Geoscience Reference
In-Depth Information
Source area
Sand dunes
Loess
Erosion
of sand
and silt
Transport
of sand
and silt
Silt and sand
deposition
followed by
re-erosion of silt
Transport
of silt
Silt
deposition
10-20 km
100-1000 km
Figure 18.24 The process of eolian sorting.
After sediments are deflated from a source area, silts and clays
are transported great distances in suspension. Coarse sands, in contrast, travel a relatively short distance,
forming dune fields.
also owes its origins to the thick deposits of glacial till and out-
wash deposited during the Ice Age. Following the deposition
of these sediments by the glacier, the ice retreated, exposing
surfaces that were unvegetated and easily deflated by strong
winds. These winds picked up the silts contained within these
deposits and carried them downwind.
Loess is a distinctive deposit for two reasons. First, it is
often found over very broad areas as a mantle of windblown
sediment. Approximately 10% of the Earth's land surfaces are
covered in this way, including much of the central United States
and parts of Argentina, Russia, and China (Figure 18.25). These
areas are some of the most productive agricultural regions on
Figure 18.25 Significant loess depos-
its in the world.
Loess covers about
10% of the Earth's landmass and about
30% of the United States. Much of the
loess in the United States originated
from glacial sediments that were subse-
quently deflated.
180°
150° W
120° W
90° W
60° W
30° W
0°
30° E
60° E
90° E
120° E
150° E
180°
60° N
60° N
30° N
30° N
0°
0°
30° S
30° S
60° S
60° S
Loess deposits
0
3000 km
0
3000 mi
