Page 322 - Fundamentals of Geomorphology

AEOLIAN LANDSCAPES 305

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