The Work of Wind and Deserts - The Changing Earth: Exploring Geology and Evolution - page 400

?

walls confine streams, the sediment spreads out laterally,
forming a gently sloping and poorly sorted fan-shaped
sedimentary deposit (

What Would You Do

Figure 15.22). Alluvial fans are
similar in origin and shape to deltas (see Chapter 12), but
are formed entirely on land. Alluvial fans may coalesce to
form a bajada, a broad alluvial apron that typically has an
undulating surface resulting from the overlap of adjacent
fans (

◗

You have been asked to testify before a congressional
committee charged with determining whether the National
Science Foundation should continue to fund research de-
voted to the study of climate changes during the Cenozoic
Era. Your specialty is desert landforms and the formation
of deserts. What arguments would you make to convince
the committee to continue funding research on ancient
climates?

Figure 15.23).
Large alluvial fans and bajadas are frequently impor-
tant sources of groundwater for domestic and agricul-
tural use. Their outer portions are typically composed of

◗

◗

Figure 15.22 Alluvial Fan A ground view of an alluvial fan, Death Valley, California. Alluvial fans form
when sediment-laden streams fl owing out from a mountain deposit their load on the desert fl oor, forming
a gently sloping, fan-shaped, sedimentary deposit.

fi ne-grained sediments suitable for cultivation, and their
gentle slopes allow good drainage of water. Many alluvial
fans and bajadas are also the sites of large towns and cities,
such as San Bernardino, California; Salt Lake City, Utah;
and Teheran, Iran.
Most mountains in desert regions, including those of the
Basin and Range Province, rise abruptly from gently sloping
surfaces called pediments. Pediments are erosional bedrock
surfaces of low relief that slope gently away from mountain
bases (

Figure 15.24). Most pediments are covered by a thin
layer of debris, alluvial fans, or bajadas.
The origin of pediments has been the subject of much
controversy. Most geologists agree that they are erosional
features developed on bedrock in association with the ero-
sion and retreat of a mountain front (Figure 15.24a). The

◗

◗ Figure 15.23 Bajada Coalescing alluvial fans forming a bajada
at the base of these mountains in Death Valley, California.