Billy Goat Trail 2: Fancy Features

This post will hopefully explain some of the more interesting structural features that we found along the trial and explain their geological significance for the Billy Goat Trail region in the grand scheme of things.  I am going to try to follow chronological order of creation when talking about the features.

Our first feature is one of the more intriguing rocks in the park.  This is because of its mysterious background.  Along the eastern portion of the trail where metamorphism was at its most extreme there are large exposures of amphibolites.  This is a black metamorphic rock containing amphibole (Thus the name “amphibolites”).  It is recognizable by its alligator skin like texture.  It is crosscut in several places by granitic dykes indicating that it is older than granite.  Potassium argon dating of the granite shows it is roughly 450 million years old.  This means that the amphibolites predate the Taconian Orogeny.  There are two theories for the origin of the amphibolites.  First is that they are an ancient piece of oceanic crust which was obducted while the Iapatus Ocean was being subducted beneath the North American Plate.  The second and more likely option is that it formed as a mafic intrusion in the forearc basin.  The amphibole rich mafic intrusion was altered to amphibolite during the Taconian Orogeny.

 

This is an amphibolite seen in the eastern portion of the trail.  Photograph courtesy of Callan B.

The next feature was being created at roughly the same time as the aforementioned amphibolites.  Throughout the region graded beds are visible in the Metagreywacke.  The greywacke is sediment that was deposited in the Iapatus Ocean along the continental shelf.  Then, by one cause or another there was a submarine landslide or other event which disturbed a large portion of the sediment.  Then the sediment settled with the largest grains falling out first.  The smaller particles then fall out.  This creates a gradient particle size from coarse at the base to fine at the top as shown in the picture below.  It is important to note that the greywacke has been lightly metamorphosed.

Above is an amazing example of a folded graded bed.

Following the trend of increasing metamorphism, the next feature we come across is schist.  This is where all of the minerals in are aligned during pressure metamorphism.  The grains are typically aligned in a plane perpendicular to d1.  The alignment of micas and other grains is called foliation.  This is shown in the picture below.  This is a relatively low grade of metamorphism, but higher than the metagreywacke talked about above.  Sadly, none of the pictures I took, nor those I had access to of my classmates did the schist justice and so are not included here.

We next found gneiss.  This is caused by a higher grade of metamorphism than we had seen before.  In gneiss the light and dark color minerals (particularly the feldspars and the micas) align in bands.  This is called “Gneissic Banding”.

Above is a picture of gneiss courtesy of Laura S.

The next important features we saw were migmatites.  Migmatites form when under the intense temperatures and pressures of metamorphism partial melting occurs.  In Great Falls the migmatites formed as the greywacke partially melted.  The partial melt created granite.  The blebs of partial melt then start to rise toward the surface. 

  

These are migmatites caught in the process of forming. Photo courtesy of Laura S.

In addition to the diversity of rock types mentioned above there were an assortment of folds and other strain indicators.  Shown below is one of the best folds that we found.

 

Photo courtesy of Laura S.

Additionally, there was the ptygmatic folding shown below.  The shortening could be measured and used in conjunction with other markers to find the overall strain on the region.  The term ptygmatic describes the intestine-like shape of the fold.

Another feature we found in the park was boudinage.  Even more interesting was the chocolate tablet boudinage we found near the end of the hike.  A boudinage is where a somewhat brittle material is elongated and becomes tapered in places.  Sometimes it can even break.  The more malleable material surrounding then fills in.  It begins to look something like sausage links.  Chocolate tablet boudinage is where this happens in three dimensions. As seen in the picture below.

There is also the matter of jointing.  Joints occur along weak planes in the rock structure.  These fractures propagate throughout the rock.  They tend to form as sets.  There is no motion of either side with respect to the other associated with joints.

 

Above is an image of jointing on the Rocky Island.

The most recent geological feature of region are the lamprophyre dykes.  They formed during the Acadian Orogeny.  The dykes are mafic intrusions into the surrounding greywacke.  They dykes are very visible on the Virginia side of the Potomac.  However, along the Billy Goat Trail the dykes are barely noticeable unless you know where to look.  The best evidence of the dykes on the Maryland side are the straight, narrow gaps in the country rock where the dykes were and have since been eroded.  An interesting detail is that the dykes do not line up across the river.  The possible causes of this will be the subject of the next post.

 

The Lamprophyre Dykes are marked below.

 

References

Bentley, Callan. GOL 135: Geology of the Billy Goat Trail, including Great Falls, Maryland.  Northern Virginia Community College: <http://www.nvcc.edu/home/cbentley/gol_135/billy_goat/readings.htm&gt;, (20 Feb. 2012).

 

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Billy Goat Trail 1: Introduction

 

As part of my structural geology course at George Mason University we went on a Field Trip to the Billy Goat Trail near Great Falls on the Maryland side of the Potomac River.  After the field trip we were asked to make a report of what we saw and learned on the trip.  We were given the choice of any medium we wanted.  I chose to create a blog to explore the style of writing and the interaction with other people in the field or with similar interests to my own.  And thus we have reached the purpose of this entry.  In this entry and the following two I hope to give a relatively brief introduction to the area and its geologic past, discuss some of the more interesting structural features we found during our trip, and finally discuss the structural features controlling Mather Gorge and the Lamprophyre dykes.

 

      Each year thousands of people visit the Billy Goat Trail.  Only a small fraction of those people are able to appreciate the geological significance of the area.  The region covered by the trail shows evidence not one but two of the three major orogenies which have influenced the eastern portion of the North American Plate.  Also, the Potomac River has heavily altered the region by incising and creating the Mather Gorge.

