Millboro Shale, Huntersville Chert, and Rocky Gap Sandstone (VADx;0)

State	Virginia
Name	Millboro Shale, Huntersville Chert, and Rocky Gap Sandstone
Geologic age	Devonian
Original map label	Dx
Comments	Appalachian Plateaus and Valley and Ridge. From expanded explanation (ref. VA002): Millboro Shale, Huntersville Chert, and Rocky Gap Sandstone. Millboro Shale. Refer to previous description under Dmn. Huntersville Chert (Price, 1929). Chert, white, thin-bedded, iron-stained, blocky, fossiliferous with cherty, glauconitic sandstone and greenish-gray shale. The Huntersville Chert ranges from 10 to 60 feet in thickness (Bartlett and Webb, 1971). Butts (1940, p. 303) states, "The Onondaga [Huntersville Chert] persists to Mendota, Washington County, but 10 miles farther southwest.... it is absent in a fully exposed section". The Huntersville correlates with the Needmore Formation to the northeast and the upper part of the Wildcat Valley Sandstone in Lee County. Rocky Gap Sandstone (Swartz, 1929). Sandstone, medium- to light-gray, weathers dark-yellowish-orange, coarse-grained, scattered, thin, quartz-pebble conglomerate beds, arenaceous chert in upper ten feet, calcite cement, friable when weathered. Thickness ranges from 0 near McCall Gap, Washington County to about 85 feet in Bland and Giles counties. Equivalent in part to the Wildcat Valley Sandstone of Lee County and the Ridgeley (Oriskany) Sandstone and Helderberg Group north of Craig County. The lower contact is disconformable. The upper contact with the Huntersville Chert appears to be conformable.
Primary rock type	black shale
Secondary rock type	chert
Other rock types	sandstone
Lithologic constituents	Major Sedimentary > Clastic > Mudstone > Shale > Black-shale Incidental Sedimentary > Clastic > Sandstone Sedimentary > Chemical > Chert
Map references	Digital Representation of the 1993 Geologic Map of Virginia", 2003, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, viewing software not included. This is a digital version of "Geologic Map of Virginia" published in 1993. Available from: https://www.dmme.virginia.gov/commerce/ [https://www.dmme.virginia.gov/commerce/]
Unit references	Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000 Digital Representation of the 1993 Geologic Map of Virginia", 2003, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, viewing software not included. This is a digital version of "Geologic Map of Virginia" published in 1993. Available from: https://www.dmme.virginia.gov/commerce/ [https://www.dmme.virginia.gov/commerce/]
Geographic coverage	Bland - Giles - Montgomery - Pulaski - Russell - Scott - Smyth - Tazewell - Washington - Wythe