Environmental Engineering Reference
In-Depth Information
YARDANGS AND DEFLATION HOLLOWS
Abrasion is likely to pick out lithological weakness and structural discontinuities aligned
close to the primary air flow, faceting and fluting the rock into regular or irregular
shapes. These in turn channel and locally accelerate the air flow, which increases
abrasion rates and polishes rock surfaces. Fluted channels can run for 10
2-3
km and are
flanked by residual
yardangs
as ridges or pillars of surviving rock. The extent to which
wind can actively enlarge flutes into large-scale landforms has been in doubt despite the
presence in many hot deserts of depressions covering areas of 10
3-4
km
2
. Wind can
abrade, but not quarry, rock and its capacity to develop
deflation hollows
requires the
coincidence of weak rock, a deep water table and enduring arid conditions. This appears
to apply to the suite of large, structurally aligned deflation hollows flooring over 75,000
km
2
of the Egyptian desert west of the Nile. Resistant surface rocks have been penetrated,
perhaps by streams in pluvial periods, to expose weak Pliocene shales which now bear
clear signs of wind abrasion down to the water table. Deflation products form extensive
leeward dune fields or smaller
lunettes
, an Australian counterpart found on the leeward
shore of ephemeral lake basins in South Australia. The material is deflated as the lakes
dry out and similar features are found in most deserts.
SAND SEAS AND LOESS SHEETS
Coalescence of aeolian sand into a sand sea or
erg
creates a large-scale depositional
landsystem whose surface is further ornamented by the wind. Single dunes and other bed
forms occur wherever deflated material is deposited, but the vast bulk of desert sand is
held in active ergs within the desert cores of North Africa, Arabia, Namibia, central
Australia and Mexico. Sand volumes of 10
3-4
km
3
are common, and ergs exceeding
25,000 km
2
in area account for some 90 per cent of desert sand. The largest, Rub'al Khali
in Saudi Arabia covers 560,000 km
2
. Erg development generally commences in sheltered
topographical depressions where boundary shear stress falls below threshold values, and
extends in the direction of dominant winds. Sand cover may be incomplete or thin (10
1
m) in peripheral areas but aggradation occurs in erg centres as active bed forms are
superimposed on each other. They may eventually reach thicknesses of 10
2-3
m.
Stabilized relic ergs often fringe active, mobile sand deserts and represent the
consequence of regional climatic change.
THE ENVIRONMENTAL SIGNIFICANCE OF LOESS SHEETS
applications
Loess sheets of silt and clay particles offer two interesting applications for palaeo-
environmental and climatic reconstruction. They might be expected downwind of sand
seas, as the finest winnowings of deflatable materials. This happens to some extent, but
loess and related
cover sands
are a specialized form of aeolian sediment, almost
exclusively formed in cool to cold desert landsystems. Their particle size - primarily
medium and coarse silt from 6 µm to 60 µm - reflects two possible sources. Either they
are deflated products of glacial sediments frost deserts and river terraces; or they are of
