History of Billy Goat Trail

It all started around 510 ma, when an island arc was moving towards ancestral North America.  Between the two was an ancient ocean, the Iapetus Ocean.  At the bottom of the ocean, there were graywacke sediments.  The graywacke seen at the Billy Goat Trail exhibited graded bedding which are associated with turbity currents.  Turbidity currents form in deep ocean environments when sediment flow along the bottom, depositing the coarse grains first and the finer grains on top.

Here's some graded bedding found on the trail.














The Taconian Orogeny occurred 450 ma when the island arc collided with North America.

Map by Ron Blakely of Taconian Orogeny

 The Iapetis Ocean basin crust was subducting under the volcanic arc, which caused metamorphic activity within the rocks.  The three we see at Billy Goat is meta-graywacke, gneiss, and schist.  The meta-graywacke formed when the temperature and pressure aligned the minerals within the graywacke.  Schist was formed from the oceanic mud.  The schist here is abundant in muscovite mica, which has also oriented in the same direction.

Extreme metamorphism can be seen in the gneiss.  The gneiss is quartz, feldspar, and biotite rich.  The gneiss has been folded which indicates two separate events.  One for the foliation and one for the folding.

Another occurrence in metamorphic activity is partial melting.  When partial melting occurred in the graywacke, the felsic minerals, which had a low melting temperature, seperated from the mafic materials, which had a higher melting temperature.  When this happened, a rock called migmatite formed.  We saw an abundant amount of migmatite on "Bear Island."


Ancestral North American experienced another orogeny 360 ma called the Acadian Orogeny.  A microcontinent by the name of Avalonia, collided with the eastern side of ancestral North America.  This orogeny formed the lamprophyre dikes at Billy Goat Trail.  This is a view of the dikes on the Virginia side.

Again, the purpose of this trip is to explain why Mather Gorge is so straight.  The lamprophyre dikes seen above are said to be evidence that it is due to a fault.  This will discussed in the next post.