The Los Pinos Formation, consisting of upper Oligocene to upper Miocene volcaniclastic rocks with interbedded ash-flow tuffs, flow breccias, and basalt flows, is exposed in the Tusas Mountains of north-central New Mexico. The Los Pinos Formation contains two penecontemporaneous members: the Esquibel and the Cordito. The Esquibel Member is a detrital apron of intermediate volcanic clasts derived from the north, primarily the San Juan Mountains of Colorado. It thins southward. In the southern Tusas Mountains, it is locally absent or underlies the Cordito Member. The Cordito Member, composed of predominantly rhyolitic clasts, is present only in the southern Tusas Mountains and was derived from source areas to the east (the volcanic center near Questa and another center possibly buried beneath the Pliocene Taos plateau basalts in the Rio Grande rift). The Abiquiu Tuff of Smith (1938) is considered equivalent to the Cordito Member.