It is proposed that the Yates formation be redefined to include all the section between the Tansill and the Seven Rivers formations, and that Sumrall's Douglas well No. 5 in the Yates field, Pecos County, be designated as the type section. The Yates formation is divided on the basis of lithological dominance into carbonate, sandstone, anhydrite, and red siltstone facies. All contain frosted quartz grains of sand size; their uppermost occurrence ranges from 5 to 30 ft. below the top of the formation. The overlying Tansill sediments grade from dolomite to anhydrite and halite. The top part of the underlying Seven Rivers is dolomite, dolomitic anhydrite, and anhydrite. The sediments were deposited in a shallow, back reef evaporating basin. The clastic material was derived from the near parts of the southern and eastern shelves and distributed across this lagoonal base by water current and wave action. Yates anticlines, formed in post-Tansill time, reflect deeper and older anticlines, buried hills, and possibly, also, sand bar, reefs, and halite flowage.