1. Deserts receive less than 25 cm of rainfall (or snowfall) a year, but
can have either hot or cold climate. Most deserts are associated with falling,
dry air masses concentrated in a belt 10 degree - 15 degree wide centered on 30 degrees north and
south latitude and at the poles. Other controls include rain shadow effects,
great distance from the ocean, and proximity to cold ocean currents. Deserts
typically lack through-flowing streams and have internal drainage that may
develop local base levels.
2. Running water is the dominant agent of landscape development in deserts
because of limited vegetation. Thunderstorms commonly cause flash floods and
mudflows in deserts, although stream valleys, called arroyos, are usually
dry.
3. Resistance of rocks is controlled by climate and rocks that might be weathered
quickly in other environments (e.g. limestone) are resistant in desert settings.
Soils are coarse and rocky, and creep is common.
4. Resistant rocks in desert settings form flat-topped plateaus, mesas, and
buttes depending on their size, although those features can be found in humid
climates as well. Dipping beds form hogbacks and cuestas. In contrast to humid
regions, desert landscapes are typically angular, although shales form gentle
slopes of badland topography.
5. Internal drainages in desert settings develop playa lakes, that dry into
playas, covered with either mud or salt. Coalescing alluvial fans form bajadas
near the bases of mountains. Erosion of mountains produces sloping surfaces
covered by a veneer of gravel called pediments that may pass into bajadas.
6. Wind is responsible for erosion of fine sediment in deserts. Like water,
the faster the wind blows, the more sediment it can move, but its density
limits erosion to only the finer sediment sizes. Once eroded by deflation,
silt and clay can remain suspended for long periods and can be carried great
distances (Saharan sediment reaches the West Indies). Sand, in contrast, moves
by saltation near to the ground. Sandblasting forms ventifacts and sculpts
bedrock exposures. Blowouts are depressions caused by wind erosion (deflation).
7. Wind deposited silt and clay is called loess and it is usually associated
with deflation of outwash plains formed during the Pleistocene throughout
the northern hemisphere.
8. Sand dunes are mounds of loose sand deposited by wind. The sand grains
can be of any composition, but are typically well sorted and well rounded.
The dunes are asymmetric, steep on their downwind side and gentle on their
upwind side. The downwind side or slip face is steep because the slope is
at the angle of repose for dry sand (34 degrees). Slip faces form the cross-bedding
preserved in dune sandstones. Movement of the dune is in the direction of
the slip face, because of erosion on the upwind side and transportation along
the dune surface.
9. Dune geometry is controlled by wind velocity, direction, sand supply,
and vegetation cover. Large dune fields are sand seas. Many dunes are irregular,
but some patterns may be recognized. Low sand supply is characterized by barchan
dunes, with horns pointing downwind. High sand areas may form transverse or
longitudinal dunes that are elongate, or parabolic dunes around blowouts,
with horns pointing upwind.
