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.
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
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.
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