Geology of the Shenandoah National Park Region - Csmres Jmu ...

younger carbonate rocks. The sinuous nature of the mountain front likely reflects the low-angle nature of the
Blue Ridge fault and offset associated with younger transverse faults.
Hightop forms the mountain immediately to the east of the overlook (Fig. 14). The “great unconformity”
between the Mesoproterozoic basement and Neoproterozoic cover sequence occurs just above the Skyline
Drive. The basement is overlain by arkosic phyllite of the Swift Run Formation and at least 250 meters of
Catoctin metabasalts. On Hightop, individual Catoctin lava flows dip gently to the southeast, such that the
steep northwest flank of the mountain forms the anti-dip slope. The basement and cover sequence are
folded into a broad open anticline (Roundtop anticline). Sandy Bottom is underlain by a northwest-striking
transverse fault that places basement against the Catoctin Formation (Fig. 14).
Figure 14. Inset from the Geologic map of the Swift Run Gap 7.5’ quadrangle with Stops 2 and 6 (from Bailey and
others, 2009).
The large roadcut exposes a distinctive orthopyroxene-bearing monzogranite. This charnockite is part of a
compound pluton that occurs from south of Swift Run Gap to north of Skyland. The rock is composed of 10-
30% alkali-feldspar, 30-50% plagioclase, 15-25% quartz, 10-15% orthopyroxene with minor amphibole and
clinopyroxene. Rock textures range from porphyritic with alkali-feldspar megacrysts to equigranular. The
unit is commonly massive, but does carry a weak foliation at some locations. A U-Pb zircon age of 1,049±9
Ma was obtained from rock at this location and is interpreted as a crystallization age (Southworth and others,
2009). Charnockite exposed immediately east of Hightop Mountain (4 km to the east) yielded a Ar-Ar age
from amphibole of 950 Ma indicating that the basement complex in the western Blue Ridge has not been
heated above the amphibole closure temperature (450˚ to 500˚ C) since the waning stages of the Grenvillian
orogeny.
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