Structural Geology

Faults

Uplifting and downdropping, and horizontal slipping of crustal blocks in the Joshua Tree region has occurred along fractures (faults) in the earth's crust. The Pinto Mountain fault is one of the most prominent. The topographic break marking the fault zone is closely followed by the Twentynine Palms Highway (State Highway 62) between Morongo Valley and Twentynine Palms. The fault trends nearly east-west along the north side of the Pinto Mountains.

On the south side of the Monument the Blue Cut fault extends east-west through the Little San Bernardino Mountains, about one-half mile south of Keys View, under Pleasant Valley and into the Pinto Basin. The Blue Cut fault branches from the Dillon fault which is even further south and trends southeastward through the Little San Bernardino Mountains. The Blue Cut and Pinto Mountain faults are both left-lateral faults. They may belong to a conjugate fault set that includes the north-northwest trending right-lateral faults of the Mojave Desert (Powell, 1982, p. 109). Pleasant Valley is a graben formed by the Blue Cut fault and one of its branches (Photo 9).

Photo 9. A branch of the Blue Cut fault forms the steep, straight, southern edge of the Hexie Mountains where they border the playa of Pleasant Valley on the Geology Tour Road.

South of the Dillon and Blue Cut faults is the San Andreas fault zone. The trace of the San Andreas fault is clearly visible from Keys View (Photo 10). The San Andreas fault south of the Monument is divided into two main branches, the Banning and Mission Creek faults. The trace of these faults is marked by the Indio Hills, a small uplifted block that is wedged between the faults, and by a number of springs and palm oases along the faults.

Photo 10. The northern end of the Salton Trough from Keys View. The high peak in the distance is Mt. San Jacinto. 10,786 feet (3287 m) elevation. Palm Springs and Cathedral City are at the base of the mountains. The Indio Hills, running from left to right in the middle distance, mark an uplifted block wedged between two branches of the San Andreas fault.

In addition to the major faults are many hundreds of minor faults throughout the region. Fault zones are important factors in localizing springs. Movement by faults causes impervious zones of shattered rock fragments to form an underground dam, which forces ground water to rise. The oasis at Cottonwood Springs, for example, appears to be due to a fault zone which has provided the fissures along which ground water reaches the surface. The oasis at the Visitor Center at Twentynine Palms marks the Pinto Mountain fault.

Rock Jointing

Joints are simply small fissures cutting rocks. They may occur in sets of parallel joints and systems of two or more intersecting sets. The White Tank monzogranite has a system of joints that is primarily responsible for the spectacular landforms in the Monument. The joint system in the White Tank monzogranite consists of three dominant joint sets. One set oriented horizontally has been caused by the release of pressure due to the removal of overlying rocks by erosion. These joints, sometimes called lift joints, cause rock sheeting and are caused by expansion and the release of stress in rocks, somewhat analogous to a seat cushion resuming its shape after the person sitting on it arises. Lift joints form dome-like outcroppings where vertical joints are widely spaced (Photo 3).

Another set of joints is oriented vertically, and roughly parallels the contact of the White Tank monzogranite with its surrounding rocks. The third set is also vertical, but it is approximately perpendicular to the other vertical set. The resulting system of joints forms rectangular blocks. Especially good examples of the joint system are at Jumbo Rocks, Wonderland of Rocks, and Split Rock (Photos 4 and 11).

Photo 11. Inselbergs at Hidden Valley Campground display the joint system that is prevalent in the White Tank quartz monzogranite.

Joints are often closely spaced along fault zones, where either there may be no apparent order to their pattern, or the major joint set may closely parallel the orientation of the fault.

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