Recognition of lateral facies changes in the Wolfcampian, Leonardian, and early Guadalupian series of the Glass Mountains is of aid in interpretation of the tectonic features. New interpretations suggest that the western part of the Glass Mountains is underlain by a hitherto unrecognized thrust sheet or numerous thrust slices which lie beneath the Dugout Creek thrust. This thrust sheet became an area of shelf deposition during late Wolfcampian, Leonardian, an early Guadalupian time and shifted the southeastern edge of the Delaware basin northward. The early Leonardian reefs apparently separated this shelf area from basinal deposits to the NE. During Leonardian and early Guadalupian time the southeastern margin of the Delaware basin migrated to the NW. so that in the northeastern Glass Mountains the earl Leonardian basin facies is overlain by late Leonardian reefs which in turn are overlain by early Guadalupian backreef deposits. Wolfcampian units are the Neal Ranch and Lenox Hills formations, Leonardian units the Leonard Formation whose lower two-thirds intertongues with the Hess, and lower Guadalupian beds are in the Word Formation with its northeastern upper member, the Vidrio biohermal dolostone. The depositional patterns of the Lenox Hills, Leonard, and Word formations show that the southwestern part of the Glass Mountains area was the site of shallow-water high-wave-energy deposition on a shelf. Reefs or biohermal deposits in each of these units are concentrated either on or adjacent to the flexures caused by late orogenic cross-folds. These flexures apparently were persistent features bounding the southeastern end of the Delaware basin.