California Division of Mines and Geology
Self-Guided Geologic Tour in Joshua Tree National Monument
Joshua Tree National Monument is a popular southern
California tourist attraction. It is also a favored area to teach
beginning earth science students about geologic processes. The Geology
Tour Road log in this article is adapted from an eight page self-guided
tour brochure titled Geology And Man (Wanrow, 1975), and is
reprinted in part with permission of the author and the National Park
Service. This brochure can be purchased for $0.25 at Monument Visitor
Joshua Tree National Monument was established in 1936
and contains 870 square miles, most of which is high desert. The
Monument encompasses some of the most interesting desert geomorphic
features in California. Rugged mountains, prominently exposed granitic
monoliths, and exposed fault scarps reveal how geologic processes
shaped, and continue to shape, this stark desert landscape (Photos 1 and
2). Much of the Monument is above 4,000 feet and steep mountain
escarpments border this region to the north and south. The Monument is
located about 140 miles east of Los Angeles and is accessible by highway
from the north and from the south (Figure 1).
Photo 1. View to the northeast along the main
Monument road at Covington Flats showing outcrops of the Cretaceous
White Tank monzonite. Note the horizontal pattern of joints in the
outcrop to the left. Photos by author except as noted.
Photo 2. Southward view illustrating typical
exposures of rounded and jointed White Tank monzonite in the Monument.
Photo courtesy of the National Park Service.
Figure 1. General location map of Joshua Tree National Monument.
The name, Joshua Tree National Monument, is derived
from the extensive stands of conspicuous Joshua trees that grow within
its boundaries. Joshua trees grow at elevations of 2,000 to 6,000 feet
and have keenly pointed bayonet-like leaves that bristle at the end of
large club-like branches. Until young Joshua trees attain a height of
six feet, their trunks have leaves that reach the ground. As the plant
ages and grows in height, these first leaves droop, die, and become
closely pressed into a thatch-like mat that covers the trunk and limbs.
Some trees have reached heights of 54 feet. The Joshua tree is one of
four species of yuccas that are native to California (Vasek and Barbour,
1988). Its unusual and sometimes bizarre shapes have made it a symbol
of the California high desert (Photo 3).
Photo 3. Joshua tree forest at the Monument.
Much of the Monument is part of the Transverse
Ranges, however its easternmost portion extends into the Mojave
Desert. There are several distinct mountain ranges within the Monument
(Figure 2). Valleys between these mountain ranges were formed by two
different processes: (1) some valleys were formed over geologic time by
erosion, and (2) some valleys were formed by down-dropped motion along
faults that formed basins (called "graben valleys"). Queen Valley, at
4,400 feet in elevation in the central part of the Monument, is an
example of a valley formed by erosion (Photo 4, Figure 2). Pleasant
Valley, located between the Little San Bernardino and Hexie mountains,
is an example of a graben valley formed by faulting. Both valleys are
discussed in the Geology Tour Road log (Figure 2).
Photo 4. View north across Queen Valley to the Pinto
Mountains. This valley was caused by extensive weathering of the
Figure 2. Location maps showing some geomorphic
features in the Monument and the route of the Geology Tour Road.
Last Updated: 15-Sep-2011