The Lower Permian Yeso Group in the Fra Cristobal and Caballo Mountains, Sierra County, New Mexico
— Spencer G. Lucas and Karl Krainer

Strata of the Lower Permian Yeso Group exposed in the Fra Cristobal and Caballo Mountains of Sierra County can be assigned to the Arroyo de Alamillo and overlying Los Vallos formations. The Arroyo de Alamillo Formation is 42-87 m thick and consists primarily of red-bed sandstone, which is very fine grained, silty, often gypsiferous and mostly thinly laminated or ripple laminated. Siltstone (mostly massive) is much less common, and there are a few beds of dolomite and gypsum in the Arroyo de Alamillo Formation. At Massacre Gap in the Fra Cristobal Mountains, the overlying Los Vallos Formation is ~ 234 m thick and can be divided into the Torres Member (~ 134 m of interbedded dolomite, gypsiferous siltstone, gypsum and siltsone to fine-grained sandstone), Cañas Member (~ 58 m of gypsum with lesser beds of gypsiferous siltstone and dolomite) and the Joyita Member (~ 42 m of red-bed siltstone and fine-grained sandstone) overlain by the Lower Permian Glorieta Sandstone. A generally similar but much covered section of the Los Vallos Formation is exposed in the McLeod Hills of the southern Caballo Mountains and directly overlain by the San Andres Formation (the Glorieta Sandstone is not present in the Caballo Mountains). However, at nearby Hidden Tank, the Los Vallos Formation is represented by only a 75-m thick section of the Torres Member. Six, dolomite-dominated intervals of the Torres Member can be readily correlated from the Yeso Group type section in Socorro County to the Fra Cristobal Mountains. Correlation farther south, into the Caballo Mountains is more difficult, due to facies and thickness changes within the Los Vallos Formation and stratigraphic relief of the unconformity at the base of the overlying San Andres Formation. Deposition of the Arroyo de Alamillo Formation took place by eolian and fluvial processes on an arid coastal plain during a time of regional low sea level. Deposition of the Torres and Cañas members of the Los Vallos Formation was in cyclically shallow marine and sabkha environments. Joyita Member deposition again records eolian and fluvial processes during a time of regionally low sea level.