13.6: Erosional Desert Land Forms

Alluvial Fans and Bajadas

Erosional processes are powerful forces that shape the land in the desert. These features can be considered as depositional (composed of sediment accumulations) or erosional (remnants of bedrock). Many of the desert landforms in this region were formed by processes active today, but many are remnants of the past.

As streams emerge into the valleys from the adjacent mountains, they create desert landforms called alluvial fans. When the stream emerges from the narrow canyon, the flow is no longer constrained by the canyon walls and spreads out. At the lower slope angle, the water slows down and drops its coarser load. As the channel fills with this conglomeratic material, the stream is deflected around it. This deposited material deflects the stream into a system of radial distributary channels in a process similar to how a delta is made by a river entering a body of water. This process develops a system of radial distributaries and constructs a fan shaped feature called an alluvial fan. Alluvial fans continue to grow and may eventually coalesce with neighboring fans to form an apron of alluvium along the mountain front called a bajada (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): A large bajada developed at the base of the Hexie Mountains in the southern Mojave Desert, Southern California. The bajada forms a continuous sloping alluvial fan blanket that marks the transition between the mountains and the adjacent basin. “Bajadas coming down from the Providence Mountains. Seen from Joshua Tree, California” by G310MEAGHAN, is licensed under CC BY-SA 3.0.

Query \(\PageIndex{1}\)

Pediments and Inselbergs

As the mountains erode away and their sediment accumulates first in alluvial fans, then bajadas, the mountains eventually are buried in their own erosional debris. Such buried mountain remnants are called inselbergs, “island mountains,” as first described by the German geologist Wilhelm Bornhardt (1864–1946). These features are quite common in these desert provinces, and are found throughout the Mojave and Colorado Desert Provinces (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Inselbergs in Joshua Tree National Park. Inselbergs of granitic rock stand as higher hills and pillars among the low lying desert that surrounds them. “Lost Horse Valley from top of Ryan Mountain” by USGS, is licensed under public domain.

The “ocean” that these islands rise from is made up of pediments. A pediment is a gently sloping erosion surface or plain of low relief formed by running water in an arid or semiarid region at the base of a receding mountain front. As the front of the mountains is eroded by physical and chemical weathering, it retreats or is worn backward; a pediment is left on the land that the mountain front once occupied.

Large areas within the Mojave Desert are pediment surfaces. These pediments reflect both the antiquity of some mountain structures in the region and the persistent arid climatic conditions in the region. Perhaps the most notable pediment in the region is Cima Dome, a very broad, shield-shaped upland area within the Mojave National Preserve (Figure \(\PageIndex{3}\)). This great, gently-sloped upland area represents a region where desert-style weathering and erosion has stripped away most of the relief to the point that the erosion keeps pace with surface weathering. The surface gradient is gentle enough to prevent gully-style down cutting.

Figure \(\PageIndex{3}\): The broad, gently sloped Cima Dome is an example of a pediment surface. The Cima Dome can be seen on skyline behind the relatively flat desert surface in the foreground. The flat plain in the foreground is also a pediment with a thin veneer of alluvium. "Cimadome" by LCGS Russ, is licensed under CC BY 3.0.

In the Mojave Desert, many of the pediment surfaces are developed in granitic rocks, possibly due to the more-or-less uniform weathering and erosion characteristics of these large, homogeneous granitic intrusions.

Spheroidal Weathering

Some of the granitic bedrock exposed as inselbergs or elsewhere in this desert environment exhibits rounded shapes that are typical of spheroidal weathering. Spheroidal weathering is a type of exfoliation that produces rounded features and is caused when chemical weathering moves along joints in the bedrock. Imagine a trickle of rainwater or groundwater slowly moving along fractures within granite. As the water comes into contact with a potassium feldspar, a common mineral which is known for the ease with which it reacts with water and hydrogen, a new, soft clay mineral (kaolinite) replaces the feldspar. Kaolinite tends to build up around edges of the granite. There is an abundance of surface area for the clay to build up in because of the abundant joints, or cracks, in the rock. Because the clay is soft, the clay-rich edges of the granite erode and become softer and rounded (Figure \(\PageIndex{4}\)). Eventually, this process can round out large amounts of rock in strange and picturesque ways (Figure \(\PageIndex{5}\)) . Thin, rounded concentric sheets of rock also have a tendency to peel off in spheroidal masses.

Figure \(\PageIndex{4}\): The process of spheroidal weathering. Fractures in bedrock are infiltrated by water, CO₂, and organic acids causing chemical weathering. This work by Phil Stoffer is in the public domain.

In humid regions, spheroidal weathering of granite typically occurs in the subsurface, as the bedrock breaks down into soil. In contrast, in arid regions the rate of chemical weathering is slow relative to the rate of surface erosion. As a result, in granitic terrains knob-shaped outcrops and spheroidal blocks accumulate on the surface. In the Mojave region, granite typically breaks down to form fairly uniform quartz and feldspar-rich, coarse-sandy sediments.

Figure \(\PageIndex{5}\): Spheroidally weathering granitic rocks in Joshua Tree National Park. Joshua trees in the foreground are ~3 meters tall. This work by Allison Jones is licensed under CC BY 4.0.

References

1. Dohrenwend, J. C. (2007). Tertiary and Quaternary Deposits. In Geology and Mineral Resources of the East Mojave National Scenic Area, San Bernardino County, California (pp. 73-75). USGS.
2. Geology of Joshua Tree National Park | U.S. Geological Survey. (n.d.). USGS.gov. Retrieved September 3, 2023, from https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-joshua-tree-national-park
3. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
4. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2023). An Introduction to Geology. Salt Lake Community College. https://slcc.pressbooks.pub/introgeology/
5. Natural History of the Granite Mountains. (n.d.). Sweeney Granite Mountains. Retrieved September 2, 2023, from https://granite.ucnrs.org/natural-history-of-the-granite-mountains/
6. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved August 31, 2023, from https://pubs.usgs.gov/of/2004/1007/intro.html