The Arundel Formation, or Arundel Clay, is a clay-rich unit of Maryland’s Potomac Group dating to the Early Cretaceous, probably the early Albian. Formed in freshwater river landscapes as clay filled oxbow swamps and sluggish channels, it preserves an unusually terrestrial fauna for the region. Once used as a source of iron ore, it is now significant as the only major source of Early Cretaceous terrestrial vertebrates in eastern North America, offering the clearest view of dinosaurs there at that time. Its fossils include Acrocanthosaurus, Astrodon, other dinosaurs, fishes, turtles, crocodilians, pterodactyloid tracks, fossil wood, spores, and pollen that reflect a swampy, wooded environment.