desert pavement

desert pavement Unusual features abound in dry landscapes. One is the desert pavement. This is a veneer of stones lying at the ground surface, in some instances so tightly packed together that there is almost no soil visible. Indeed, the surface takes on the appearance of being finely paved or cobbled with stone (Fig. 1).

The stones in a desert pavement are usually about halfway embedded in the soil surface, with only their upper surfaces exposed, and are fairly tightly held in place. More rarely, the stones may be loose. Below the stones, there is finer material that may be silty, dry, and powdery, or of heavier clay texture. This material itself contains few, if any, stones. Vascular plants are generally absent in areas of pavement, but the sheltered environment at the interface between the stones and the finer material beneath may be extensively colonized by hypolithic algae, especially beneath translucent stones.

Desert pavement is significant in landscape behaviour because it acts as an efficient seal, preventing ready entry of rainwater into the material below. It also provides a relatively smooth surface across which the surplus water flows, and which facilitates wind transportation of materials. The limited entry of water between the stones entraps air beneath, and the resulting positive pressure deforms the moist soil to form a distinctive vesicular zone containing abundant spherical pores up to 1 mm or so in diameter.

The stones in a pavement are usually coated with a desert varnish of materials deposited from water (see rock coatings). The older the pavement, the more developed and thicker is the coating. This can be used as a means of determining the relative ages of pavements within an area.

Desert pavement can arise in several ways. Wind and water erosion may in places gradually sift out and carry away finer materials from an earlier mixed deposit, leaving more and more of the immovable stones lying at the surface. Eventually, so little fine material is exposed that winnowing ceases, and a stable pavement, formed as a lag deposit, results.

Dry landscapes are periodically subject to blowing dust, which can settle across favourable sites as a growing surface mantle. Dust is washed from the upper surfaces of stones to accumulation sites between them (and beneath any overhanging edges). Upon drying out, layers of these fine materials crack, and newly settled dusts both fall and are washed into these openings, which may be too narrow to receive stones. The wind-blown materials thus work their way under the stones. Desert pavements may develop in this way during multiple episodes of deposition and in climatic conditions different from those of today. The pavement stones are not a lag deposit left by erosion, but rather are from the original landscape below the dust, and have been kept at the surface by the downward infiltration of finer grains as they accumulate. This mechanism for pavement development affords an explanation of the rarity of stones in the finer sub-pavement material.

The development of a nearly complete desert pavement may effectively terminate the developmental process in either of the mechanisms just outlined. Further loss of fine material is prevented by a dense pavement, while dust accretion is also restricted by rapid and nearly complete run-off of rain. Thus, well-developed desert pavement may be an indicator that the surface on which it lies has been stabilized. Landscape activity may then switch to other areas, perhaps to the stream channels that must carry away the water shed from the pavement surfaces.

David L. Dunkerley