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.
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>, (20 Feb. 2012).