A thin sequence of Cambrian and Ordovician beds lying on Precambrian granite and overlain by Pennsylvanian Magdalena beds exists at the Eaton ranch locality along the southeastern edge of the San Mateo Mountains. The section includes representatives that are widespread in southern New Mexico: the Bliss formation, El Paso group, and Montoya group. This is the northernmost known occurrence of these rocks in south-central New Mexico. The occurrence suggests that Cambrian-Ordovician strata originally may have extended more directly northwesterly toward similar rocks in northeastern Arizona and northwestern New Mexico. The position of the Montoya on very low beds of the El Paso emphasizes the mid-Ordovician unconformity in the region.