The Inyan Kara group, composed of Lakota and Fall River Formations, consists of sandstone, shale, conglomerate, variegated siltstone, and claystone and forms the lower part of the Cretaceous in the Black Hills and adjacent areas. The Lakota is dominantly continental and the Fall River is deltaic and other marginal marine deposits, littoral to neritic sandstone, and offshore shale deposited during a southwestward transgression of the Cretaceous sea. Variations in thickness and facies of these two formations are explained by the encroachment of the sea and the shifting of the delta along its margin. Most oil is trapped in sandstone reservoirs behind updip permeability barriers. The sand is believed to have been deposited in meandering channels which are approximately parallel with present-day structural contours.