Eric Butler - NPS Photo
Shenandoah straddles the
As in all mountain ranges throughout time, the pressures finally eased, the peaks stopped rising, and erosion took over. For hundreds of millions of years, the once-lofty Grenville range slowly wore away, until it was no more than a landscape of rolling hills and occasional higher points, rather similar to the Virginia Piedmont found east of Shenandoah today. Evidence of this past terrain can still be seen in the shape of contacts between the old Grenville rocks and the younger rocks above. There were no plants or animals, however, for these had not yet evolved from the primitive life just now beginning to flourish in the oceans.
This stable scene ended violently around 570 million years ago, however, as the underlying tectonic plates began to move apart. Vast quantities of lava rose through a developing system of rifts, erupting onto the surface along rift zones that stretched for thousands of miles along what is now eastern
As this ocean (named Iapetus) spread over the land, it left behind thick deposits of sediments that hardened into layered sedimentary rocks. The youngest rocks in Shenandoah are river, lagoon, and beach deposits left behind by the early stages of the Iapetus. In the valleys and ridges to the west, thick sequences of limestone and other marine rocks preserve evidence for the rest of this underwater period.
The ancient Grenville rocks, the lava flows, and the sediments represent the three main geologic units found within Shenandoah. Yet all of these events occurred over 400 million years ago, and the
By 200 million years ago, however, these forces had abated once more, and the
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Gaithright II, T.M. 1976. Geology of the Shenandoah National Park, Bulletin 86. Virginia Division of Mineral Resources, Charlottesville, Virginia.
Harris, A.G. and E. Tuttle. 1990. Geology of National Parks. Kendall/Hunt Publishing Company, Dubuque, Iowa.
Harris, D.V. 1976. The Geologic Story of the National Parks and Monuments. Colorado State University Foundation Press, Fort Collins, Colorado.
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Websites that provide helpful information about geology are: