Mahantango Formation (Willard, 1935). Shale, siltstone, and sandstone. Shale, gray, pale-green, and black, weathers silvery gray, silty with minor interbedded siltstones; lower contact gradational with underlying Marcellus Shale. Siltstone, greenish-gray with interbedded shale. Sandstone, greenish-gray to grayish-brown, fine-grained, fossiliferous. Spheroidal weathering common in all lithologies. Thickness ranges from 0 in southwestern Shenandoah County (Butts, 1940) to approximately 1100 feet in Frederick County (Butts and Edmundson, 1966). Southwest of Shenandoah County the Mahantango Formation is indistinguishable from the underlying Marcellus Shale. They are combined and mapped as a single unit, the Millboro Shale, from southern Shenandoah County to Southwest Virginia (Rader, 1982).