This map depicts the Maryland side of Great Falls; the blue circle marks our study area.  (Photo Courtesy of the National Park Service)

The Billy Goat Trail is located within the Chesapeake and Ohio Canal National Historical Park which runs from Cumberland, MD to Washington D.C..  The canal was the brain child of George Washington who sought to overcome the topography created by the very geology that makes the region so special.  Up until the canal was built it was impossible to transport goods by water past the “fall line”.  This is the name given to the dividing line between the Piedmont and the Coastal Plane Regions of Virginia.  This marks a transition from easily eroded rocks to the more resistant rocks of the Piedmont and western regions.  The more resistant material creates waterfalls and shallow rocky regions making the area impassible for boats.  Thus another means of transporting goods was required and the canal was begun.  Unfortunately, with the advent of the steam engine not long after completion of the canal soon made it obsolete.  The canal fell into disuse until a Supreme Court Justice, one William Douglas who led the fight to get the entire canal made into a National Historical Park which people would be able to enjoy.

The fantastic geology found along the trail is a product of the tectonic activity which has taken place over the course of the last 500 ma or so.  To better understand the features visible along the trail and to see some of the more subtle overall trends one must first understand the tectonic history of the region.

510 million years ago the eastern what would be the eastern seaboard of the United States was the continental shelf of the North American.  The Instead of the Atlantic Ocean it was the Iapetus Ocean.  Not far off the coast was the Chopawamsic Terrain, an ancient volcanic island arc which was moving toward the North American Continent.  The oceanic crust along the edge of the North American Plate was being subducted since the oceanic crust was less dense than the continental crust under the island arc.  As the slab was pulled down into the mantle rising temperature and the presence of volatiles caused partial melting to occur.  The melt then moved up to the surface above the sinking slab supplying magma to the volcanic arc, this added to the size of the volcanic island arc.  Then, after roughly 60 million years of suspense, the oceanic crust between the two landmasses ran out and they collided.  In the words of Professor Bentley, it was like a Volkswagen Beatle getting into a head-on collision with a tractor trailer.  There was some strain on the part of the east coast of the ancient North American Plate but on the whole it remained relatively undamaged.  The Chopawomsic terrain however was smashed onto the leading edge of North America.  In other words it was accreted.  The deformation caused by the collision of the two landmasses created the majority of the geologic features found along the Billy Goat Trail.  The collision is known as the Taconian Orogeny.  The 450 Ma date of the orogeny was confirmed by potassium argon dating of the granite migmatites.  The Taconian Mountains were the end result of the collision.  The level of metamorphism of the features along the trail increases from west to east indicating that the heart of the Taconian Mountains was east of Great Falls near Washington D.C..  The Taconic Mountains in New York are the only remaining mountains of the ancient mountain range.  However, the rocking and hilly nature of the Piedmont Providence of Virginia can be attributed to being the roots of the Taconian Mountains.

Below is an image of the North American Plate before the Taconian Orogeny. (Figure Courtesy of Callan B.)

 

This is a picture of the North American Plate just after the Chopawamsic Terrain collided with it.  The micro-continent Avalonia can be seen moving in from the North East. (Figure Courtesy of Callan B.)

 

For the next 100 million years the east coast was quiet with very little happening, that is until the Acadian Orogeny.  Around 350 Ma another continent, Avalonia, swept in from the North and collided with the North American Plate.  This date is corroborated by the radiogenic dating of the lamprophyre dykes.  The primary impact occurred in the North from roughly what is New York up into Canada.  For what would eventually be the D.C. Metropolitan area, the collision was a glancing blow.  There was very little metamorphism in the area associated with the Acadian Orogeny.  In fact, along the trail the only piece of evidence of the mountain building event is the lamprophyre dykes.

Since the Acadian Orogeny there has been very little activity in the region aside from weathering.  The Potomac River has likely had the biggest impact on the topography of the area.  In the past it was a meandering river which wandered back and forth across what is now the Virginia-Maryland State Line.  Channel Lag and other indicators of the Potomac’s ancient path ranges deep into Virginia.  For instance I have seen places in cascades south of VA-Route 7 where the larger sediment deposited by the Potomac has caused the area to resist erosion and become a large hill.  Other evidence of the past elevation of the river are straths or terraces.  These show the ancient floodplains of the river from when it was meandering.  Straths look something like a giant’s set of stairs on either side of the river leading down to the river.  They are caused by the river incising.  Then the river meanders creating a flat area around it.  Later the river incises again and repeats the process.  The majority of our hike took place on the Bear Island Strath.

Later, during the past ice ages sea level was much lower than it is today.  This caused the river to have more potential energy (p=mgh, where p is potential energy, m= mass, g is gravity, and h is height).  This increase in potential energy caused the river to erode the sediment rather than deposit.  The augmented erosional power of the river caused it to incise and become entrenched as it is today.  The incision created Mather Gorge.

References

Anonymous, Feb. 2012,  Great Falls Park.  U.S. National Park Service: < http://www.nps.gov/choh/planyourvisit/upload/greatfallstrailmap.pdf&gt;, (27 Feb. 2012).

Bentley, Callan. GOL 135: Geology of the Billy Goat Trail, including Great Falls, Maryland.  Northern Virginia Community College: <http://www.nvcc.edu/home/cbentley/gol_135/billy_goat/readings.htm&gt;, (20 Feb. 2012).