Geology Reference
In-Depth Information
Slipface
Barchanoid
Crest
Traces of former
dune positions
Linguoid
Block
slumping
Court
Brink
Bottle slides
Apron
Figure 12.5 The main features of a dune.
Source: Adapted from Livingstone and Warren (1996, 65)
Linear dunes have slip faces on either side of a crest
line, but only one of them is active at any time, and
sand transport runs parallel to the crest. They may be
divided into sharp-crested seifs , also called siefs and sayfs
(Plate 12.6), and more rounded sand ridges. Both are
accumulating forms that either trap downwind sand from
two directions or lie parallel to the dominant wind. Lin-
ear dunes occur in all the world's major sandy deserts.
They stand from less than a couple of metres high to
around a couple of hundred metres high and may extend
for tens of kilometres. They often run parallel but many
meander with varied spacing and may join at 'Y' or
'tuning fork' junctions.
Dune networks and star dunes possess a confused
set of slip faces that point in several directions. Dune
networks, which are very widespread, usually occur in
a continuous sand cover. They are composed of dunes
no more than a few metres high and spaced 100 m or
so apart. Stars dunes bear several arms that radiate from
a central peak (Plate 12.7). They may be up to 400 m
high and spaced between about 150 and at least 5,000 m.
Found in many of the world's major sand seas, star dunes
cover a large area only in the Great Eastern sand sea of
Algeria.
Sheets of sand come in two varieties - zibars and
streaks. Zibars are coarse-grained bedforms of low relief
with no slip faces. Their surfaces consist exclusively of
wind ripples and local shadow and shrub-coppice dunes.
They are common on sand sheets and upwind of sand
seas. Streaks , also called sand sheets or stringers ,are
large bodies of sand that bear no obvious dune forms.
They occupy larger areas of sand seas than accumulations
with dunes.
Anchored dunes
Several types of dune are controlled by vegetation, topog-
raphy, or local sediment sources. These anchored or
impeded dunes come in a variety of forms (Table 12.3;
Figure 12.7). Topographic features cause several distinct
types of anchored dune. Lee dunes and foredunes are
connected to the pattern of airflow around obstacles.
Wind-tunnel experiments have shown that the growth of
climbing dunes (Plate 12.8) and echo dunes depends
 
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