More than 1,300 ft (396 m) of Upper Cretaceous rocks overlie shales of the Upper Triassic Dockum Formation and are exposed on the Sevilleta Grant, near La Joya, Socorro County, New Mexico. The Upper Cretaceous sequence extends from the Dakota Sandstone to the Dilco Member of the Crevasse Canyon Formation and consists largely of shales and sandstones. The included Tres Hermanos Sandstone Member of the Mancos Shale includes sandstones, shales, and two coal beds.
The lowermost sandstone unit is a moderately sorted, calcareous sandstone which coarsens upward and with small- and medium-scale tangential, wedge and trough-shaped sets of cross-beds. The sandstone is about 11 ft (3.4 m) thick and contains fossil-rich lenses of poorly sorted, dark yellow-brown-weathering sands that are friable to moderately indurated and calcite cemented. Shale galls are present in the fossil-rich lenses and, together with wood fragments, abundant turtle bone fragments, some crocodile tooth and scute fragments, amid vertebrae and teeth, indicate a nearshore environment with a nearby source of freshwater. Selachian teeth and probable coprolites are richly varied and abundant. The following genera have been recognized: Hybodus, Lonchidion, Squalicorax, Squatina, Brachaelurus, Scapanorhynchus, Odontaspis, Cretoxyrhina, Cretolamna, Plicatolamna, Paranomotodon, Ischyrhiza, Ptychotrygon, Rhombodus as well as several as yet indeterminate genera. The dominant invertebrate genus is the oyster Crassostrea soleniscus although at least two genera of gastropods are present.