Billy Goat Trail - Part II - History

Five hundred million years ago the East Coast of the United States was fairly different than what is generally recognized today, mostly because it didn’t really exist. At this point in time the eastern coast of what is now considered America was somewhere along the border of Ohio and West Virginia. While the mid-Atlantic region was for the most part present, it was buried beneath a warm shallow sea, and not yet the natal home of Willard Scott. This was all about to change though as off the coast a volcanic island arc was approaching on a collision course.

As the proto-Atlantic ocean was closing due to subduction, the volcanic island arc was depositing graded-beds of sandstone (graywhacke) onto the seafloor. These will be significant as they end up being the major constituent of the Washington D.C. area’s bedrock, and can be seen predominantly throughout the park. The volcanic islands made landfall with the North American plate around 460 million years ago and caused the suturing of the two terranes. This tectonic event is what caused the majority of deformation we witnessed along our hike/study, including metamorphosing the graywhacke into METAgraywhacke. It was now ready to take on Godzilla for the rights to torment tiny Japanese cardboard buildings.

Evidence for this tectonic event, known as the Taconic Orogeny, can be found beyond that of just metamorphosed sandstone, but also in the much more picturesque folding of that sandstone, as seen below. We also saw evidence of metamorphism in the presence of boudinage, which look similar to rock sausage links.

*side note: Microsoft Word is not a fan of geologic terminology. All of my words have red squiggly lines underneath them.

When the graywhacke was originally being deposited in the along the base of the volcanic terrane it collected in a pattern of coarse grains to fine grains as the energy of the water lessened. Even though these graded beds have gone through metamorphic stresses they can still be seen. The beds are also a good indicator of what orientation “up” was when the rocks were being deposited as the coarsest grains are always on the bottom. Geopetal structures like graded bedding can come in useful when dealing with folded areas which may have been overturned at some point.

Hiding amongst the plethora of metagraywhacke was an indicator of even older tectonic forces. Down near the end of Mather Gorge where the Potomac River broadens and turns we found amphibolite, a metamorphosed mafic rock characterized by its rough texture with a alligator-skin-like look to it. The amphibolite was dated to around 530 ma and can be attributed to ocean floor of the Iapteus Ocean. When the volcanic island arc pushed the graywhacke up onto the continent the amphibolite was sent packing with it. There is a noticeably large boudin hiding amongst the metagraywhacke that was probably broken off during the Taconian or Acadian Orogenies. We know that the Acadian had an impact on the area as located up river from the amphibolite are sets of lamprophyre dikes striking their way through the bedrock. (These will be further discussed in Part III in relation to the fault).

While the Alleghenian Orogeny had a much larger impact on the North American continent there is not much evidence to be seen along the trail or in the park, and if not for the lamprophyre dikes there is not much evidence for the Acadian Orogeny either. DC tapped out early in the Appalachian Orogeny events.

Up next we go fault exploring.