STRUCTURAL ANALYSIS OF THE BOLAR ANTICLINE AT SOUNDING KNOB, VALLEY & RIDGE PROVINCE, HIGHLAND COUNTY, VIRGINIA

The Bolar Anticline is a 2^nd-order asymmetric fold that exposes Ordovician to Devonian strata in the Valley & Ridge province of Highland County, Virginia. Sounding Knob (elev- 1,340 m) forms a distinctive highpoint along the crest of the Bolar Anticline, and its summit is underlain by a Paleogene basaltic volcanic plug. Previous researchers have mapped the geology of the Bolar Anticline with contrasting results. We use structural contour analysis and balanced cross sections to evaluate these differing interpretations.

In the Sounding Knob area, the Bolar Anticline plunges gently to the northeast and three Paleozoic units are exposed: these include 1) interbedded reddish sandstone and mudstone of the late Ordovician Juniata Formation, 2) erosionally-resistant quartz arenite of the Silurian Tuscarora Formation, and 3) hematitic sandstone with greenish-gray to red shale of the Silurian Rose Hill Formation. The outcrop pattern of these units is the crux of the discord between previous interpretations of the Bolar Anticline.

Daniel (1990,1:24k map), interpreted the older Juniata Formation to be exposed along much of the ridge NE of Sounding Knob with the Tuscarora Formation cropping out in two strike belts on either side of the main ridge. This map pattern is, in essence, a breached anticline. In contrast, on a regional-scale 1:100k map (Rader and Wilkes, 2001) and on a 1:24k map (Wilkes, 2011), interpret the ridge NE of Sounding Knob to be capped by a carapace of Tuscarora Formation that is flanked by younger strata of the Rose Hill Formation, with only limited exposures of the older Juniata Formation occurring near Sounding Knob.

Structural contour analysis, on both the base and top of the Tuscarora Formation, revealed problems and inconsistencies with both map patterns. The Rader/Wilkes map pattern is untenable, yielding cross sections that don’t match the surface geology. The Daniel map, with some modifications of contact locations based on structural contours, yields viable cross sections. Our study provides an example of the importance of structural contour analysis in constructing viable regional to local cross sections and geologic maps.