ABSTRACT: The Reef Trail Member, Bell Canyon Formation, represents the uppermost subdivision of the Guadalupian Series (Capitanian Stage) of theMiddle Permian in its type area. Previouswork in the PattersonHills and in other parts of theDelaware basin have produced conodonts (e. g., Clarkina hongshuiensis) that provide a direct biostratigraphic correlation to latest Capitanian age rocks in the Lengwuan Stage (=Capitanian, latest Guadalupian, Middle Permian) in China. The conodont species Clarkina postbitteri, whichmarks the basal part of the Lopingian Series (Upper Permian), has not been found in theMiddle Permian ofWest Texas. For the first time we describe complete detailed sections of the Reef Trail Member in order to provide a lithostratigraphic framework on which to base further biostratigraphic studies.We designate and describe a formal primary reference section for theReef TrailMember because of the absence of a complete section in the type area.We also describe another complete section and two other important but incomplete sections, all in the Patterson Hills. The strata of all of the sections described were deposited in a basinal setting near the toe-of-slope, and are dominated by carbonate materials. Nearby outcrops contain toe-of-slope debris that provide evidence of mass wasting of the Capitan reef immediately prior to onset of evaporite deposition of rocks of the Castile Formation in the basin. Biostratigraphic and lithostratigraphic evidence indicate that evaporate deposition occurred simultaneously throughout the Delaware basin.