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Geology of Sonoran Desert National Monument - Sonoran National Monument is located within the Desert Region of the Basin and Range geologic province of southwestern Arizona.  The region is characterized by steep, rocky, alternating mountain ranges separated by broad, gently sloping to nearly flat, deep, broad valleys formed by faulting that occurred approximately 5 to 15 million years ago.  Most of the mountain ranges have been formed by faulting, flooding, or volcanism.  The broad valleys are generally underlain by thick deposits of gravel, sand, and silt.

The monument also contains numerous areas of rocky "desert pavement," where fine alluvial material has been removed.  Desert pavements are slow to form and extremely fragile.  A thin, hardened surface layer called "desert varnish" may occur on desert pavement.  Desert varnish acts as a cement, holding surface soils and protecting them from wind and water erosion.  When the crusty layer of desert varnish is disturbed, underlying soils are subject to erosion and compaction of underlying soils.  The time r3qured to form desert soils can range from hundreds to thousands of years.  Without such soil, most plant life cannot survive.

The "basin and range" country includes numerous high peaks and low valleys.  The 4,373-foot, basalt-capped Table Top Mountain dominates the topography on the east end of the monument.  Its flat-topped summit is easily seen and recognized as far away as the outskirts of Phoenix, 45 miles to the north, and Casa Grande, 20 miles to the east.

The Maricopa Mountains are lower, with one peak as high as 3,300 feet, but dozens between 2,000 and 3,000 feet in elevation.  The alluvial valleys near the Maricopas descend to as low as 800 feet above sea level.  The relief is similar in the Sand Tanks, where the highest point of Javelina Mountain (confusingly named "Maricopa Peak") reaches over 4,000 feet.  The wide, flat wash that cuts through the heart of the Vekol Valley descends only about 300 feet in elevation over 12 miles, from the monument's southern boundary to Interstate 8.


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