Cover Image: Tonuco Mountain (a.k.a. San Diego Mountain)
— Jeffrey Amato
View looking northwest at Tonuco Mountain (a.k.a. San Diego Mountain), located along the eastern Rio Grande Valley between Las Cruces and Hatch in Doña Ana County, NM. Early Spanish settlers and explorers traveling between Chihuahua, Mexico and Santa Fe, New Mexico followed the Camino Real, which ran between the river and Tonuco Mountain in this area. The Tonuco Mountain horst is part of a structurally complex uplift with evidence for Neogene as well as older Laramide deformation (see Seager et. al, 1971, NM Bureau of Mines and Mineral Resources, Bulletin 97). The steep western side of the mountain includes interbedded volcanic, volcaniclastic and alluvial deposits of the Lower Santa Fe Group (including the Neogene Rincon Valley and Hayner Ranch formations), which are downfaulted against Proterozoic basement and overlying Paleozoic sedimentary rocks to the east. Fluorite and barite mineralization, largely in veins in Proterozoic rocks, were exploited to a limited degree at Tonuco Mountain during the early twentieth century. Pale-colored bluffs and cuestas eroded into the lower part of the Santa Fe Group are visible in the middle and foreground. The photo was taken during fieldwork in an area where exposures of the Upper Santa Fe Group (Plio-Pleistocene; Camp Rice Formation) have yielded an assemblage of late Pliocene vertebrate fossils (Tonuco Mountain Local Fauna), including a recently discovered early Blancan tapir, discussed by Morgan and others in this issue. Photo and caption by Jeffrey Amato, NMSU.