Landforms of the Desert

There are major differences between the landforms in arid and in humid regions. This is because:

Typical arid landforms encountered in the high desert are:

(1) arroyos or dry washes, stream courses that contain water only a few hours or perhaps a few days per year;

(2) playas, lakes that may contain water a few weeks a year during the rainy season;

(3) alluvial fans, fan-shaped deposits of sediment formed at the base of mountains in arid regions;

(4) bajadas, the broad sloping aprons of sediment that result from the coalescing of many alluvial fans;

(5) pediments, erosional features on gently sloping bedrock surfaces that have been carved along the base of desert mountains.

Pediments are a curious desert landform, typical of the southwestern United States and many other desert areas of the world. Superficially, pediments look like bajadas (depositional features) rather than products of erosion of bedrock. The slopes of pediments are slight, from 1/2° to about 6° and they are usually carved on homogeneous crystalline rock such as granite. Pediments may be covered with a thin mantle of gravel, but if more than ten feet of gravel cover a pediment the resulting landform is considered depositional and is called a bajada. In order to determine whether the sloping surface is a pediment or a bajada the thickness of the gravel veneer in the drainage channels must be observed.

Apparently pediments are formed by the retreat of the mountain front leaving an extensive bedrock surface that marks the path of the retreating foot of the slope. Rill wash, sheetfloods, winds, and lateral planation by streams sweep the pediment clean of debris except for local accumulations of gravel.

A pediment can be seen at Malapai Hill (Stop 6 on the "Geology Tour Road" through the Park). Large expanses of bare granite pavement and bold dikes weathered out of the granite are exposed on the surface of the pediment (Photo 14 and 15).

Photo 14. Pediment on the Geology Tour Road through Joshua Tree Monument. The pediment is eroded on the White Tank monzogranite. Malapai Hill is in the distance.

Photo 15. Remnants of an aplite dike exposed in monzogranite on the pediment near Malapai Hill.

Some investigators regard pediments as the only true desert landforms which can be attributed solely to arid conditions operative at present (Bradshaw and others, 1978, p. 307-308). Others regard pediments as features that have evolved in a sequential manner over a period of perhaps several million years (Ollier, 1975; Oberlander, 1972). At issue are the relative roles of past and present processes in explaining the development of these arid region landforms.

The origin of pediments may be closely linked to the origin of inselbergs, prominent steep-sided residual hills and mountains rising abruptly from erosional plains. Inselbergs studied in Uganda are thought to be residuals of deep chemical weathering during the more humid environments of the late Tertiary and Quaternary Epochs (Figure 3A) (Ollier 1975, p. 206-207). Subsurface weathering is more intense in areas of closely spaced jointing but less so in areas of wider joint spacing. Pediments are developed by removal of these deeply weathered rock materials leaving the sparsely jointed rock residuals as inselbergs.

Figure 3. Two theories of pediment and inselberg development.

A. Pediment and inselberg development in Uganda: (after Ollier, 1975)

(1) Subsurface jointing in the original substrate.

(2-4) Deep and complete weathering of the rock with closely spaced joints, but unconsumed cuboidal blocks in regions of widely spaced joints.

(5) Removal of weathered rock leaves pediments and inselberg remnants.

B. Pediment and inselberg development in the southwestern United States from a combination of deep weathering of a horst upland, stream erosion, and rising base level in the adjacent down-faulted basins (after Garner, 1974, and Bradshaw and others, 1978).

The origin of inselbergs in Uganda is not totally applicable to the deserts of southwestern United States where, unlike Uganda, tectonism has been active for millions of years up to the present. Tectonism has created fault block mountain ranges and downdropped basins. The internal drainage of the basins causes the basins to fill gradually with sediments derived from the adjacent uplands. As a result the local base level of erosion slowly rises. Possibly stream erosion with rising local base levels is important in forming pediments in the Mojave Desert (Figure 3) (Garner, 1974; Bradshaw and others, 1978).

