Geoscience Reference
In-Depth Information
Hamada
Desert
pavement
Yardang
Mesa
Lag gravel
Butte
Pediment
Deflation
hollow
Talus
cone
Erg
Bajada
Playa
Alluvial apron
Buried
evaporite pan
Airflow
Utah, United States, sourcing material for the aeolian system.
Gravel fans, draped across the bajadas below the mountains,
give way to finer sands and silts as debris is swept towards
the adjacent playa, from where it is deflated to form sand seas
and dunes.
Photo: Ken Addison
Desert landsystems
Desert pavements, lags and ventifacts
Deflation initially produces remnant landforms in areas
stripped of sand and silt. Non-deflatable, coarse-grained
lag gravels
loosely protect underlying abraded bedrock
surfaces or
hamada
to form
desert pavement
. Residues
from the restricted chemical weathering in deserts are
rapidly deflated. Lag clasts themselves are abraded
in situ
as deflation continues and, if large enough to remain
static, sandblasted facets develop on the windward side
of these
ventifacts
, often giving a three-faced or
dreikanter
appearance. Desert pavements, known also by their
alternative Arabic or aboriginal names
reg
and
gibber
,
eventually become sterile unless further fines are
introduced to stimulate abrasion.
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.
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
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 develop-
ment generally commences in sheltered topographical
km
2
. Wind can abrade, but not