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Fig. 5.23
Cross-bedded Jurrasic sandstone in Zion National Park, Utah. Fine-scale layers represent sand deposited upon a sand dune, while the
larger sets of layers, ranging from 2 to 12 m in height, record the passage of multiple sets of sand dunes. Photo J. Zimbelman
Sands National Monument (Fig.
5.26
). Recently, high-
albedo features (Fig.
5.27
) seen at an equatorial dune field
on Mars have been interpreted (Gardin et al. 2011) to be
analogous to these White Sands 'barchan scars'. This is
noteworthy since it would imply the existence of liquid
water (perhaps from the subsurface) in amounts and for
durations enough to cement the sand at the foot of the
slipface.
enough not to be sticky, it can be saltated, perhaps mixed
with sand, forming intricate multiple layers (Fig.
5.28
).
Subsequently, further saltation may bury the snow under
more sand.
The burial of snow by sand can both insulate the snow
from heat, and retard the diffusion of moisture, allowing the
snow to persist inside the dune. Depending on the temper-
ature history, the snow may melt, moistening the sand and
reducing its mobility (Fig.
5.29
). This moisture may then
refreeze, forming a very hard, cemented layer. On the other
hand, if the dune accumulates rapidly, the snow may be
preserved for long periods with no melting at all, and very
slow sublimation if any. One example, in the Killpecker
dune field in Wyoming, showed 25 cm-thick lenses of snow
1.5 m beneath the surface of a dune, some 4 months after
5.7
Niveo-Aeolian Processes
An interesting complication can occur with sand dunes in
cold regions on Earth in that both snow and sand can be
present. Once snow falls under conditions that it is cold
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