In the Sangre de Cristo Mountains of northern New Mexico, Precambrian rocks are overlain by a thin but complex sequence of sandstone and limestone of pre-Pennsylvanian age. These rocks, formerly classed variously as the lower limestone member of the Sandia formation, and the Arroyo Penasco formation, are here divided into 2 new formations on lithologic bases. The lower formation, consisting of sandstone, sandy dolomitic limestone, and crystalline and clastic limestone, is here named the Espiritu Santo formation. The Espiritu Santo formation ranges in thickness from a trace to more than 80 ft., with an irregular but general increase in thickness from S. to N. No fossils of undoubted stratigraphic significance have been found in this formation. On the basis of stratigraphic position and lithologic similarity to rocks of Devonian age in Colorado, the Espiritu Santo formation is classified as Devonian(?). Overlying the Espiritu Santo formation, with widespread erosional unconformity, is a sequence of limestone breccia and conglomerate, crystalline limestone, and calcarenite here named the Tererro formation. The Tererro formation has been divided into 3 members. In ascending order these are the Macho, Manuelitas, and Cowles members. The Macho member is composed of limestone breccia derived in large part from solution and collapse of cavernous parts of the upper part of the Espiritu Santo formation, and in minor part from collapsed parts of the overlying Manuelitas member. The Manuelitas member rests unconformably on the Macho member and is composed of crystalline limestone, limestone pebble and cobble conglomerate, and calcarenite. At many places these beds were involved in collapse into large sinkholes in the Macho member and Espiritu Santo formation during deposition of the Manuelitas member. The Cowles member rests unconformably on the Manuelitas member and is composed of a lower unit of calcarenite and an upper unit, present locally, of marly quartz siltstone and intercalated limestone beds. The Tererro formation ranges in thickness from a trace to 130 ft. in the southern part of the Sangre de Cristo Mountains and from 6 to 48 ft. thick farther N. A sparse faunule found in the Manuelitas member indicates Early Mississippian age for that part of the Tererro formation. The Tererro formation is overlain with erosional unconformity by sandstone and coaly shale beds of the Sandia formation, as here restricted, of Early Pennsylvanian age.