Geology Reference
In-Depth Information
patterns that form as a result of wind activity and are
visible as contrasts in albedo, or surface re
ectivity.
Repetitive imaging shows that many of them disappear,
reappear, or change their size, shape, or position with
time. Mapping the orientations of variable features has
been used to derive near-surface wind patterns on Mars
and Venus.
3.5.5 Periglacial processes
Figure 3.38. Sand dunes form from the accumulation of grains
moved primarily in saltation. Stoss refers to the upwind side of
dunes, while slipface refers to the downwind side of the dune
formed by sands sliding into their angle of repose from the dune
brink; the dune crest refers to the highest part of the dune. This view
of two barchan dunes was obtained by E. C. Morris in the Sechura
Desert of Peru (courtesy of US Geological Survey).
Periglacial refers to processes, conditions, areas, climate,
and topographic features in cold regions or in any environ-
ment where frost action is important. A review of
the various environments in the Solar System shows
that all planets and satellites except Venus experience
Figure 3.39. Sand dunes are classi
ed primarily on
their shape, shown in these diagrams for common
dunes on Earth; arrows indicate the prevailing
wind directions responsible for their formation
(from McKee,
1979
).
