stress and or strain.
A little on Mather Gorge:
Below is a satellite image of Mather Gorge at Great Falls which illustrates both the narrowing and straight trend of the otherwise wide, meandering Potomac River. The gorge is roughly 6.5 miles long and returns to meandering almost as abruptly as it stopped. Flowing water is very powerful, however it is always searching for a weak point or a way to use less energy to achieve the same task.
This is the mechanism driving the idea that a fault or series of faults could be present under the river. When the river was down-cutting it located weak, crushed up rock that is analogous with a fault zone and this zone or area of weakness essentially controlled the course of the river. Therefore, if there is an area of weakness due to a fault line underneath Mather Gorge it would seem logical that the Potomac would follow its path rather than try to cut into the much stronger metagraywacke (metamorphosed sandstone that contains mud) that surrounds the river.
In the beginning of the expedition many of the students were absolutely lost. Even though many had acquired a wealth of knowledge inside the classroom, few knew what to do with it. After a quick lecture and Brunton compasses in hand the class embarked on their first real experience measuring folds, foliation, bedding, joints and dikes. But more on this later, first it is important to understand the geology and diagenesis in this area before moving on.
A little on Mather Gorge:
Below is a satellite image of Mather Gorge at Great Falls which illustrates both the narrowing and straight trend of the otherwise wide, meandering Potomac River. The gorge is roughly 6.5 miles long and returns to meandering almost as abruptly as it stopped. Flowing water is very powerful, however it is always searching for a weak point or a way to use less energy to achieve the same task.
This is the mechanism driving the idea that a fault or series of faults could be present under the river. When the river was down-cutting it located weak, crushed up rock that is analogous with a fault zone and this zone or area of weakness essentially controlled the course of the river. Therefore, if there is an area of weakness due to a fault line underneath Mather Gorge it would seem logical that the Potomac would follow its path rather than try to cut into the much stronger metagraywacke (metamorphosed sandstone that contains mud) that surrounds the river.
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| Note the pink arrows pointing at the abrupt beginning and ending of Mather Gorge |
In the beginning of the expedition many of the students were absolutely lost. Even though many had acquired a wealth of knowledge inside the classroom, few knew what to do with it. After a quick lecture and Brunton compasses in hand the class embarked on their first real experience measuring folds, foliation, bedding, joints and dikes. But more on this later, first it is important to understand the geology and diagenesis in this area before moving on.
It all started . . . . a long, long, geologically long time ago. The time was roughly 510 million years ago during the late Cambrian and there lived an ancient ocean called Iapetus (named after Atlas's father) that surrounded Ancestral North America.
 |
| The pink arrows are pointing to (from left to right) present day Chesapeake & Ohio Canal National Historical Park, Iapetus Ocean and Chopawamsic Terrace respectively. |
After the break up of Rodinia, another super-continent in Earth's history, Ancestral North America was traveling in a north west direction and was oriented much differently than it is today. In order to give you a better idea as to how different, you would need to rotate modern North America 90 degrees to the south, that is, today's east coast was situated along the south coast.
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