Desert Landscapes and Life

Desert Landscapes and Life 

The popular media commonly portray deserts as endless vistas of sand, punctuated by the occasional palm-studded oasis. In reality, not all desert landscapes are buried by sand. Some deserts  are vast, rocky plains; others sport a stubble of cacti and other hardy desert plants; and still others display intricate rock formations that look like medieval castles. Explorers of the Sahara, for example, traditionally distinguished among hamada (barren, rocky highlands), reg (vast, stony plains), and erg (sand seas in which large dunes form). 

In this post, we’ll see how the erosional and  depositional processes described above lead to the formation of such contrasting landscapes.

Rocky Cliffs and Mesas 

In hilly desert regions, the lack of soil exposes rocky ridges and cliffs. As noted earlier, cliffs erode when rocks split away along joints. When this happens, the cliff face steps back into the land but retains roughly the same form. The process, commonly referred to as cliff retreat, or scarp retreat, occurs in fits and starts. A cliff may remain unchanged for decades or centuries, and then suddenly a block of rock falls off and crumbles into rubble at the foot of the cliff. Cliffs exposing alternating layers of strata with contrasting strength develop a step-like shape; strong layers (sandstone or limestone) become vertical cliffs, and weak layers (shale) become rubble-covered slopes. 

Mesas and buttes form in deserts as cliffs retreat over time.

With continued erosion and cliff retreat, a plateau of rock slowly evolves into a cluster of isolated hills, ridges, or columns. Flat-lying strata or flat-lying layers of volcanic rocks erode to make flat-topped hills, which go by different names depending on their size. Large examples (with a top surface area of several square km) are mesas, from the Spanish word for table. Medium-sized examples are buttes (figure above a, b), and small examples, whose height greatly exceeds their top surface area, are chimneys (figure above c). Natural arches form when erosion along joints leaves narrow walls of rock. When the lower part of the wall erodes while the upper part remains, an arch results. 

 The formation of cuestas and inselbergs, due to erosion and deposition in deserts.

In places where bedding dips at an angle, an asymmetric ridge called a cuesta develops. A joint-controlled cliff forms the steep front side of a cuesta, and the tilted top surface of a resistant bed forms the gradual slope on the backside (figure above a). If the bedding dip is steep to near vertical, a narrow symmetrical ridge, called a hogback, forms. 

After a long period of erosion, all that may remain of a once broad region of uplifted land is a relatively small island of rock, surrounded by alluvium-filled basins. Geologists refer to such islands of rock by the German word inselberg (island mountain; figure above b). Depending on the rock type or the orientation of stratification in the rock, and on rates of erosion, inselbergs may be sharp-crested, plateau-like, or loaf-shaped (steep sides and a rounded crest). Inselbergs with a loaf geometry, as exemplified by Uluru (Ayers Rock) in central Australia (figure above c), are also known as bornhardts.

Desert Pavement 

Desert pavement, and a hypothesis for how it forms by building up a soil from below.

In many locations, the desert surface resembles a tile mosaic in that it consists of separate stones that fit together tightly, forming a fairly smooth surface layer above a soil composed of silt and clay. Such natural mosaics constitute desert pavement (figure above a, b). Typically, desert varnish coats the top surfaces of the stones forming desert pavement. Recently, researchers have suggested that pavements form when windblown dust slowly sifts down onto the stones and then washes down between the stones. In this model, the pavement is “born at the surface,” meaning that the stones forming the pavement were never buried, but have been progressively lifted up as soil collects and builds up beneath (figure above c). Over time, the rocks at the surface crack, perhaps due to differential heating by the desert sun. Sheetwash, during downpours, may wash away fine sediment between fragments, and when soils dry and shrink between storms, the clasts settle together, locking into a stable, jigsaw-like arrangement.

Stony Plains and Pediments 

The coarse sediment eroded from desert mountains and ridges washes into adjacent lowlands and builds out to form gently sloping alluvial fans. The surfaces of these gravelly piles are strewn with pebbles, cobbles, and boulders, and are dissected by dry washes (wadis or arroyos). Portions of these stony plains may evolve into desert pavements. 

When travelers began trudging through the desert of the southwestern United States during the 19th century, they found that in many locations the wheels of their wagons were rolling over flat or gently sloping bedrock surfaces. These bedrock surfaces extended outward like ramps from the steep cliffs of a mountain range on one side, to the alluvium-filled valleys on the other (see 2nd figure). Geologists now refer to such surfaces as pediments. Pediments develop when sheetwash during floods carries sediment away from the mountain front, during mountain-front retreat. The moving sediment grinds away the bedrock that it tumbles over.

Seas of Sand: The Nature of Dunes 

The types of sand dunes and the cross beds within them.

A sand dune is a pile of sand deposited by a moving current. Dunes in deserts may start to form where sand becomes trapped on the windward side of an obstacle, such as a rock or a shrub. Gradually, the sand builds downwind into the lee of the obstacle. Dunes display a variety of shapes and sizes, depending on the character of the wind and the sand supply (figure above a). Where the sand is relatively scarce and the wind blows steadily in one direction, beautiful crescents called barchan dunes develop, with the tips of the crescents pointing downwind. If the wind shifts direction frequently, a group of crescents pointing in different directions overlap one another, creating a constantly changing star dune. Where enough sand accumulates to bury the ground surface completely, and only moderate winds blow, sand piles into simple, wave-like shapes called transverse dunes. The crests of transverse dunes lie perpendicular to the wind direction. Strong winds may break through transverse dunes and change them into parabolic dunes whose ends point in the upwind direction. Finally, if there is abundant sand and a strong, steady wind, the sand streams into longitudinal dunes, whose axis lies parallel to the wind direction.

In a sand dune, sand saltates up the windward side of the dune, blows over the crest of the dune, and then settles on the steeper, lee face of the dune. The slope of this face attains the angle of repose, the slope angle of a free-standing pile of sand. As sand collects on this surface, it eventually becomes unstable and slides down the slope, so geologists refer to the lee side of a dune as the slip face. As more and more sand accumulates on the slip face, the crest of the dune migrates downwind, and former slip faces become preserved inside the dune. In cross section, these slip faces appear as cross beds (figure above b, c).
Figures credited to Stephen Marshak.

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