Desert Pavement

Desert pavements are concentrations of stones on the land surface of arid areas, produced by wind and running water erosion and upward movement of stones through the soil. Stone pavements may signal serious soil erosion that must be addressed before the land can be irrigated for agriculture.

Occurrence of Desert Pavement

Desert pavements are extensive stony surfaces in arid areas that occur on slopes and on a range of lowland surfaces, including water-eroded and deposited areas. The stones are closely packed on flat or moderately inclined plane surfaces. Stone pavements are rendered prominent by their lack of vegetation, but, in any case, they form most readily by processes typical of arid regions where plant roots are not normally established to bind the soil to any extent. The pavements are most striking in areas of low relief, where the largely flat, stony surface itself, rather than the contours of the land, impresses the observer.

Desert stone pavements range from rocky or boulder-strewn surfaces to smooth plains of fine gravel. Given the prevailing aridity across North Africa and the Middle East, terminology for different types of desert pavement commonly derives from Arabic. Thus, the term “hamada” (Arabic for “unfruitful”) describes a boulder-strewn terrain, and “reg” (meaning “becoming smaller”) indicates a finer pavement of small stones. The term “serir” is a synonym for “reg” in the central Sahara.

Most hamada pavements are residual, consisting of stones derived from the bedrock beneath, or constitute boulders transported only short distances. Most regs consist of transported stones. These distinctions of size and transport distance become blurred over time as the larger rock fragments are progressively weathered to finer sizes and transported farther by wind and water. Nevertheless, a residual origin of many hamada pavements is evident in several places by the angularity and lack of sorting of the rock fragments and by the similarity to the bedrock beneath. The water-deposited nature of a reg pavement can be indicated by sorted and rounded gravels of mixed composition and distant origin and by the pavement’s occurrence close to dry stream channels. Residual regs, usually closely associated with hamadas, commonly consist of angular flakes of local bedrock or a less weatherable residue. Reg pavements can be composed of two elements in varying proportions: a compact mosaic of small stones embedded in soil and a more uneven component of larger fragments lying loose on that surface or protruding through it.

Wind and Water Erosion

Desert pavements are polygenetic, which means there is more than one process by which they can be produced. The most traditional explanation, however, is that the stones are concentrated through wind erosion or deflation of the fine particles. Deflation may eventually settle the pebbles into such stable positions that they fit together, almost like the pieces of a mosaic or the blocks of a cobblestone street.

The effectiveness of deflation on desert pavements is less, however, than has been commonly believed. Under natural conditions, soils that scientists have tested can resist wind erosion because they are silty and cohesive and tend to form crusts due to repeated wetting and drying. A larger grain size of the soil also makes it more resistant to wind erosion. Soil cohesiveness is an important factor. Even where particles are spherical, the magnitude of the cohesive forces between particles less than 0.1 millimeter in diameter is greater than the particle's weight. Small particles, however, are usually more irregular in shape, or even plate-like, a factor that increases their cohesion. Since cohesion is so important between small particles, silts and clays should resist wind erosion. That is observed to be true, but only where the surfaces are smooth, the clays and silts are uniformly fine, and no other material is blown onto them. Otherwise, large grains are dislodged from fine soils as aggregates of particles or knocked loose by the impact of other large sand grains, a process known as saltation. In any case, the amount of lowering and stone concentration by wind action is limited. Deflation diminishes markedly as the protective stone cover increases, and deflation becomes virtually ineffective when stones cover 50 percent of the surface. Undisturbed stone pavements are among the most windstable of desert surfaces.

Rain and water flow appear more effective than wind in eroding fine-textured soils on sloping desert pavements. In test plots cleared of stones in one experiment on 5-degree slopes, water wash accounted for most of the 5- to 50-centimeter surface lowering that occurred in five years. During this time, the stone pavement was renewed with stones from below this differential erosion.

