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Vegetation/environment relationships in a lithologically diverse locale in the northern Sonoran Desert were examined to determine whether variation in slope angle and soil texture associated with bajada sequences, which is commonly cited as a major control of vegetation patterns at a local scale, retains its dominant expression at a sub-regional scale. For ninety-nine sites sampled in Organ Pipe Cactus National Monument, Arizona, slope angle and soil texture showed a strong relationship to overall vegetation patterns expressed by ordination and classification analysis. Sites were ordered along the first ordination axis from rocky hillslopes with coarse-textured soil to lower bajadas and alluvial flats with relatively fine-textured soil. The classification analysis generated compositionally distinct groups of sites that tended to occupy either rocky uplands or alluvial flats. Furthermore, slope angle consistently showed a significant relationship to the distribution of the individual species modelled with regression analysis. Variation in geologic substrate and aspect were secondary controls of vegetation patterns. The second ordination axis was interpreted as a nutrient gradient associated with substrate variation in the monument (volcanic versus granitic bedrock). Granitically derived soils tended to be lower in magnesium and coarser in texture than volcanically derived soils. The upland groups of sites generated by the classification analysis were subdivided on the basis of lithology and, to some extent, aspect. Available magnesium, pH, and aspect were significant explanatory variables in distribution models of many common species. Although the influence of the bajada sequence is apparent at the relatively broad scale adopted in this study, variation in lithology and aspect must be included to account for vegetation patterns observed at a sub-regional scale.