In the Blue Ridge Province of northern Virginia, Maryland, and southern Pennsylvania the established lower Cambrian beds are underlain by a thick sequence of greenstone and interbedded sedimentary rocks known as the Catoctin formation. In an effort to determine the thickness of the formation, its relationship to overlying and underlying rocks, and the original nature of the lavas from which the Catoctin greenstone was derived, an area near Luray, Virginia, was studied. There the Catoctin formation lies unconformably on granitic rocks. Its basal sedimentary layer ranges from a few inches to 150 feet thick and contains pebbles of underlying plutonic rocks. The erosion surface beneath the Catoctin is irregular, and in several places hills up to 750 feet high were buried beneath the Catoctin lavas. No important time break is indicated between the deposition of the Catoctin formation and of the overlying Cambrian sediments, and the Catoctin may be early Cambrian.
Mineralogy, chemical composition, and primary structures indicate that the original Catoctin lavas were basaltic in composition and were probably normal plateau basalts. Columnar joints, amygdules, sedimentary dikes, flow breccias, low-dipping primary joints, and other primary structures are well preserved, and the flows in this area are believed to have been subaerial.