What we see now are the ancient roots. Before the continental colision, there was a sea and subduction zone. This gave us sandstones, diorite, and granite... All of which were crushed at incredible pressure and temperature by the continental collision. At the deep roots of the mountains, this transformed the rock into gneiss, marble, and other extremely hard rock. Additionally, the forces were so great that the very bottom melted and became fresh granite.

All of these stones are very hard and resistant to erosion, and are what we see todayas the Appalachians