Manassas Sandstone is a Late Triassic unit in Virginia and Maryland composed mainly of coarse sandstone and conglomerate, preserving the oldest exposed Triassic rocks in the Culpeper Basin. Lying above an unconformity between the Chatham Group and older eroded rocks, it records steep streams, debris flows, braided rivers, and alluvial fans entering the basin from several source areas. Its named members reflect those changing environments and differing sediment sources. Along the Potomac River, stone from the Poolesville Member was quarried as Seneca sandstone for landmarks such as the Seneca Aqueduct and Smithsonian Castle. The unit also preserves reptile footprints and fish fossils.