Upward Displacement

Where subsoils are so clay-rich that they do not erode easily by either wind or water and the subsoils are also largely stone-free, the formation of stone pavements by deflation is more difficult to understand. In these situations, the mechanism of formation seems to have been forces of expansion and contraction within the soil that cause upward displacement and concentration of stones on the surface. Soils exhibiting this phenomenon contain expansive clays in alkaline chemical conditions and are subject to swelling and heaving upon wetting and shrinkage and cracking upon drying. Periods of heating followed by cooling and freezing followed by thawing also contribute to the expansion-contraction cycles, which cause stones to move upward and concentrate on the surface.

The exact mechanism for upward stone movement in deserts is not precisely known. However, some details are understood by analogy to known stone movement in areas of intense freeze and thaw cycles. It is possible, for example, that the stones shift upward as the underlying soils swell in wet periods and that, as the soil shrinks in dry periods, fine particles fall into cracks beneath the stones and prevent the return of the rock fragments. Stones may also induce differential swelling in the soil by speeding the downward advance of a wetting front around and over them. The stones are likely to be displaced upward, away from the dry zone beneath them, and held tightly by the wet and sticky soil above. Soil may squeeze into the space left and thus prevent a return movement of the stones. These processes also resemble those postulated for certain sorted, patterned ground types, where stones become arranged in polygons or striped zones. In these cases, it is clear that no process other than upward movement along certain zones could produce such patterns.

Stone pavements of the upward-movement type occur on and in the topsoils of weakly salty soils on the stony tablelands of arid Australia, where the subsoils are almost stone-free. The pavement stones are silica-rich, originally precipitated within the soils by mobilization and concentration of silica in unusual chemical reactions. For those stone pavements to have originated as a residue from erosion would have required a stripping of more than 1.5 meters of erosion-resistant, clay-rich soil. Such a process is unlikely, so an upward movement is far more probable, given salty clays' swelling and cracking potential. Similar pavements have been noted in deserts in Nevada and California and the Atacama Desert of Peru.

An alternative form of displacement is possible where the soil material has been transported, particularly by the wind. For example, in South Australia, a stone pavement occurs on a layer of clay rich in gypsum (calcium sulfate), which is thought to have been blown into place by the wind. The stone pavement on the surface resembles a buried pavement. It is possible that the original pavement first trapped a small amount of windborne dust among the stones. Some of the rock fragments were then displaced upward, little by little, through wetting and swelling of the aeolian (wind-deposited) clays during their accumulation, and thus, the stones were never deeply buried.

The concentration of stone pavement through winnowing by wind or wash can be relatively rapid, but the contribution by movement through the soil may take longer. Once formed, a pavement is relatively stable. The closely spaced stones act as a drag on the surface wind, restrict the entrainment of finer intervening materials, and so limit deflation. On moderate slopes, runoff water is spread over the surface by the stone mantle and thus does not tend to cut a deep gully. Selective erosion is countered further as the stone concentration increases. Whether residual or transported, pavements naturally consist of materials resistant to weathering and serve as “armor” for deserts.

Surface Weathering

Another manner in which stone pavements can be generated is by relative concentration through surface weathering. Rock fragments in a desert soil's relatively moist subsurface environment are more susceptible to weathering than those on the arid surface, particularly where the soil is impregnated with salt or gypsum. Consequently, a stone pavement may survive above soil with few stones because they disintegrate at depth over time. The phenomenon is pronounced in granitic gravels. Some terrace sediments, for example, have larger stone pavements above horizons of small granite fragments formed by the chemical weathering breakdown of boulders beneath the surface. These subsurface zones of fine particles are the deepest and most free of subsurface stones on the highest and oldest terraces, where they may be more than 50 centimeters thick. In such cases, it is thought that stone pavements first formed from the abundant rock fragments in the area, but as subsurface weathering progressed, all the buried stones were destroyed. At the same time, those on the surface remained relatively unweathered.

Despite their status as a protective armor on desert surfaces, stone pavements are subject to further evolution over thousands of years as the stones weather and their secondary products are redistributed. This evolution is generally toward an increasingly even surface of small grain-sized particles characterized by greater compaction. Also common is progressive darkening by surface weathering and the formation of rock or desert varnishes of manganese and iron stains precipitated on the stone surfaces by microbial and weathering action.

