The California Desert can be
divided into three large geologic regions which geologists call geomorphic
provinces. The lands within each of these provinces shares a similar landscape,
geologic structure, and geologic history. Of California's 12 geomorphic
provinces, the three which cover nearly all of the California Desert are the
Basin and Range, the Mojave, and the Colorado Provinces.
BASIN AND
RANGE GEOMORPHIC PROVINCE
Covering the northern portion of the
California Desert Conservation Area (CDCA), this province extends from the
Sierra Nevada eastward into Nevada and ends at the Garlock Fault on the south.
This area is characterized by internal drainage since water does not flow out
of the region. Another key feature are the elongated, north-south oriented
mountain ranges and valleys. It includes some of the greatest relief in North
America with high desert ranges rising more than 10,000 feet above adjacent
valleys.
Garlock Fault This fault separates the Basin and Range
Province and the Mojave Desert Province. It has had at least 40 miles of
lateral movement and exhibits numerous surface offsets and disruptions.
Dry Lakes and Dry Stream Channels: During the last ice age (2
million to 10,000 years ago), extensive lakes and connecting streams occupied
valley floors. Today, with a warmer, drier climate, the valley floors now
contain salt flats, dry lake beds (playas), and intermittent streams (arroyos).
Many desert basins are rimmed with miles of remnant lake shorelines cut into
the surrounding ranges. An excellent place to observe these fossil shorelines,
beaches, and sandbars is in Searles Valley.
Trona Pinnacles: Over
500 tufa pinnacles, some up to 140 feet high, create a rugged, stark landscape
at the southwestern shore of Searles Lake. These columns formed under water
when a lake existed. They are composed of calcium which was concentrated and
deposited by algae. The algae thrived where underwater springs flowed into the
lake.
Eureka Sand Dunes: These dunes in southern Eureka Valley,
45 miles east of Big Pine, are the highest in California. Rising nearly 650
feet above the valley floor, they have developed where sand-laden winds lose
speed as they reach surrounding ranges and drop the sand to accumulate. Backed
by the rugged Last Chance Range, these dunes produce a very scenic
setting.
Redrock Canyon: Along State Highway 14, 26 miles north
of Mojave, is a series of colorful and rugged canyons. The canyons were created
when Ricardo Creek cut across rock layers as it was being uplifted along the El
Paso Fault. As the creek cut through the rising surface, several types of rock
were exposed, including lake sediments and volcanic materials. The exposed rock
has subsequently been eroded into scenic cliff faces.
MOJAVE DESERT
GEOMORPHIC PROVINCE
This province, covering the eastern portion of
the CDCA, is bordered by the San Andreas Fault, Garlock Fault, Nevada State
line, Colorado River, and the Riverside County line. Surface structure is
related to movement along the San Andreas and Garlock Faults. Geologic |
features in this province include
faulting, volcanism, and a large variety of landforms produced by erosional and
depositional forces. Until development of the Garlock Fault approximately 35
million years ago, this province was part of the Basin and Range Province.
Today, the region is dominated by broad, sediment filled basins that are
burying the older, more mountainous landscape. Just as in the Basin and Range
Province to the north, this province has many striking, parallel faults
oriented northwest to southwest. However, the more dominant fault the movement
is lateral, and very little vertical displacement is occurring to push the
earth's crust upward. Since new mountains are not being formed and older
mountains are being eroded, this region is being buried in alluvial sediment.
Cima Dome and Cima Volcanic Field: This unique high desert
landscape lies within the Bureau of Land Management's 1.4 million-acre East
Mojave National Scenic Area between I-15 and I-40. The Dome itself is a
granitic pediment (erosion surface) covered with areas of lava and volcanic
cones. This elevated area of high desert has been important geologically in the
study of desert erosion and how uplifted slopes retreat in an arid environment.
The Dome is believed to have formed from an upwarping of the crust when igneous
(granitic) rock rose into the area 100 million years ago.
Volcanic
Cones: An interesting alignment of volcanic cones exists from Barstow to
Amboy. These include the Newberry, Pisgah, and Amboy cones.
Lake
Manix: In the last glacial period, a large lake filled the broad basin
around Baker on I-15. This former lake, named Lake Manix, covered about 200
square miles and has left many fossil lake features. Much evidence of
prehistoric man has been found around the ancient shoreline.
Kelso
Dunes and Devil's Playground: These dunes, whose tallest peak is 560 feet
high, cover a large area south of Baker. The sand here was derived from the
Mojave River and sand bars left by Lake Manix. These dunes have been called
"booming dunes" because of the deep, rumbling sound they make when sand slides
occur.
COLORADO DESERT GEOMORPHIC
PROVINCE
This province covers the southern region of the CDCA. It is
bounded by the Colorado River, Mexican border, Peninsular Ranges, and the San
Bernardino County line. The principal structural features are the Salton Trough
and numerous parallel fault lines which transect it. The Salton Trough is a
large crustal depression extending from Palm Springs to the Gulf of California.
The San Andreas Fault extends through the depression, and along with other
parallel fault lines, has created the down-dropped block.
Orocopia
Mountains: The rocks in this geologically important range give striking
evidence of the lateral land movement along the active San Andreas Fault. An
unusual grouping of ancient rock types which occurs in the Orocopia Mountains
is exactly duplicated in the |