Virginia Appalachians

The rocks of Virginia's Blue Ridge remember three worlds. The oldest — the Old Rag Granite and the gneisses of the Pedlar Formation — crystallized over one billion years ago during the Grenville orogeny, deep in the roots of a mountain chain raised when an earlier supercontinent, Rodinia, was assembled (USGS; NPS). Scrambling over Old Rag's bald summit, you are handling basement rock from a mountain range that vanished long before the first animals with shells existed. When Rodinia tore apart, the second world arrived in fire. About 570 million years ago, floods of basaltic lava poured across the rifting landscape; metamorphosed to greenstone, these Catoctin Formation flows now form the sheer, jagged cliffs of Stony Man, Hawksbill and many other summits in Shenandoah National Park (NPS). The rift became the Iapetus Ocean, and beach and shallow-marine sands along its new shore hardened into the tough Chilhowee Group quartzites that litter the southern district's summits as talus. The third world is the one the brief is about: the Alleghanian orogeny. Between roughly 300 and 250 million years ago (Pennsylvanian to Permian; some syntheses count deformation pulses back to ~325 Ma), ancestral Africa collided with ancestral North America, closing the proto-Atlantic oceans and welding the supercontinent Pangea together. The collision drove thrust sheets east-over-west: the Blue Ridge basement itself — Old Rag included — was uplifted and shoved bodily to the northwest, while the sedimentary layers beyond crumpled into the long parallel ridges of the Valley and Ridge province (Historical Geology / opengeology.org; USGS). The NPS states plainly that rugged mountain ranges 'similar to the Himalayas' rose along these collision zones — the ancestral Appalachians among them — 'now much lower thanks to 250 million years of erosion.' Here is the detail that reorders your sense of time on the trail: when these mountains stood at their full height, there was no Atlantic Ocean. Pangea only began to split at the very end of the Triassic, about 201 million years ago, when vast lava fields erupted along the future seaboard and the North Atlantic started to open (NPS). The ocean that now defines 'the East Coast' is tens of millions of years younger than the mountains that overlook it. Everything you hike in Virginia today — greenstone cliffs, granite domes, quartzite scree — is the deep, erosion-resistant root of that Himalayan-scale range, still standing after a quarter of a billion years of being worn down.