Billy Goat Trail > Lamprophyre Dikes on the Virginia Side, Visible From the Billy Goat Trail by Robin Rohrback-Schiavone

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. This is a view of the Virginia side of the river, as seen from the Billy Goat Trail (which is on the Maryland side). Width of widest dike (second from left) is about two feet.