These are the lamprophyre dikes on the Virginia side of the Potomac River. There are dikes located on both sides of Mather Gorge. But while Mather Gorge is straight, the dikes on the Virginia side and the Maryland side are not aligned. It is unknown why they do not line up.
There are two hypotheses that were discussed in class. The first and most popular hypothesis is that the dikes were originally straight but they were cut by a fault. The reason this hypothesis is so popular is because of how straight Mather Gorge is. It is thought that the river followed the fault, which would be the weakest and easiest path for it to take. The picture to the left shows "A" as the first theory and "B" as the second theory. The second hypothesis is that the dikes were never straight and were originally jagged.
The structure students took the strike and dip of foliation, bedding, joints, fold limb, and lamprophyre dikes and the trend and plunge of fold hinges. If Mather Gorge is indeed caused by faulting, the strike and dip of the joints would give us some clues. When the class data of the joints was plotted on a stereonet, this is what it looked like.
There appear to be about three trends that the joints follow. However there is still not enough data to conclude anything. Then I plotted the stereonet of all the foliation, bedding, joints, fold limbs, lamprophyre dikes, and fold hinges. The result is this.
The purple represents the foliation, the red is the bedding, blue is joints, pink is fold hinge, orange is fold limbs, and the green is the lamprophyre dikes. However, looking at this the only trend is the green of the dikes. I believe that the data is scattered because it was a majority of the classes first time measuring strike and dip, therefore there much of the data received is not correct. And by looking at the data collected it is obvious to see that it is not conclusive enough to state that Mather Gorge is formed from a fault.
Where is the trend of Mather Gorge on your stereonet? How does it compare to the joint sets and foliation that you measured?
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