Weathering of pavement stones occurs not only beneath the stones, where they are in contact with the protected, mildly corrosive soil layers but also on their exposed surfaces. Pavement stones can be wetted frequently by dew, which also contributes salts to the surface and the subsurface in the weathering process. Because pavements are generally unchanneled and provide little runoff, the features are particularly subject to episodic or seasonal cycles of shallow wetting by rainfall and evaporative drying, through which weathering is activated. Bare and generally dark-colored pavements, among the most strongly heated surfaces of the desert, are areas of considerable evaporation, as (mostly saline) moisture is drawn upward through narrow cracks. As a result, thin salt crusts are widespread, the soil itself is impregnated with chlorides and sulfates of calcium and sodium, and salt weathering is significant. In this process, the growth of various salt crystals in the pores of the stones generates forces sufficient to disrupt the rocks.

Pavement stones also trap windborne dust in fissures and cracks, and dirt cracking can result from the expansion of the dust particles when they are wetted. Lichens and algae also exploit the shaded and relatively moist environments under the stones, adding an organic element of chemical decomposition to the weathering process.

In the breakdown of pavement stones, there tends to be a further selective concentration of resistant fine-grained material. For example, siliceous flint or chert pebbles may accumulate on the surface as relatively soft limestones are weathered. Such stones can eventually break down by incorporating water into their microcrystalline atomic structures, spalling (chipping), blocky fracturing (crazing), or complete cleavage and radial splitting. Coarser-grained stones undergo granular disintegration and pitting. Fracturing of pavement stones has also been attributed to differential expansion and contraction caused by solar heating, but the idea is controversial. Such thermal fracturing may be only a partial cause, as much of the broken stone has been superficially altered chemically, and dirt-cracking expansion may be a more important factor. In some cases, stones below the surface, where solar heating is impossible, can also be seen to have been pried apart in this fashion.

Wind abrasion is a form of natural sandblasting. Its effectiveness is related to wind velocity, the hardness of the sand and dust carried by the wind, and the hardness of the rock fragments being eroded. It is probably most effective in certain polar areas where cold, dense air can carry large particles at very high velocities and where, at winter temperatures, even ice has the hardness of some minerals. The “dry valleys” of Antarctica, for example, have extensive stone pavements that have been affected in this way.

Boulder and Pebble Forms

Boulders and pebbles in stone pavements may be fluted, scalloped, and faceted by wind abrasion. Flute and scallop forms vary in length from a few millimeters to several meters, but large flutes are not common in hard rocks. They are thought to be produced by turbulent helical (helix-shaped) flows carrying dust and sand, and that the scallops grow downwind. Where they are cut on large, immobile boulders, they are clear indicators of the strongest (dominant) wind directions. For this reason, they are most commonly reported from places where wind directions have remained largely unchanged for thousands to hundreds of thousands of years.

Some of the best-known wind-eroded stones in pavements are ventifacts (a general term for wind-faceted pebbles and boulders). A rock face abraded by wind may be pitted if there is a range of hardness in the minerals of the face, or it may be smooth where the rock is fine-grained or composed of only one mineral. Thus, rocks such as coarse-grained granite have pitted surfaces, but fine-grained quartzite generally produces only smooth, polished surfaces. Ventifacts have a great range of surface shapes, with plane and curved faces and two or more facets. The German term “dreikanter” is used for ventifacts with three facets, and “einkanter” is used for two-faceted stones. Multiple facets indicate that there was more than one wind direction or that the stones have been turned over through time. As the sizes of a stone pavement’s fragments are progressively reduced, a matrix of increasingly fine particles is supplied to the pavement. The proportion of such secondary material reveals the maturity of the development of a desert stone pavement. However, in practice, it may be difficult without sophisticated analysis to distinguish between a new pavement in the process of being formed and an old one being degraded.

