Partial sections of the Paleozoic and Mesozoic sequence of southwestern New Mexico are exposed amid the Tertiary igneous rocks of the Tres Hermanas Mountains. Several hundred feet of the Silurian Fusselman Dolomite occur in a small fault block. Only the upper 360 ft of the Mississippian Escabrosa Limestone is exposed beneath 20-40 ft of the Paradise(?) Formation. The overlying Pennsylvanian is about 560 ft thick and consists of limestone, a few shale lenses, and a 60-ft thick sandstone unit. The Wolfcampian Hueco Formation is at least 525 ft thick, is erosionally unconformable on the Pennsylvanian strata, and is composed of lower chert-limestone conglomerates and upper dark gray fossiliferous limestones. Lower Cretaceous rocks are more than 1530 ft thick and consist of 4 lithic units: 1) lower 375 ft of chert conglomerate, sandstone, pale red siltstone and silty limestone; 2) 395 ft of massive limestone; 3) 425 ft of limestone conglomerate and reddish sandstone; and 4) upper 340 ft of sparsely fossiliferous limestone.