Soil Surfaces: Desert Pavements

Large, flat areas devoid of vegetation and covered by a layer of tightly packed small stones are conspicuous features of extremely arid landscapes. These desert pavements are rare or absent in the moister parts of the Sonoran Desert, but become increasingly pronounced in the driest parts (see the photo below ).

 

Closeup of tightly packed stones in a desert pavement. The dark stones are varnish-covered pieces of volcanic rhyolite; the white pieces are quartz on which varnish usually does not form.

Some of the most extensive and well-developed areas of desert pavements occur on stony alluvial fan deposits flanking the rugged, low mountains in the extremely arid lower Colorado River Valley. Geologically young deposits (Holocene-aged, less than 11,000 years) lack the flat-surfaced pavements. Surfaces of these young deposits are typically cluttered with large stones and rocks irregularly piled in elevated bars; these low bars are separated by intervening swales. This stony jumble of bars and swales is the topographic imprint of the surface’s creation by the powerful tumult of moving water laden with rocky debris. Over time, though, this imprint disappears as the vertical relief of these coarse, rocky deposits is leveled out, eventually forming the flat pavement of small stones. The best-developed pavements are those that have formed over the passage of several tens of thousands to a few hundreds of thousands of years.

Research conducted within the last fifteen years in the Mohave Desert by a team of soil scientists and geologists (L.D. McFadden, S.G. Wells, and M.J. Jercinovich) provides a detailed picture of how desert pavements form on rocky parent materials such as these. Physical weathering of the large rocks on the surface produces the smaller stones that eventually form the pavement surface. These smaller stones tend to accumulate in topographic lows on the original, uneven surface. Special soil characteristics found directly beneath pavements provide clues to an additional process involved in creation of pavements. If you carefully remove the layer of stones from a pavement surface, you will find a distinct, fine-grained soil horizon called a vesicular A (or Av) horizon (see the photograph below). The name “vesicular” refers to the many vesicles or large pores found throughout the horizon. “A” denotes its position as the topmost mineral layer of soil. The Av horizon is typically a few centimeters (about an inch) thick, and contains mostly silts and clays; it lacks coarse materials, even though small stones of the pavement cover the Av horizon and rocky materials occur in the soil underlying it.

Side view of a piece of fine-textured Av horizon removed from beneath a desert pavement showing the air-filled vesicles.

The origin of the fine-grained Av horizon is an important key to understanding how the overlying flat-topped pavement develops. Examination of the minerals contained in the Av horizon at one site in California demonstrated that the materials in this horizon did not originate from the weathering of the rocky parent materials. Instead, dust deposited on the stony surface is the source of the silts and clays of the Av horizon. These fine-grained materials accumulate beneath a layer of surface stones, separating these stones from the rest of the underlying rocky materials. Over time, the further accumulation of fine-textured materials in the Av horizon literally lifts the mono-layer of stones of the pavement and levels the surface (see the illustration below).

Prior to this work, it was commonly believed that most desert pavements originated through the selective erosion of fine materials from the surface by either wind or water, a process called deXation. However, such a process cannot explain the development of pavements in stony parent materials that initially lacked fine-grained materials (such as many coarse-grained alluvial fan deposits or areas of exposed bedrock, including basalt flows). Nor can it explain the presence of the fine-grained Av horizon that separates the surface pavement from underlying rocky materials. In coarse parent materials, atmospheric additions of fine materials (rather than their selective removal) and incorporation of these materials into the Av horizon below a layer of stones are responsible for creation of the pavement surface.

 

Formation of desert pavement.

More rarely, however, stone pavements can also be created by a process of deflation in certain environments where the original parent material consisted of small stones and pebbles mixed with abundant fine-grained sands, such as in beach deposits that ring ancient lake beds in some parts of the region. In such cases, the selective removal of the sand by wind and/or water leaves behind a lag of pebbles that resists further deflation. Once the surface lag stabilizes the fine-grained deposit, the pebbles act as a dust trap and airborne materials accumulate to eventually form a silt- and clay-rich Av horizon beneath the pebble pavement in the same manner as that which occurs on rocky parent materials. These kinds of pavements exist in a limited number of locations in the Sonoran Desert region, but are less common by far than pavements that developed on coarse, rocky parent materials as described previously.

