I’ve contacted my old GeoPhysics professor (I used to double-major in GeoPhysics and Comp Sci until I dropped GeoPhysics because I had a different hopelessly liberal professor who was a nincompoop) to find out this exact formation but it is some sort of folded limestone that has been eaten away by the river and lichen. I thought this was one of the more interesting rock formations I have seen on the island.