Following the informal usage of Lee (1979, 1980), the lowest basalt flow in the Culpeper basin is here named the Mount Zion Church Basalt of the Culpeper Group. It comprises one or two separate flow sheets and extends discontinuously for more than 55 km (35 mi) in the west central part of the basin. The basalt is partly porphyritic and vesicular, columnar jointed, tholeiitic, with lenses of sandstone and siltstone. It disconformably or paraconformably overlies the Catharpin Creek Formation and underlies the Midland Formation. Thickness ranges from 3 to 180 m (10 to 590 ft). Age is Early Jurassic (Hettangian), based on palynoflora in underlying and overlying formations.

Source: GNU records (USGS DDS-6; Reston GNULEX).