Formations of Mississippian age are exposed along the west face of the Sacramento Mountains of New Mexico from a short distance north of Alamogordo southward to the vicinity of Grapevine Canyon. Detailed sections have been measured and the faunas collected throughout the entire area of exposure. The name Caballero is proposed for the gray, nodular, marly limestone formation of Kinderhook age lying at the base of the Mississippian section. The Caballero formation has heretofore been considered as a part of the Lake Valley formation. The Lake Valley formation has been divided into three members, Alamogordo at the base, followed by Arcente, and capped by Dona Ana. The fauna of the Caballero formation is closely related to that of the Chouteau formation of the upper Mississip i Valley region. The Lake Valley formation is entirely of early Osage age. Both Fern Glen and lower Burlington faunas can be recognized. Spectacular, large bioherms characterize the Alamogordo member, making it necessary to subdivide the member into several facies.