Both Aridisols and Mollisols are mapped in part of Presidio County as part of the progressive soil survey. Aridisols are currently mapped on alluvial fans and fan remnants from 800 to 1300 m in elevation with an estimated average annual precipitation of 300 to 350 mm. Mollisols are primarily mapped in areas of higher elevation (1400 to 2000 m) with increased precipitation (350 to 400 mm). Vegetation is tentatively used to distinguish between ustic aridic and aridic ustic soil moisture regimes. Studies along transects on alluvial fans of similar age and lithology indicate that almost all soils have a relatively shallow petrocalcic horizon. Argillic horizons occur in most Mollisol pedons but are absent in Aridisol pedons.