The visual and map view appearance of Black Mesa is as an inverted valley because erosion has removed the relatively soft sedimentary strata from either side of the resistant Raton basalt of the lava which originally had occupied and filled a river valley. The mesa is capped by erosion-resistant basaltic lava formed by a volcanic eruption 3 to 5 million years ago. The lava erupted from a vent in the Raton-Clayton volcanic field in northeastern New Mexico and southeastern Colorado. The volcanic cap to the mesa is thick, long and from 0.5 miles to 8 miles wide, which is to the north-northwest of Oklahoma. The erupting lava filled a stream channel in the Pliocene age Ogallala Formation. During the years since the eruption, the adjacent rock of the Ogallala and older formations have been removed leaving the valley-filling basalt perched atop a long ridge. Strata exposed along the mesa below the basalt and Ogallala include the Cretaceous Dakota Sandstone and the Jurassic Morrison Formation.