>>12893396They were formed by a collision between the North American tectonic plate and an oceanic plate resulting in the development of a subduction zone and the growth of the Appalchians in a manner not too dissimilar to that of the present day Andes in South America. All of this happened during the creation of the supercontinent Pangaea, and the build-up of the mountains ceased when Pangaea started to break apart.
What you're seeing in the photos are the folded ridges formed by those collisions. They stand out more than normal due to humans preferring to farm in the relatively flat valleys between the ridges, leaving the steep ridges forested.
A few hundred million years ago the Appalachians were as mountainous as the Rockies and even the Himalaya. Erosion is a hell of a thing.
