  Geologists tell us how the rounded, forest-covered mountains of the Blue Ridge record more than a billion years of the earth’s history. The two types of granitic basement rocks are the oldest. These rocks were formed many thousands of feet beneath the surface of the earth, as large masses of magma (molten rock) cooled and crystallized very slowly. One type of granitic rock (hypersthene granodiorite) is exposed at Marys Rock Tunnel, and along the crest and on the western flank of the Blue Ridge. The other type is a much coarser grained granite. It is named Old Rag granite for exposures found on that mountain and in the area east of the crest of the Blue Ridge. Both of these granitic rocks were changed by heat and pressure (metamorphism), which accounts for their layered or laminated (gneissic) textures at many locations. Hundreds of millions of years ago, long before the present landscape was formed, erosion carved an ancestral terrain of mountains, hills, and canyons into the ancient granitic rock. There were no trees, wildflowers, or animals to relieve the Then, from a series of long cracks or fissures in the earth, layer after layer of lava poured out until most of the granitic hills were submerged in a vast, level plain of lava. These basaltic outpourings formed the principal rocks of the Catoctin formation. Long after the lavas had hardened, they were radically altered by intense metamorphism into completely new groups of minerals, which give the rock its present characteristic green color, and its name—greenstone. This extremely durable greenstone schist caps many of the highest peaks in Shenandoah. The lava plateau sank slowly beneath advancing Paleozoic seas. Some 30,000 feet of sea-floor sediments were deposited in a vast trough, or geosyncline. Only the very lowest or oldest rocks from this age are now found in the north and south districts of the park. They belong to the Chilhowee series, but throughout the central district even these have been stripped by erosion from the crest of the Blue Ridge. Near the end of the Paleozoic era (some 180 million years ago), the sediments of the Appalachian geosyncline were subjected to intense compression by tremendous unknown forces acting from the southeast. These intense pressures slowly wrinkled, folded, shoved, and fractured the ancient granites, lavas, and sea-floor sediments into a great original Appalachian mountain system, which stood several times higher than the present mountains. Most of the younger rocks were shoved into parallel mountain ridges to the west of the Blue Ridge and now form the Allegheny Mountains. Continuous erosion has stripped thousands of feet of material from the former alplike mountains. Vast quantities of this material have been transported to the sea. Some of it was deposited in local basins to the east during the Triassic period. The Triassic redbeds are now being uncovered by erosion along Lee Highway (U.S. 211) and in the road cuts west of Bull Run or Manassas Battlefield. Even today the geologic forces continue, almost unnoticed, to change and shape the land. |
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