By: magic-gigapans
Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection
License: Creative Commons Non Commercial ⧉

Uploaded: 8 May 2020
Last Updated: 15 May 2020

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The Acadian Orogeny took place mainly to the northeast of here. (As the name implies, Acadian-aged rocks are well-exposed in Acadia National Park in coastal Maine.) Along the Billy Goat Trail, they left one set of important structures: a series of four parallel dikes filled with the igneous rock called lamprophyre. ("Lamprophyre" is a fancy name for what is essentially a mafic igneous rock, like more-familiar basalt or diabase). Biotite mica from these dikes has been dated using isotopes of potassium-40 and argon-40 to be about 360 million years old. It's worth noting that the mafic-composition rock which makes up the lamprophyre dikes is less stable than the older metagraywacke that it cuts across: as a result, it weathers out more easily -- making the dikes show up as a series of "slashes" in the surface of the rock face on the Virginia side. Corresponding gaps in the rock on the Maryland side--that is, on the Billy Goat Trail--represent the extension of the dikes across the span now filled by the Potomac River. There is no lamprophyre left in these gaps; it has all weathered out and tumbled downslope to the river. There is a pronounced lack of alignment between the Virginia side and the Maryland side. There are two hypotheses for the offset: (A) An early theory, and still a favorite possibility, is that the dikes were originally straight, and later cut by a fault, which offset the Virginia side to the right, relative to the Maryland side. Such a fault might also help to explain the extremely straight trend of Mather Gorge. (B) Another possibility is that the dikes were not originally straight. After all, cracks in rock can take on many shapes; they do not have to be planar. Instead, hypothesis B suggests that their original shape was jagged, and they therefore the offset dikes do not show good evidence for a fault underneath Mather Gorge.