More on mountain building events. . .

Now that you have had time to grasp that concept we can move on.

The outer part of the continent is called the continental shelf. A continental shelf is a shallow, seaward-sloping platform that is part of the continent, extending to the continental rise (it's under water). It was somewhere on this continental shelf platform that would be the future home of the Chesapeake and Ohio (to be referred to as C&O Canal) Canal and the Billy Goat Trail. The C & O Canal is notated on the previous post.The reason that this once underwater area is dry and above sea level today is because of a discrete mountain building event.

During the Middle Cambrian to Early Ordovician (~530Mya) was a Volcanic Island Arc named Chopawamsic Terrane that was located off the coast of Ancestral North America. Unfortunately, the volcanic island arc had been drinking and driving and was headed for a head on collision with poor Ancestral North America. The driving mechanism for the movement was the subduction of the more dense oceanic crust underneath the less dense volcanic island arc. This subduction created lava and ash flows along with movement that eventually closed the Iapetus Ocean basin around 460 million years ago. This was not a fast process, however and allowed for volcanic sediments to be accumulated on the ocean basin which will be of importance later in discussion.

Diagram of the oceanic crust being subducted underneath the volcanic island
arc causing the two land masses to collide. Illustration courtesy Callan Bentley.

In the time leading up to this collision an impure sandstone known as graywacke was accumulating on the bottom of the Iapetus Ocean basin. These sediments were deposited, primarily, as graded beds which are usually associated with turbidity currents. Because of the dark color of the beds it has been interpreted that they were most likely formed in a low-oxygen environment, i.e. the deep sea. This is important to remember because graywacke is the original rock type or protolith of the metagraywacke that is along the Billy Goat Trail and in the Washington DC area. The force and eventual strain caused by the collision of these two land masses caused the originally horizontal graded graywacke to metamorphose into metagraywacke or migmatite. Migmatite refers to a rock that has undergone some metamorphism, however cooled before the process was complete. The result is a rock that has both igneous and metamorphic (granite and metagraywacke).

A key characteristic of a rock that has gone through some sort of deformation, as seen in the photo below, is foliation and folding. Another important feature measured on the Billy Goat Trail were joints. These were used to determine the validity of the Mather Gorge hypothesis in that we were suggesting that joints ( i.e. a fault zone or set of faults) alone could explain its presence. The differential deformation that is observed in the metagraywacke all along the trail is evidence for the mentioned tectonic event which, to geologists, is known as the Taconian Orogeny. The term orogeny refers to a mountain building event. Thus when the Chopawamsic Terrane closed the Iapetus Ocean and collided with Ancestral North America the surrounding rock was put under immense pressure/heat which caused the rock to metamorphose and deform. There is still some horizontal graded bedding present that indicates which way was up and this is due to the nature of a turbidity current. In a turbidite, larger grains settle out first followed by smaller grains and thus the larger grains will be deposited at the bottom, hence giving you the rocks original orientation.

Example of jointing. Yellow arrows pointing at two sets of joints
perpendicular to one another.

Graded bedding. Blue arrow pointing at larger sized grains. Blue
 line separates the boundary between fine and coarse grains
 indicating the younging direction (coarse grains represent the
 bottom whereas fine grains represent top).

Example of folding on the Billy Goat Trail. Purple
lines outline the major theme of foliated beds

At this point on our journey we had encountered rocks that were around 460 million years old! What if I told you we found rocks that were roughly 530 million years old? At the edge of Mather Gorge, just before the Potomac gets wider and turns, the class found deposits of a mafic rock called amphibolite. They are given this name because of their unique alligator skin like texture and in this area were large tabular masses. How did these very old, mafic rocks get to be deposited in the area of Billy Goat Trail?

There are two major clues here, the mafic nature of the rock and its age. Since we know that it is older than the Taconian Orogeny and it is mafic (oceanic crust) then we can deduce that it most likely came from the bottom of the Iapetus Ocean. This was possible due to accumulations of graywacke sediments that were scrapped up off of the ocean floor at the head of the subduction zone, otherwise known as an accretionary wedge. Later, when the Taconian island arc collided with Ancestral North America, these sediments were deposited and with them pieces of the oceanic crust. 

As you probably already know there has been more than one tectonic and or mountain building event here on the modern east coast of America. I am bringing this up because as our journey into geologic past progressed we came upon sizable exposure that seemed out of place in the presence of all the metagreywacke. Boudinage is a term used by geologists for structures that form as a response to layer parallel extension (or layer perpendicular flattening) of stiff layers that are surrounded by softer layers (boudin refers to individual these pieces). This is common among highly deformed sequences of rock types of varying strength. A picture of a boudin is posted below. Evidence for the Acadian Orogeny can be seen along the Billy Goat Trail by way of a series of roughly parallel dikes filled in with mafic igneous rocks that have been dated around 360 million years ago. These structure's are called lamprophyre dikes and are going to be very important in our later discussion regarding Mather Gorge and the fault hypothesis. 

The yellow arrows are pointing at amphibolite - note the alligator skin like texture.
Photo was taken at the Billy Goat Trail. 

Illustration of an accretionary wedge. Image courtesy Callan Bentley. 

Example of a Boudin. Note the difference in rock type
between the index and pinky finger. Photo from Luke O'Neal.

It is amazing to me, and hopefully now you, that rocks can tell such a story, and of course that I get to put my hand on something that is over half a BILLION years old.