The Santiago Formation in Orange County and northwestern San Diego County is a geologic unit of siltstones, mudstones, and sandstones laid down from the Late Eocene to Late Oligocene. Its importance comes chiefly from a rich fossil record that includes Walshina esmaraldensis, Diegoaelurus vanvalkenburghae, and other mammal and invertebrate remains, helping document ancient terrestrial environments in southern California. A paleoenvironmental study of land snails from Member C near Oceanside, at San Diego Natural History Museum locality 3276, interpreted the area as subtropical to tropical, with shell evidence indicating paleotemperatures above 25 °C and a climate warmer than today.