The Bull Run Formation is a Late Triassic stratigraphic unit in Virginia and Maryland's Culpeper Basin, part of the Newark Supergroup, preserving ancient river, lake, and basin environments. Its members reflect changing conditions across the basin, from the widespread fluvial sandstones and shales of the Balls Bluff Member to the playa-lake shales and paleosols of the Groveton Member, with other local units recording debris from nearby uplands. It is especially important for trace fossils, including abundant dinosaurian and reptile trackways, while rarer body fossils include fish remains, coelacanth scales, and phytosaur teeth and bones. Despite debates over its formal classification, the name remains useful for a significant set of rocks bearing evidence of early Mesozoic life and environments.