The fine particles in a degraded pavement are redistributed by wash and rain, a process that contributes to the smoothing and compaction of the pavement. Any exposed soil is puddled and sealed by heavy rainfall or runoff water, so a saturated layer flows into hollows between the stones. Bare soil interspersed through a pavement is generally crusted above a bubbly or vesicular horizon, 1 to 3 centimeters thick, extending around and beneath the pavement stones. The bubbles result from the escape of entrapped air. Equally important in compaction is the gravitational settling of the stones during the expansion and contraction of the surface upon heating and cooling and (especially in saline and expansive clay soils) upon wetting and drying. Pavements of this type are generally soft and puffy after rain, but upon drying, the stones become ever more firmly embedded in a tight mosaic.

The slow downhill creep of water-saturated surface materials can also assist in either smoothing or roughening sloping stone pavement, particularly where the matrix's dispersal is accentuated by salinity. The differential flow of fine sediments can reduce microrelief on a stone pavement, and the pavement stones can become more evenly distributed and further embedded. In some cases, the stone pavements can even move into a series of rough steps aligned along the contours of a slope.

In the twenty-first century, scientists continue to make important discoveries regarding desert pavements that allow them to refine and restate previous research. Researchers studying the desert pavement at Cima Dome in the Mojave Desert have discerned that alternative methods of forming desert pavements occur. Further discoveries about the ability of soil to accumulate beneath a desert pavement have also been made. Researchers have dated some desert pavements, like the one found near the Calico Solar Project site in California, to be over 7,000 years old. The age of desert pavements can have important implications for archeological study. Scientists are also investigating the role desert pavements have in the hydrological cycle, especially regarding groundwater. Environmental activists continue to advocate for protecting desert pavement areas against industries like mining that threaten them. 

Principal Terms

creep: the slow, gradual downslope movement of soil materials under gravitational stress

deflation: the sorting out, lifting, and removal of loose, dry, silt- and clay-sized soil particles by turbulent eddy action of the wind

dirt cracking: a process in which clays accumulate in rock cracks, take on water, and expand to rupture the rock

expansion-contraction cycles: processes of wetting-drying, heating-cooling, or freezing-thawing, which affect soil particles differently according to their size

salt weathering: the granular disintegration or fragmentation of rock material affected by saline solutions or by salt-crystal growth

thermal fracture: the formation of a fracture or crack in a rock as a result of temperature changes

ventifact: any stone or pebble that is shaped, worn, faceted, cut, or polished by the abrasive action of windblown sand, generally under desert conditions

Bibliography

Alden, Andrew. “Desert Pavement Theories.” ThoughtCo, 2 Oct. 2019, www.thoughtco.com/theories-of-desert-pavement-1441193. Accessed 28 July 2024.

Clarke, Chris. “How a Flat Expanse of Boring Gravel is Vital to You and the Desert.” PBS SoCal, 6 Dec. 2011, www.pbssocal.org/redefine/how-a-flat-expanse-of-boring-gravel-is-vital-to-you-and-the-desert. Accessed 28 July 2024.

Cooke, Ronald U. “Stone Pavements in Deserts.” Annals of the Association of American Geographers, vol. 60, 1970, pp. 560-577.

Cooke, Ronald U., Andrew Warren, and Andrew Goudie. Desert Geomorphology. London: UCL Press, 1993.

Laity, Julie J. Deserts and Desert Environments. Hoboken, N.J.: John Wiley & Sons, 2008.

Mabbutt, J. A. Desert Landforms. Cambridge, Mass.: MIT Press, 1977.

Mainguet, Monique. Aridity: Drought and Human Development. Berlin: Springer-Verlag, 2010.

McGinnies, William G., B. J. Goldman, and P. Paylore, editors. Deserts of the World. Tucson: University of Arizona Press, 1968.

Middleton, Nick. Deserts: A Very Short Introduction. Oxford: Oxford University Press, 2009.

Nicholson, Sharon E. Dryland Climatology. New York: Cambridge University Press, 2011.

Parsons, A. J., and Athol D. Abrahams. Geomorphology of Desert Environments. Springer Science+Business Media B.V., 2009.

Schaetzl, Randall J., and Sharon Anderson. Soils: Genesis and Geomorphology. Cambridge, England: Cambridge University Press, 2005.

Thomas, David S. G. Arid Zone Geomorphology: Process, Form and Change in Drylands. 3d ed., Chichester: Wiley-Blackwell, 2011.