Tucson Mountains are an excellent example of the tilted, lava-capped ranges of southern Arizona. They consist of rocks comprising what appears to be one of the most complete columnar sections in the Basin Range province of Arizona. The study of the mountains has shown the presence of sediments ranging in age from Cambrian to Cretaceous, and Cretaceous, Tertiary, and Pliocene or Pleistocene effusives.
Laramide intrusives cut Laramide folds and a Laramide thrust fault, which is remarkable in its preservation. The fault was practically parallel with the early Tertiary erosion surface, and only a thin veneer of the overthrust block is preserved below the Tertiary lavas. The events in the historical sequence have been dated by the discovery of invertebrate remains in the Cretaceous sediments and plant remains in a Tertiary tuff.
The great reduction of the range by erosion and the burial of its outer edges have concealed the hypothetical Basin Range border faults, but the predominance of steep-angle normal faulting in the Tertiary rocks of the range indicates a similar structure of the buried borders. A complicated erosional history is indicated by the presence of pediments, probably of three different ages, and evidence for a former greater burial of the range in its own débris.