||The devil's backbone is a volcanic dike. A huge cloud of volcanic stuff goes up in the air, cools quickly and drops to earth as fairly light, porous and soft rock. Lave is forced up through the cracks in this rock, and because everything is still hot, cools very slowly. This allows the molecules to settle, the rock gets very dense, and you get walls of hard stuff amid the soft stuff. A lot of erosion later and you have a dike.