Stock Photo - Leesburg Limestone Conglomerate - Frederick county Maryland, Formed in a Triassic half grabben created when the Africa plate split apart from the North American plate, Member of the Bull Run Formation Limestone conglomerate exposed near Leesburg, Va., (loudon county) as well as Frederick and Montgomery county Maryland. Consists of fragments of lower Paleozoic limestone, dolomitic limestone, quartzite, quartz, schist, slate, and greenstone in a clayey matrix cemented by calcite. It is sometimes metamorphosed into a light-gray marble called Potomac marble. It occurs in the northwestern part of the Culpeper basin where it is intercalated with Balls Bluff Siltstone. Thickness ranges from 190 m (630 ft) in the Frederick valley, MD., to 1,070 m (3,500 ft) in Montgomery Co., MD., used to make the pilllars in the statue room of the US capitol building