Erosion processes in an arid region differ from those in a more humid climate. Because of the low precipitation, sparse vegetation, lack of soil cover, and accumulation of broken, poorly weathered surface debris, the topography lacks the smooth, gentle curves characteristic of the moist climates. Limestone, a non-resistant rock in the humid climate, is notably resistant in climates deficient in the water to weather and dissolve it. Deserts are defined by their limited precipitation. Characteristically, when the rain does fall, it arrives as thunderstorms, short periods of heavy precipitation. Thus, running water has a very obvious and significant influence on the arid landscape. When a storm falls in a watershed, it makes its way over the surface rearranging the loose materials and flowing in previously dry channels called ARROYOS. The runoff makes its way rapidly through the stream system suddenly turning the dry valleys into rushing streams full of rocks and debris.
Two different types of arid/semi-arid topographies are especially important. In areas of horizontal or nearly horizontal sedimentary rocks, a resistant layer, the cap rock, can protect underlying layers. Where streams cut through, deep canyons are formed. The flat-topped uplands or plateaus are MESAS, and when streams dissect these into small, isolated uplands, they are called BUTTES (rhymes with shoots).
The second type of topography, best associated with the Basin and Range physiographic region, develops in arid areas of block faulting. When a block is uplifted relative to another, the streams erode canyons into it and carry their waters to the interior basins where they collect and evaporate. These temporary lakes are called PLAYAS. When the streams emerge from the canyons carved into the uplift block, their gradients drop, and they deposit some of their load as ALLUVIAL FANS. If many streams emerge along a mountain front, the alluvial fans may coalescence and form a BAJADA. As the mountains erode further, the alluvial fans and debris protects the mountain front and the angular nature of the fault scarp is preserved as it wears back. Beneath the fans and deposits, a gently sloping surface called a PEDIMENT forms between the fault trace and the fault scarp. As the erosion processes continue, the pediment grows larger. Eventually, the uplifted block will be worn away, but before complete elimination, the last elevated portions or the remnants form INSELBERGS (German for island mountains). These are surrounded by the pediments that have merged from both sides of the block.
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