Geoscience Reference
In-Depth Information
Table 17.3
Possible nuclei for sand dune development.
dominates over sand accumulation, sand sheets occur and
dune forms are normally absent.
Figure 17.3 shows the global distribution of major ac-
tive and fixed sand seas. The term 'fixed' is applied here
to sand seas where dune forms are identifiable, but are
presently inactive or have limited surface activity, because
current environmental factors do not favour the significant
operation of aeolian processes. This is largely evidenced
by the presence of a significant vegetation cover on dunes,
e.g. in large parts of Australia and the northern Kalahari
sand sea in southern Africa, but also through other evi-
dence such as the occurrence of soils on dune surfaces,
and degradation of dune forms. Although it is the inter-
action between atmospheric (
erosivity
, by wind energy)
and surface (
erodibility
, linked in particular to vegetation
cover) conditions that determines the potential for sand
transport (Lancaster, 1988; Thomas, Knight and Wiggs,
2005), most of the major sand seas that are obviously
active today lie within the 150 mm mean annual isohyet
(Wilson, 1973) (Figure 17.4), where given a supply of sed-
iment and suitable wind conditions, aeolian transport is
uninhibited by plants. The implication of fixed sand seas
is that climatic and environmental conditions in the past
have been more conducive to aeolian sand transport than
today, particularly during the Late Quaternary Period.
1. Topographical obstacles
2. Smaller obstacles
Pebble or boulder
Plant or bush
Animal carcass, e.g. dead
camel
Human artefact, e.g. oil
drum
3. Remnant of previous
bedform
4. Surface irregularities
and hollows
Source:
After Wilson (1972b) with modifications.
Ahlbrandt (1979) and others, an
active sand sea
may be
defined as an area of at least 125 km
2
where no less than
20 % of the ground surface is covered by windblown
sand. Lancaster (1995) has defined sand seas as 'dynamic
sedimentary bodies that form part of local- and regional-
scale sand transport systems in which sand is moved
from source zones to depositional sinks'. Dune forms are
commonly present, but in areas where sand throughflow
SAND SEAS
Active
Probably limited activity
Fixed
LOESS
Major deposits
Thin or uncertain deposits
Figure 17.3
The global distribution of aeolian deposits (after Snead, 1972, modified and supplemented with data from
