Engineering Geology

E n g i n e e r i n g G e o l o g y
However, downward lowering is almost invariably arrested when the water table is reached
since the wind cannot readily remove moist rock particles. What is more, deflation of sedimentary
material, particularly alluvium, creates a protective covering if the material contains
pebbles. The fine particles are removed by the wind, leaving a surface formed of pebbles that
are too large to be blown away. The suspended load carried by the wind is comminuted
further by attrition, turbulence causing the particles to collide vigorously with one another.
When the wind is armed with grains of sand, it possesses great erosive force, the effects of
which are best displayed in rock deserts. Accordingly, any surface subjected to prolonged
attack by wind-blown sand is polished, etched or fluted. Abrasion has a selective action, picking
out the weaknesses in rocks. For example, discontinuities are opened and rock pinnacles
developed. Since the heaviest rock particles are transported near to the ground, abrasion is
there at its maximum and rock pedestals may be formed. In deserts, flat smoothed surfaces
produced by wind erosion are termed desert pavements.
The differential effects of wind erosion are illustrated in areas where alternating beds of hard
and soft rock are exposed. If strata are tilted steeply, a ridge and furrow relief develops,
because soft rocks are more readily worn away than hard. Such ridges are called yardangs.
Conversely, when an alternating series of hard and soft rocks are more or less horizontally
bedded, features known as zeugens are formed. In such cases, the beds of hard rock act as
resistant caps, affording protection to the soft rocks beneath. Nevertheless, any weaknesses
in the hard caps are picked out by weathering, and the caps are breached eventually, exposing
the underlying soft rocks. Wind erosion rapidly eats into the latter and, in the process, the
hard cap is undermined. As the action continues, tabular masses, known as mesas and
buttes, are left in isolation (Fig. 3.32).
Desert Dunes
About one-fifth of the land surface of the Earth is desert. Approximately four-fifths of this
desert area consists of exposed bedrock or weathered rock waste. The rest is mainly covered
with deposits of sand. Bagnold (1941) recognized five main types of sand accumulations,
namely, sand drifts and sand shadows, whalebacks, low-scale undulations, sand
sheets and true dunes. He further distinguished two kinds of true dunes, the barkhan and the
seif (Fig. 3.33a and b).
Several factors control the form that an accumulation of sand adopts. Firstly, there is the rate
at which sand is supplied; secondly, there is wind speed, frequency and constancy of direction;
thirdly, there is the size and shape of the sand grains; and fourthly, there is the nature
of the surface across which the sand is moved. Sand drifts accumulate at the exits of the
gaps in the landscape through which wind is channelled and are extended downwind.
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