Billy Goat Trail 3: A Non-Linear Quandary…

The final pondering point of our day’s adventure was the lamprophyre dykes from the Acadian Orogeny.  There is a distinct disconnect between the Virginia and Maryland Side.  On the North side of the river the dykes are roughly 100 meters downstream from where they enter the water on the Virginia side.  Over the years there have been two theories.  One theory is that when the dykes formed they were continuous across Mather Gorge.  Later there was a right lateral fault which ran parallel to the length of the gorge.  This offset the dyke on either side of the river.  The opposing theory is that there is no fault.  Instead, the dykes were jagged.  The dykes were joint controlled and not planer.

Unfortunately, the turbid waters beneath Great Falls make direct observation of the submerged sections of the dykes impossible.  Because of this another method must be employed to solve this mystery.  During our trip my classmates and I took a series of strike and dip measurements.  We would then plot all of these strike and dip measurements as shown below.


Below is the stereonet of the Foliations.  There appear to be at least two primary orientations of foliation.  One dipping just South of East, and the other to the southwest.


Note: North is up.

Above is a stereonet showing the orientation of the Lamprophyre Dykes.  The dyke set is dipping toward the North-East.  We only had data from the North side of the river, for this reason we are assuming that the dykes have the same orientation on the Virginia Side.  This appears to be the case from what we could see.  On the right is a stereonet showing the orientation of Mather Gorge.

Note: North is up.


Above is a stereonet showing the joint sets running through the Great Falls area.  There was so much data that the great circles were a little busy to look at by themselves.  Because of this I decided to have the program plot the poles as well.  A pole is the line perpendicular to the plane you are interested in.  In this case it shows three main joint sets, one dipping to the east, one dipping to the south-west, and one dipping to the north-east.  When joints form they tend to do so in response to forces applied across a region.  This causes joints to form in sets all oriented the same direction.


Note: North is up.

This is a stereonet showing the metamorphic foliation found along the Billy Goat Trail.  There appear to be two primary foliation orientations.  One to the North-West and one to the South-East.


            Based on the data collected by my class I do not believe that the Mather Gorge is fault controlled.  As igneous intrusions form they follow the path of least resistance like any liquid trying to find a pressure or potential energy equilibrium.  For this reason it would make sense that the dyke complex would follow the preexisting joint sets.  The data above shows three sets of joints that crisscross the region.  Joint Set 1 has the same orientation as the dykes on either side of the river.  For this reason it is likely that the exposed sections of the dykes above the water line followed joint set 1 on its way toward the surface.  Now that that is settled the question is why the dykes don’t line up on either side of the river.  It is my belief that the dykes may have followed another set to where joint set 1 crossed it again.  For instance, my hypothesis is that the dykes followed Joint Set 1 on the Virginia Side of the river.  Then, where the river is now they followed Joint Set 2 (Which is at roughly a 30 degree angle to Mather Gorge) East.  Then at some point under the present river the dykes intersected and joint from Joint Set 1 and continued North to the exposure on the Maryland shore.


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