9.3: A Brief Summary of Desert Features
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Because eolian transport of sand, and the construction of eolian dunes large and small, is perhaps the most striking characteristic of deserts (even though, as noted above, fluvial sediment transport is much more effective in shaping the landscapes of deserts), I’ll concentrate in the following sections on eolian saltation and eolian dunes. But in this section I’ll mention just briefly several other aspects of deserts.
An alluvial fan is a fan-shaped or cone-shaped body of loose sediment deposited at the foot of a steep stream valley. Alluvial fans usually consist mostly of sand and gravel. Fans are deposited because the slope of the stream, and therefore the sediment-transporting ability of the stream, decreases abruptly where the stream leaves it mountain channel and spreads into a valley. At the head of the fan, the stream passes from being laterally confined to laterally unconfined, so on average it becomes wider and shallower, and therefore of gentler slope—although the transition is not really as abrupt as I indicated above.
The contrast in slope between the confined mountain stream and the unconfined fan surface is greatest in the very early stages of fan development, when there’s only a small fan at the base of a steep mountain front, and much less in the later stages, when the fan has built to greater size and the stream has had a chance to cut a deep canyon and extend its headwaters far into the mountain mass.
Once on the fan, the stream progrades the fan locally, and in doing so the elevation of the stream bed is raised somewhat above the level of the adjacent fan surface. (By progradation I mean the tendency for the depositional surface to be built forward, in the direction of sediment transport.) At some point the stream 414 shifts its position to a lower area of the fan. In this way, over a long time period the stream sweeps irregularly across the entire fan, building it in a symmetrical, conical shape.
Alluvial fans are characteristic of deserts with high relief, especially in the southwestern US, but they are not common in many other deserts. You should also keep in mind that alluvial fans are not restricted to arid or semiarid regions: they form even in humid regions where there is high relief and rapid weathering. In such areas, however, alluvial fans tend to be heavily vegetated and not as noticeable.
Rigid solid surfaces tend to be abraded by the impact of flying sand grains. The upwind-facing surfaces of outcropping bedrock or pieces of gravel are abraded to smooth and sometimes fluted surfaces. Such rocks are called ventifacts, or wind-worn stones. Loose pieces of gravel sometimes show a faceted shape formed by abrasion from two or three different directions—either by variability of wind direction or shifting of the position of the clast relative to the prevailing sand-transporting wind direction.
On old alluvial fans it’s common to see a ground surface with a strikingly densely packed veneer of gravel. The substrate beneath consists of much finer sediment as well as gravel. The explanation is that the surface of an alluvial fan can become stranded, or cut off from fresh supply of sediment. This happens whenever there is tectonic uplift of the fan, causing the streams to be incised into the fan. Then slow deflation (removal of sediment by wind action) winnows the finer part of the surficial sediment, leaving only the coarsest fraction as a surficial residue. If you dig just below the surficial coarse layer you find a mixture of coarse material with much finer sediment. Only if the coarse surface layer is somehow disturbed can the wind act upon that underlying fine material.
Rock surfaces in the desert commonly show a dark coating, dark brown to almost black and sometimes even with a suggestion of darkest violet. The coatings, a small fraction of a millimeter thick (0.05–0.1 mm), are known from actual chemical analyses to be rich in iron and especially manganese, along with aluminum, silicon, and oxygen, but these elements seem not to be well crystallized into minerals.
Rocks most susceptible to desert varnish are relatively resistant to weathering: mafic and felsic volcanics, metamorphics, and well cemented sandstones. Rocks like felsic intrusives, which tend in deserts to weather fairly readily by granular disintegration, don’t show good desert varnish, because the surfaces are degraded too fast by mechanical weathering. The same is true of limestones and even dolostones, because they weather by solution too fast.
The source of the Fe and Mn, as well as the mode of precipitation, are not entirely clear. The general consensus seems to be that Fe and Mn can be derived either from the interior of the rock itself or from soil in which rock fragments rest. (Other things—rock type and locality— being equal, rock fragments in regolith show better desert varnish than bedrock.)
Films of moisture must be invoked for transport of the Fe and Mn in solution; rainwater is an obvious possibility, but coatings of dew are likely to be even more important, because most desert rocks are wet for a much greater part of the time from dew than from rain. The Fe and Mn in solution could be transported by actual flow of the film, or by diffusion across it. The details of chemistry are not yet clear.
By various lines of evidence it’s known that the time scales for development of a good coating of desert varnish vary greatly, from just a few decades in the most favorable circumstances to many centuries or even millennia.
In geomorphological usage, a playa is the barren, flat, and usually dry area at the lowest part of a basin of interior drainage. (In Spanish, playa means just beach or shore.) The typical playa is dry most of the time, except for occasional inundation by infrequent heavy rain in the watershed area. Playas vary in size from those you can walk across in a few minutes to those many kilometers across. With increasing frequency and magnitude of rainfall in the drainage basin, playas grade over into what might better be termed ephemeral lakes, and then into permanent interior-drainage lakes.
Most playa surfaces are virtually nonvegetated, except possibly at the very periphery, where relatively salt-free water emerges from drainage from the distal slopes of alluvial fans, which typically terminate at the playa edge. The lack of vegetation is clearly related to the elevated content of salts in the sediments of the playa.
The nature of playa surfaces varies widely, depending partly upon the ratio of dissolved load (salts in solution) to fine particulate sediment (mainly suspended load) in the flows reaching the playa, and partly upon the groundwater regime of the playa. Some playas, especially those that are topographically closed but have throughgoing flow of groundwater, are dry except soon after rains, because the water can drain out of the basin through an aquifer. Such playas have little or no salt in their sediments. Other playas, which are closed with respect to groundwater flow as well as to topography, tend to be moist long after rains, especially where the groundwater table, or at least the capillary fringe thereof, is always fairly near the playa surface. Such playas are characterized by much salt in their sediments, and some consist mostly or entirely of evaporites. (Evaporites are sediment deposits that originate from partial or complete evaporation of water that contains salts in solution.) The salt content of such playas is derived partly from weathering of rocks in the watershed (that salt may be from pore waters originally included in sedimentary rock or it may be from constituents of the minerals themselves) or from washing out of atmospheric salts, ultimately derived from the ocean. That not all evaporitic playas show the same content is good evidence that the salt content in the watershed is usually the more important of the two sources. Halite, gypsum, and borates are some of the common compositions of playa evaporites. The floor of Death valley is probably the most-visited playa, still in its natural state, in the United States.