Climatic conditions during the late Tertiary and the Pleistocene must have been significant in the development of pediments and inselbergs. The present climate of this region is relatively new, having been established during the Pleistocene Epoch which began only about 2.5 to 3 million years ago. Botanical evidence indicates that progressive deterioration of vegetative cover took place throughout the Mojave Desert during the Miocene and Pliocene (from about 25 million to about 3 million years ago) (Axelrod, 1950; 1958).

The change in climate and the corresponding change in plant cover left increasing areas of surface unprotected by vegetation which promoted accelerated denudation of the soil. Furthermore, the renewal of soil during the Pleistocene Epoch was slowed by decreased rainfall causing the rate of soil erosion to exceed the rate of soil formation.

Eight million years ago the landscape of the Mojave Desert was one of rolling hills covered with a soil mantle that had developed in a hot, semi-arid to humid climate. At that time the rates of soil formation and soil erosion were closely balanced. The climate and the amount of vegetative cover then were similar to that existing today along U.S. Highway 395 between Temecula and Escondido (Oberlander, 1972).

Increased erosion removed the residual soils from the steeper hillsides leaving behind the subangular and spheroidal boulders that formerly had been the subsurface corestones which had been isolated by chemical decomposition along joint planes (Figure 4). These corestone features, called boulder mantled slopes (Oberlander, 1972), can be seen along the road between the northwest entrance to Joshua Tree National Monument and Hidden Valley Campground (Photo 16).

Figure 4. Schematic diagram illustrating the formation of inselbergs at Joshua Tree National Monument.

A. Vertical section through granitic rocks with a varied spacing of joints some 20 million years ago.

B. During the Pliocene after a period of sub-humid climate and decomposition of the rock by ground water that percolated downward along joints to the water table. Rotted and decomposed rock is shown in black.

C. Boulder-mantled slopes developed in the past few tens of thousands of years of the Pleistocene Epoch by the removal of the decomposed rock under arid conditions. Present day examples: along the Fortynine Palms Oasis trail and along the highway between the town of Joshua Tree and Hidden Valley Campground.

D. The present. In higher elevations with longer exposure to conditions of arid weathering, the boulder mantle has been largely decomposed leaving steep-sided bold outcrops rising abruptly above the surrounding surface. A thin veneer of grus covers the horizontal surface. Examples are at Hidden Valley, Caprock, Ryan Campground, and Jumbo Rocks.

Photo 16. Boulder mantled slopes along the road between Joshua Tree City and Hidden Valley.

The boulder mantles gradually crumble away in the present arid climate leaving inselbergs, the cores of relatively unweathered, sparsely jointed granite that form the spectacular prominences at Hidden Valley, Cap Rock, Jumbo Rocks, and along the Geology Tour Road (Photos 9 and 10). The presence of these masses of undecomposed rock is evidence that the renewal of boulder mantles by present-day weathering processes is not taking place. Thus, the granitic landscape in Joshua Tree National Monument, and elsewhere in the Mojave Desert, is a fossil landscape which has evolved over a time span of several million years (Figure 4).

Evidence for this interpretation comes from sites in the Mojave Desert such as at Old Woman Springs where reddish iron oxide and calcite-rich soils and corestones in a grus matrix have been preserved beneath remnants of lava flows. The lava flow at Old Woman Spring has a radiometric age of eight million years. Similar soils form today in warm regions under the cover of heavy brush where the average rainfall exceeds 10 inches annually. Continuity between these relict soils, corestones, and grus beneath the basalt remnants and the present boulder-mantled slopes clearly establishes the boulder mantle as a feature inherited from a time of deep weathering in the late Tertiary Period (Figure 5).

Figure 5. Diagrammatic sketch of soil conditions at Old Woman Spring. After Oberlander, 1972.

A. Immediately after the Pliocene episode of volcanism about 8 million years ago.

B. Present conditions showing the paleosol (ancient soil) protected from erosion by the remnant of the 8 million-year old basalt flow, and the boulder-mantled slope of corestones that formed beneath the ancient soil.

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