November 2025 Tidbits from the Museum Series

Tidbits from the Museum Series

Episode #1: Rio Rico Geology

FROM:  Chris Novak

The Bajada: The Hills and Ravines of Rio Rico

The distinctive landform features we see around Rio Rico are the bajadas. A bajada is a series of confluent alluvial fans at the base of a mountain range.

As mountains are formed from the Earth’s crustal extension, faulted mountain blocks form. These faults are a zone of weakness to erosion and form saddles, which are dips between ridges. These zones of pulverized rock are now to suspectable to forming gullies. Throughout the course of time, flash flooding and normal monsoonal rains with the assistance of gravity, sand, gravels and boulders are eroded out of the mountain along the zone of weakness, forming a deeper drainage.  When this debris, in combination with gravity, are rapidly moving downslope, when suddenly its energy is depleted all this detritus spreads out like a fan at the base of the mountain. This feature is called an alluvial fan. Confluent alluvial fans form our bajada (Spanish for lower or descent).

One can see these gravels, sand and boulders in road cuts, in canyons like Peck and Agua Fria. Rio Rico has one of the best examples of the bajada feature outside of Death Valley.

Board Member and geologist, Chris Novak

Photo top: Cross section of road cut showing alluvial fan including sand, gravel to boulders (Rock Corral drainage)

Bottom photo: Graphic depiction of alluvial fans forming a bajada.