Wind-deposited materials hold clues to past as well as to present wind
directions and intensities. These features help us understand the
present climate and the forces that molded it. Wind-deposited sand
bodies occur as sand sheets, ripples, and dunes.
_Sand sheets_ are flat, gently undulating sandy plots of sand surfaced
by grains that may be too large for saltation. They form approximately
40 percent of eolian depositional surfaces. The Selima Sand Sheet, which
occupies 60,000 square kilometers in southern Egypt and northern Sudan,
is one of the Earth’s largest sand sheets. The Selima is absolutely flat
in some places; in others, active dunes move over its surface.
Wind blowing on a sand surface _ripples_ the surface into crests and
troughs whose long axes are perpendicular to the wind direction. The
average length of jumps during saltation corresponds to the wavelength,
or distance between adjacent crests, of the ripples. In ripples, the
coarsest materials collect at the crests. This distinguishes small
ripples from dunes, where the coarsest materials are generally in the
troughs.
[Illustration: Wind-blown sand moves up the gentle upwind side of the
dune by saltation or creep. Sand accumulates at the brink, the top of
the slipface. When the buildup of sand at the brink exceeds the angle of
repose, a small avalanche of grains slides down the slipface. Grain by
grain, the dune moves downwind.]
Accumulations of sediment blown by the wind into a mound or ridge, dunes
have gentle upwind slopes on the wind-facing side. The downwind portion
of the dune, the lee slope, is commonly a steep avalanche slope referred
to as a _slipface_. Dunes may have more than one slipface. The minimum
height of a slipface is about 30 centimeters.
Sand grains move up the dune’s gentle upwind slope by saltation and
creep. When particles at the brink of the dune exceed the angle of
repose, they spill over in a tiny landslide or avalanche that reforms
the slipface. As the avalanching continues, the dune moves in the
direction of the wind.
Some of the most significant experimental measurements on eolian sand
movement were performed by Ralph Bagnold, a British engineer who worked
in Egypt prior to World War II. Bagnold investigated the physics of
particles moving through the atmosphere and deposited by wind. He
recognized two basic dune types, the crescentic dune, which he called
“barchan,” and the linear dune, which he called longitudinal or “sief”
(Arabic for “sword”).
[Illustration: Sand dunes in Death Valley, California (photograph by
Richard Frear).]
[Illustration: Ripples on a dune in Eureka Valley, California
(photograph by Terrence Moore).]