7 ½’ X 15’ quad: Whitetop Mountain, VA
Grayson and Smith Counties
Lewis Fork Wilderness National Recreation Area
Blue Ridge Mountains
Metamorphic (metamorphosed volcanic)
5,729 ft (1,746 m)
Weakly metamorphosed rhyolitic lava, rhyolitic ash-flow tuff and sericitic phyllite. Rocks at the summit are the Wilburn Ridge Member, part of a welded ash-flow sheet, grayish purple to maroon, typically with quartz and perthite feldspar phenocrysts. An eruption from a volcano centered at Mt. Rogers, perhaps during a caldera collapse, produced a sheet of volcanic ash up to almost 2000 feet thick. The deposit became compacted under its own weight, flattening pumice fragments into flame-like structures. Heat that was retained by the deposit served to weld the ash, crystals and larger fragments together. (Interesting note: Wilburn Ridge takes its name from one Wilburn Waters, 1812-1878, preacher and bear-hunting pioneer who killed 120 bears, according to a marker in nearby North Carolina!)
South of the summit area phenocryst-poor rhyolitic lava and tuff are exposed. Mt. Rogers lies in the Stone Mountain thrust sheet, which overlies the Pond Mountain thrust sheet, exposed in a small area north and west of the summit. Together these thrust sheets overlie Cambrian rocks in the Unaka belt on Iron Mountain, which in turn is thrust west over the Valley and Ridge belt. Metamorphic assemblages in greenstones of the Mount Rogers Formation are in the epidote-actinolite facies. The phyllites are no higher than biotite grade.
The rhyolites are associated with granites of the Crossnore Complex, dated at 765 to 740 Ma, which may represent the rifting of very thick continental crust that preceded the rifting which opened the Iapetus Ocean. The crust that was rifted had been deformed and metamorphosed during the Grenville Orogeny (1100 Ma ago), but the Mount Rogers Formation itself was not metamorphosed until parts of Iapetus closed in the Taconian Orogeny around 450 Ma ago. Thus the geological history of the Blue Ridge parallels quite closely that of the Green Mountains in Vermont.
For a discussion of all the volcanic highpoints, see http://www.unh.edu/esci/volcanic_highpoints03.pdf.
Soil Series: Ashe silty loam: Very light brown to brownish-yellow, well drained, silty loam with numerous bedrock fragments. Local pockets of Porters loam: dark brown loam, in depressions where more organic material has collected.
- Devereaux, R.E. and G.W. Patterson, 1934, Soil Survey of Grayson County, Virginia: U.S. Soil Conservation Survey.
- Novak, Steven W. and D.W. Rankin, 2004, Compositional zoning of a Neoproterozoic ash-flow sheet of the Mount Rogers Formation, southwestern Virginia Blue Ridge, and the aborted rifting of Laurentia, in. Tollo, R.P. and others, Proterozoic tectonic evolution of the Grenville Orogeny in North America: Geological Society of America Memoir 197, p.571-600.
- Rankin, Douglas W., 1993, The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation -- two Late Proterozoic units in southwestern Virginia: U.S. Geological Survey Bulletin 2029, 26p.
- Rankin, Douglas W., G.H. Espenshade and R.B. Neuman, 1972, Geological map of the west half of the Winston-Salem quadrangle, North Carolina, Virginia and Tennessee: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-709-A, 1:250,000.
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