| 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.