Mojave Desert, Joshua Tree
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Mojave Desert, Mojave Desert Land Trust


rocks

rocks

rocks

Photos ©
Drew Reese

Desert Landscape

The California desert is a land of contrasts – undulating sand dunes, jagged volcanic cinder cones, astounding rock formations, and many other scenic wonders.  No single geologic process is at work; rather the combined effects of water, sun, wind, biological and tectonic forces.  Because these factors combine in different ways in different places, the appearance of the California desert landscape varies as well.  29 million acres, or 28% of California’s landmass, is desert, with elevations ranging from 250 feet below sea level to nearly 12,000 feet.

This alluring landscape is characterized by unique landforms that heighten our curiosity and enlarge our understanding of geologic time.  Some characteristic desert landforms are listed below:

  • Alluvial Fans form cone shaped deposits where canyons meet basins.  California deserts are famous for dramatic alluvial fans at the base of steep mountain ranges, largely composed of debris flow deposits.

  • Arroyos or Washes are cursory stream courses that may contain water only a few hours or perhaps a few days each year during the rainy season; usually have a flat, gravel-strewn floor.

  • Bajadas form where several alluvial fans coalesce across a mountain front.

  • Basin is a depression in the earth’s surface that collects sediments.  The basins in the California Desert are controlled by faults and locally receive sediments from the adjacent uplifted mountains. Regionally, when the climate was wetter, rivers drained from distant mountains into the desert. The 110 mile long Mojave River, rising in the San Bernardino Mountains, delivered its sediments to Soda or Silver Dry Lake near the town of Baker.  The 185 mile long Amargosa River, rising in Nevada and the eastern Sierra, terminated in today’s Death Valley.

  • Buttes and Mesas form due to tectonic uplift.  Capped by resistant rock, typically basalt, these tall narrow ridges jut up from the basin floor in dramatic fashion.  Mesas form similarly, having a base broader than they are high.

  • Desert Pavement develops on surfaces no longer experiencing active deposition.  Wind erosion and soil formation processes take over.  Over tens to hundreds of thousands of years the space between pebbles and cobbles (clasts) decreases.  On young pavements, some vegetation grows in the space between clasts.  On well-developed pavements, tightly packed clasts form a smooth surface, lacking or devoid of vegetation.  Desert varnish darkens as the surface ages.

  • Desert varnish, also called rock varnish, is a thin biogenic layer partly composed of manganese and iron oxides, which forms a coating on rocks in arid regions.  Many petroglyphs are created by chipping through the dark coat of varnish to expose a lighter colored underlying rock.  Because the varnish registers cumulative biologic activity over time, there is great interest in finding a method to reliably age date the varnish.

  • Dunes are an accumulation of wind transported sediment.  Stand alone dunes are seen in basins and climbing dunes lap up against a mountain range.  The Kelso and Algodones Dunes are the largest dune fields in the California Desert.

  • Inselbergs are the resistant bedrock cores of ancient hills.  They are the distinctive features of Joshua Tree National Park.

  • Pediments are gently sloping exposed bedrock surfaces that extend outward and away from the base of desert mountain ranges.  They resemble bajadas, but are carved features that are eroded on the bedrock surface; their lower margins may merge with bajadas.

  • Playa is a basin that is seasonally or intermittently wet.  The wetting and drying process frequently brings salts, such as calcium and sodium, to the surface.  Salt mining is an economic activity in Searles Dry Lake, Bristol Dry Lake, Dale Dry Lake and others.  Playas are the largest contributor of dust (PM 10 size) into the atmosphere.

For further reading on the forces that shape the California Desert, please visit the following pages:

 
Painting © Diane Best
Landscape painting of the great Mojave Desert by Diane Best.

© 2016 Mojave Desert Land Trust. P.O. Box 1544 • 60124 29 Palms Hwy., Joshua Tree, CA 92252 • (760) 366-5440 • Fax (888) 869-4981 •
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