Page 131 - Sedimentology and Stratigraphy

118 Aeolian Environments

aeolianites, and these may be locally important com- grains will not be picked up by the air flow and where
ponents of coastal deposition (McKee & Ward 1983). few saltating grains land.
Aeolian ripples have extremely variable wave-
lengths (crest to crest distance) ranging from a few
8.4 AEOLIAN BEDFORMS
centimetres to several metres. Ripple heights (bottom
of the trough to the top of a crest) range from less than
The processes of transport and deposition by wind
a centimetre to more than ten centimetres. Coarser
produce bedforms that are in some ways similar to
subaqueous bedforms (4.3), but with some important grains tend to be concentrated at the crests, where
differences that can be used to help distinguish aeo- the finer grains are winnowed away by the wind, and
lian from subaqueous sands. Three groups can be as aeolian ripples migrate they may form a layer of
separated on the basis of their size: aeolian ripples, inversely graded sand. Where a crest becomes well
dunes and draas. Each appears to be a distinct class of developed grains may avalanche down into the adja-
bedform with no transitional forms and a plot of the cent trough forming cross-lamination, but this is less
range of sizes for each (Fig. 8.3) shows that they fall common in aeolian ripples than in their subaqueous
into three distinct fields (Wilson 1972). counterparts.

8.4.1 Aeolian ripple bedforms 8.4.2 Aeolian dune bedforms

As wind blows across a bed of sand, grains will move Aeolian dunes are bedforms that range from 3 m to
by saltation forming a thin carpet of moving sand 600 m in wavelength and are between 10 cm and
grains. The grains are only in temporary suspension, 100 m high. They migrate by the saltation of sand
and as each grain lands, it has sufficient energy to up the stoss (upwind) side of the dune to the crest
knock impacted grains up into the free stream of air, (Fig. 8.6). This saltation may result in the formation
continuing the process of saltation. Irregularities in of aeolian ripples which are commonly seen on the
the surface of the sand and the turbulence of the air stoss sides of dunes (Fig. 8.7). Sand accumulating at
flow will create patches where the grains are slightly the crest of the dune is unstable and will cascade
more piled up. Grains in these piles will be more down the lee slope as an avalanche or grain flow
susceptible to being picked up by the flow and at a (Fig. 8.8) (4.5.3) to form an inclined layer of sand
constant wind velocity all medium sand grains will (Fig. 8.6). Repeated avalanches build up a set of cross-
move about the same distance each time they saltate. beds that may be preserved if there is a net accumula-
The result is a series of piles of grains aligned perpen- tion of sand. At high wind speeds some sand grains
dicular to the wind and spaced equal distances apart. are in temporary suspension and are blown directly
These are the crests of aeolian ripples (Figs 8.4 & over the crest of the dune and fall out onto the
8.5). The troughs in between are shadow zones where lee slope. These grain fall deposits accumulate on

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Fig. 8.3 Aeolian ripples, dunes

!
and draas are three distinct types of

aeolian bedform.