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 barrenness of the wind- and
rain-swept landscape. A half-billion years ago, only a few primitive
plants (algae) may have spotted the jagged granite hills.
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.