The Sundance formation of Wyoming contains a stratigraphic break which gives evidence of the result of a period of erosion during Divesian time. The lower portion of the Sundance formation is believed to be equivalent to part of the Twin Creek formation of western Wyoming and to part of the lower Ellis formation of northwestern Wyoming, as it was originally described. The upper portion of the Sundance formation is believed to be in part equivalent to the lower Beckwith of western Wyoming (the Preuss and the Stump formations of southeastern Idaho), and to the upper portion of the original Ellis formation of northwestern Wyoming. Oil production from the Sundance is found in anticlinal structures which lie in a belt slightly more than 100 miles broad trending in a northeast- outhwest direction across the eastern portion of Wyoming. This belt is roughly parallel to the shore line of the Logan (Sundance) sea. Theories to account for the distribution are (1) "up-dip" migration before concentration in anticlines, (2) biochemical relationships, and (3) textural differences. Flushing appears to have played an important part in some structures.