BEDROCK GEOLOGIC MAPPING OF THE WESTERN HALF OF THE MONTEREY SE 7.5-MINUTE QUADRANGLE, VIRGINIA

New 1:24,000 bedrock geologic mapping efforts in the western portion of the Monterey SE 7.5-minute quadrangle were primarily focused on the west-verging Bolar Anticline and the unnamed synclinal valley to the southeast. Both trend northeast to southwest, which is concurrent with typical Valley and Ridge orientations. The Bolar Anticline includes Jack Mountain, Little Jack Mountain and the Bolar Valley (Big Valley). The uppermost stratigraphic unit present in the mapping area is the Devonian Millboro Shale, while the eroded core of the anticline exposes the oldest unit, the undivided Ordovician carbonate sequence. Significant portions of the ridge crests, particularly around Sounding Knob, are underlain by the Ordovician Juniata Formation, rather than Silurian Tuscarora, Rose Hill and Keefer Formations, as previously mapped; in fact, the Keefer is thin or absent in this area. Eocene igneous intrusions of both basaltic and felsite compositions are also present in the Bolar Anticline; the extent of these had not previously been well constrained.

Contrary to previous interpretations of a southeast-dipping thrust along Buck Hill, our mapping suggests that the fault involves primarily the Silurian Tuscarora and Rose Hill Formations and is antithetic, dipping northwest with a northeast-southwest strike. This structure results in localized duplications of the Tuscarora section and thickening of the Rose Hill. Additional evidence for the fault and its geometry include float blocks of hematitic sandstone cataclasite and an outcrop with lenses of cataclasite near continuous beds of the Silurian Williamsport Sandstone.

Part of the mapping project included scrutiny of the nomenclature of the Ordovician carbonate sequence, as the multitude of previous stratigraphic names had been primarily based on faunal content. In an effort to divide the strata into easily identifiable and mappable lithostratigraphic units, and following much field work, the various formations between the Ordovician Beekmantown and Dolly Ridge Formations were mapped as an undivided sequence of carbonate strata. This was deemed the most appropriate choice for mapping these strata until future work allows for identification of stratigraphic contacts that are suitable for field observation and identification while mapping at the 1:24,000 scale.