The Recreation Redbeds of the Tucson Mountain Section is one of the important sandstone members exposed in the monument. Geologists believe that its deposition occurred early in the Cretaceous Period more than 65 million years ago. Above the Recreation Redbeds lies a formation consisting mainly of limestone and commonly known as the Tucson Mountain “Chaos”—an appropriate name, in view of the geologists’ meager knowledge of it. Within the Chaos formation, believed to be lower to Middle Tertiary in age, are limestone blocks of greater age than the rocks within which they are entombed. No generally accepted explanations have been advanced as to how these relationships developed.
The processes that destroy mountains continue concurrently with those that build them. Temperature changes, weathering, and downslope creep under the influence of gravity are among the agents which destroy the rocks and eventually convert them into soil. Though these processes work rapidly from a geological standpoint, in terms of the average human life span, they progress at an imperceptible rate. Violent thunderstorms and cloudbursts, however, cause a massive, often spectacular movement of boulders, gravel, sand, and silt by torrential streams.
In desert regions, most of the runoff from storms sinks into the slopes, dropping its burden along the way. Big rocks are dropped early as the carrying power of the water diminishes with the speed of flow; smaller fragments travel farther. The finest material is carried far out into the basins between mountain ranges, gradually filling them. (The alluvial material in the Tucson Basin is estimated to be 2,000 to 5,000 feet thick.) Thus desert mountains tend to bury themselves in their own debris.
Alluvial fans (fan-shaped deposits built by rivers flowing from mountains into lowlands) form at the mouths of canyons. Sometimes the alluvial fans of adjacent canyons coalesce, forming the long, sweeping slope known as a bajada (pronounced ba-HA-da). In other places the eroded bedrock extends outward from the bases of desert mountains, forming “pediments,” which are usually covered by a veneer of alluvial material. Pediments of this kind stretch from the lower slopes of the Rincon Mountains. In the Tucson Mountains the alluvial material deepens rapidly toward the Avra Valley because the bedrock at the base of the mountain dips steeply downward.
Serious gaps remain in the story of the origin and evolution of the landscape in Saguaro National Monument. Moreover, it is difficult to establish the exact sequence in which the various events occurred. The composition, texture, and relationships of the rocks, however, do reveal much about the nature of the events and the processes that were involved.
During summer, mule deer and javelina frequent King Canyon, which has the only known permanent spring in the Tucson Mountains. The predominant rock in this major canyon northeast of the Arizona-Sonora Desert Museum is called Amole Arkose; it is of sedimentary origin and was deposited during the Cretaceous period.
GEOLOGICAL CROSS-SECTION OF SANTA CRUZ VALLEY
Faults are hypothetical