(d) Hooks. A hook, or recurved spit, is a spit with its tip turned towards the shore due to a deflection
of the current that built it or to the opposing actions of two or more currents.
2. Wind. Although wind is a less important geologic agent than water, it does exert an appreciable influence on
the topography of arid regions. Wind action is referred to as an eolian process, and all topographic features
created by such a process are referred to as eolian landforms. Eolian landforms are typically depositional in
nature; however, several types of erosional landforms exist as well.
a. Erosional Features Created by Wind Action. There are two processes by which wind accomplishes its
erosional work-deflation and abrasion. Deflation is the lifting or rolling and subsequent removal of loose, dry
sediments by wind action. The amount and size of material that is transported depends on the velocity of the wind
(as the velocity increases, the wind carries increasingly larger particles). In general, only particles that range from
fine silt or clay to coarse sand are capable of being transported by winds. As the wind-driven sand and silt
impacts on exposed rock surfaces, a form of natural sandblasting, called abrasion, occurs. Both deflation and
abrasion result in the formation of distinctive surficial features.
(1) Desert Pavement. When heterogeneous surficial mixtures of gravel, sand, and silt undergo
erosion by deflation, the finer particle are removed, leaving behind only the particles that are too large to be
transported. Eventually, a continuous remnant layer of gravel is left to protect the underlying heterogeneous
material from further erosion. The gravel layer is referred to as a desert pavement due to its resemblance to
cobblestone mosaics.
(2) Ventifacts. In windswept areas where abrasion is a predominant form of eolian erosion,
sandblasting may result in the formation of smoothly polished face on exposed rock surfaces. If the wind
direction or the position of the rock changes, additional facets may develop. A multifaceted rock shaped by wind
abrasion is called a ventifact.
(3) Other Erosional Eolian Features. Several types of exotic erosional features may result from
wind abrasion. For example, isolated rock masses have been carved into mushroom-shaped pedestals and
bridgelike arches. In addition, holes, called windows, have been cut completely through rock walls.
b. Depositional Features Created by Wind Action. The wind velocity, with all transporting agents,
eventually decrease at some point, and this decrease in wind velocity is accompanied by the deposition of
suspended sediments. Initial deposition consists of relatively large particles at short distances from the source
area. This is followed by later deposition of smaller particles farther from the place of origin. Sand-sized
particles are commonly deposited in low hills or drifts called dunes, whereas finer sediments are generally
deposited as smooth sheets.
(1) Dunes. As winds carrying a load of sediment encounter an obstruction, such as a boulder or bush, the
wind velocity decreases and the sediment is subsequently deposited in the form of a hill or an elongate ridge of
sand on the lee side of the barrier. This mound of sand is called a dune. Depending on the effectiveness of the
wind, the abundance of vegetation, and the amount of sand available, the dune may take on one of many different
shapes (see figure 2-14, page 2-22).
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