Page 133 - Sedimentology and Stratigraphy

120 Aeolian Environments

in amplitude. These structures are known as draas
and there is again evidence that they are a distinct,
larger bedform separate from the dunes that may be
superimposed on them (Wilson 1972). Draas are
usually made up of dunes on the stoss and lee sides,
but a single slip face may develop on some lee slopes.
They show a similar variability of shape to dunes with
star, linear and transverse forms.

8.4.4 Palaeowind directions

The slip faces of aeolian dunes generally face down-
Fig. 8.7 Aeolian ripples superimposed on an aeolian dune.
wind so by measuring the direction of dip of cross-
beds formed by the migration of aeolian dunes it is
possible to determine the direction of the prevailing
wind at the time of deposition (Fig. 8.9). Results can
be presented as a rose diagram (5.3.3). The variability
of the readings obtained from cross-beds will depend
upon the type of dune (McKee 1979). Transverse
dunes generate cross-beds with little variability in
orientation, whereas the curved faces of barchan
dunes produce cross-beds that may vary by as much
as 458 from the actual downwind direction. Multiple
directions of cross-bedding result from the numerous
slip faces of a star dune. In all cases the confidence
with which the palaeowind direction can be inferred
from cross-bedding orientations is improved with the
more readings that are taken.
Wind directions are normally expressed in terms of
the direction the wind blows from, that is, a south-
westerly wind is one that is blowing from the south-
west towards the northeast and will generate dune
cross-bedding which dips towards the northeast. Note
that this form of expression of direction is different from
that of water currents that are normally presented in
terms of the direction the flow is towards.

8.5 DESERT ENVIRONMENTS
Aeolian sands form one part of an arid zone facies
Fig. 8.8 Grain flow on the lee slope of an aeolian dune.
association that also includes ephemeral lake deposits
and alluvial fan and/or ephemeral river sediments
pattern of structures that are an order of magnitude (Figs 8.12 & 8.13). In these dry areas, sediment is
larger than dunes. The surface of the erg shows an brought into the basin by rivers that bring weathered
undulation on a scale of hundreds of metres to kilo- detritus from the surrounding catchment areas and
metres in wavelength and tens to hundreds of metres deposit poorly sorted mixtures of sediment on alluvial