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Sooner or later you arrive in the brightness and space that you came for. And if you turn your eyes from the distant landmarks around you, you may see another kind of pavement at your feet, called desert pavement.
It's not at all like the drifting sand that people often picture when they think of the desert. Desert pavement is a stony surface without sand or vegetation that covers large parts of the world's drylands. It's not photogenic, like the twisted shapes of hoodoos or the eerie forms of dunes, but seeing its presence on a wide desert vista, dark with age, gives a hint of the delicate balance of slow, gentle forces that create desert pavement.
It is a sign that the land has been undisturbed, perhaps for thousands—hundreds of thousands of years.
What makes desert pavement dark is rock varnish, a peculiar coating built up over many decades by windblown clay particles and the tough bacteria that live on them.
Varnish has been found on fuel cans left in the Sahara during World War II, so we know that it can form fairly fast, geologically speaking. What makes desert pavement stony is not always so clear. There are three traditional explanations for bringing stones to the surface, plus a much newer one claiming that the stones started out at the surface. The first theory is that the pavement is a lag deposit , made of rocks left behind after the wind blew away all the fine-grained material.
Wind-blown erosion is called deflation. This is clearly so in many places, but in many other places, a thin crust created by minerals or soil organisms binds the surface together.
That would prevent deflation. The second explanation relies on moving water, during the occasional rains, to winnow out the fine material. Once the finest material is splashed loose by raindrops, a thin layer of rainwater, or sheet flow, sweeps it away efficiently. Both wind and water could work on the same surface at different times.
The third theory is that processes in the soil move stones to the top. Repeated cycles of wetting and drying have been shown to do that.
Two other soil processes involve the formation of ice crystals in the soil frost heave and salt crystals salt heave in places with the right temperature or chemistry. In most deserts, these three mechanisms—deflation, sheet flow, and heave—can work together in various combinations to explain desert pavements. But where there are exceptions, we have a new, fourth mechanism.
A desert pavement is covered by closely packed, rounded rock fragments of the pebble. The rock pebbles are intermixed with silt that is devoid of any vegetation. Desert pavements are not unique to any particular region of the earth as implied by the different names for the desert pavement. In Australia, the desert pavement is called Gibber while in North Africa it is referred to as a reg. Other names of desert pavement include Sai and serir.
Once the desert pavement is formed, it becomes a shield of a sort by creating resistance to wind and water preventing further soil erosion. Some of the exceptionally old soils and rock particles are found below the desert pavement.
Desert pavements are coated with desert varnish on the surface. The varnish is dark brown or a shiny coating containing some clay minerals. Because of the slow process of the formation of desert pavements, several theories have been put forward to explain the formation process. The studies of different desert pavements have yielded different age estimates ranging from thousands to tens of thousands of years. However, all the theories incorporate the role of wind, erosion, and rains in the formation process.
A common theory suggests that desert pavements are formed through gradual removal of sand and other fine particles by the wind and intermittent rains leaving behind the large fragments.
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