Geology Reference
In-Depth Information
Loess sediments have some unique qualities that make them desirable as farmland:
Loess particles have not been altered by chemical weathering. The fact that
loess sediments originate in dry environments (including glacial deposits) means
that they have not been subjected to the heavy chemical weathering (see Chapter
7) that removes important elements from the minerals. This means these elements
are available in the loess deposits as mineral nutrients to support vegetation.
Loess particles are windblown and angular. Suspended particles carried by wind
are not rounded like particles carried by water. (Think of how rounded river rocks
are.) Rather, wind transport and abrasion create sharp, angular edges on the
particles. Because of the angular shapes, when the particles are deposited they
don't pack together very tightly, which means there is space between them for wa-
ter to accumulate. The result is that water is available as moisture for plant roots.
Paving the Desert: Deposition or Erosion?
A unique feature found in arid environments that are subjected to processes of wind
erosion and deposition is desert pavement. Desert pavement is a thin layer of pebbles
and rocks across the land surface.
For a long time, geologists thought desert pavement was the result of erosion — the res-
ult of wind removing smaller particles from the surface, leaving behind a layer of larger
pebbles and gravel. This layer left behind is called a lag deposit or lag feature. The pro-
cess of creating desert pavement with erosion is shown in Figure 14-6.
Figure 14-6: Desert
pavement created
by wind erosion.
However, evidence now suggests that desert pavement may be created by the depos-
ition of clay sediments instead of their erosion. By this process (shown in Figure 14-7),
clay and silt particles are deposited among the rocky surface materials and accumulate
through time, creating a layer beneath the surface rocks.
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