Rock Varnish

Desert pavements are frequently very dark-colored; in many cases they are nearly black. Rock varnish (frequently called desert varnish) on the stone surfaces provides this dark complexion, despite the rock’s internal color. The glossy coatings of desert varnish on stones are very thin, at most a few hundredths of a millimeter thick—about the thickness of a sheet of paper. These thin, lustrous coatings contain a variety of constituents. Clay minerals typically form about three-quarters of the bulk of the varnish and manganese oxides impart the dark color. Many other minerals are present in trace amounts. Desert varnish also contains organic matter, apparently derived from microbial activity.

How rock varnish forms is poorly understood. Many of the mineral ingredients of varnish, including clays and manganese, are probably derived from airborne materials that settle on rock surfaces. Bacteria residing on the rock surface may play a major role in concentrating and cementing these materials to form the glossy coatings. Laboratory studies have shown that rock varnish gives off considerable carbon dioxide when moistened, indicating bacterial respiration. However, bacteria are generally absent from the shiny exposed surfaces of varnish, indicating that they reside within and beneath the microscopic varnish layers.

The formation of varnish may actually be a means by which these microbes protect themselves in the exposed, extreme environment of a rock surface in the desert. Interestingly, the manganese oxides in rock varnish very effectively block the transmission of ultraviolet radiation. Perhaps the rock-dwelling microbes manufacture their own manganese-formula sun-screen!

Rock varnish forms very slowly. Surfaces of some rocks, including many coarse-grained granitic rocks, rarely sport thick coats of varnish because they weather and erode faster than varnish can form. But if rock surfaces resist weathering, varnish coatings become increasingly thick and dark with the passage of time. Rock varnish therefore provides geologists with a valuable tool for determining relative ages of different alluvial fan deposits. Stone surfaces in older deposits are generally covered by thicker, darker coats of varnish. In some cases, the thickest, darkest coatings of varnish found on older deposits may have been accumulating for many tens of thousands to over 100,000 years.

Ancient inhabitants of the Sonoran Desert used varnished desert pavements as dark-colored canvases on which they rendered gigantic artistic impressions. By removing the dark varnished stones and exposing the underlying light- colored soil, prehistoric peoples created fantastic images of human figures, animals, and abstract forms. Many of these intaglios or geoglyphs have been discovered in the vicinity of the lower Colorado River Valley near Blythe, California and Ehrenberg, Arizona. Intaglios of the Sonoran Desert are frequently ten yards (10 m) or so long. The largest yet discovered, located about seventy-five miles (120 km) west of Phoenix, is a human figure nearly the length of a football field. These gigantic works of landscape art may have been created well before European colonization of the Americas. We don’t know why the intaglios were created or the purposes they may have served. However, according to legends of indigenous peoples who occupied the area in historic times, some of the giant, human-like figures represent gods or other supernatural beings. The large, skyward-looking images, although usually difficult to recognize from the ground, certainly would be apparent to ancient peoples’ deities in skies above. Inhabitants of other desert regions of the world also created intaglios long ago on the surfaces of stone pavements, perhaps for similar reasons. One of the best known examples is located on extensive alluvial fan deposits in the Peruvian coastal desert near Nazca. These intaglios include long, straight lines that are fifteen to twenty miles (24-32 km) long, geometric forms, and images of many kinds of animals.

Intaglio (geoglyph) found northeast of Quartsite, Arizona.

Rock surfaces covered with dark desert varnish provided prehistoric desert dwellers with another medium for artistic expression. Petroglyphs are designs created by chipping away the surface varnish on large rock and boulder surfaces, exposing lighter-colored rock beneath. These are much, much smaller than the giant intaglios, but similarly include diverse shapes, geometric designs, and human and animal forms.

Darkly-varnished desert pavements take so long to form and are extremely sensitive to disturbance. The intaglios created by ancient peoples can last for centuries. So will the uninspiring and less aesthetically appealing tracks so thoughtlessly created in our time by drivers of off-road vehicles.