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Fig. 8.3 An example of a regional sand transport system, viewed
from an airliner. The Kelso dunes in southern California are seen at the
bottom of the picture, looking north: the sand accumulates in part due
to topographic blocking and in part due to convergent winds. A
dominant pathway for the sand (sourced primarily from the Mojave
river) is evident towards the top of the picture, where lee dunes and
sand streaks in the Devil's Playground show sand migrating eastwards
into the Kelso basin where it then turns south to pile up in the main
Kelso dune system. From the tip of the arrow to the rightmost end of
the dunes is *30 km. Photo R. Lorenz
the dunes. With Titan there is essentially the opposite
problem. The dunes form a long and nearly contiguous belt
around the equator, with neither an obvious beginning nor
an end. The sand source has not been identified, although
one speculation is that the sand may form in the polar seas:
if this is the case, then somehow the sand must not only
migrate *4000 km from pole to equator, but must do so
uphill. Clearly, much work remains to be done on this topic.
8.2
Dune Stabilization
Large deposits of sand can be found on Earth which are not
active, or are only weakly so, and so the dunes are not
especially large or prominent; examples include the
Nebraska sand hills, the Kalahari, and some of the Australian
deserts. This is usually because vegetation cover (see
Figs. 8.5 and 8.6 ) becomes sufficient to halt significant
movement of sand by the wind; both sand supply and sand
loss essentially go to zero. The ability of vegetation to grow
is largely a function of rainfall amount, which accounts for
the great attention paid to that quantity in terrestrial arid land
studies. As noted in Sect. 1.2, rainfall patterns can change in
both directions, and it is expected that the Kalahari desert
Fig. 8.4 Sand of different compositions (false color highlighting the
differences) 'flowing' in a larger-scale sand transport system near the
Terkezi Oasis in Libya. Image is about 120 km across. Even larger-
scale sand transport systems are visible in remote sensing of Earth
(see, e.g., Fig. 12.6 ) . US Geological Survey image from Landsat 7
 
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