We have traced a variegated claystone in the alternating facies of the Red Peak Member of the Chugwater (Triassic) Formation in an east-west direction for more than 73 mi in the southern part of the Bighorn Basin, Wyo. It averages 5.5 feet in thickness, ranging from 2 to 9 feet. The north-south extent is probably more than 110 mi. The base of the variegated claystone is from 98 to 138 feet below the base of the Alcova Limestone Member of the Chugwater Formation along a generally east-west cross section, possibly reflecting a thickening of the interval above the base of the variegated claystone toward the depositional axis of the former miogeosyncline. Contrary to published views, other persistent beds and sequences of regional extent occur in outcrops of the Chugwater Formation, as well as the widely recognized subsurface horizons.