Geoscience Reference
In-Depth Information
been accepted, and detailed field mapping revealed that the loess was associated
with deflation from the outwash plains along the southern margins of the Laurentide
and Cordilleran ice caps. The loess is not necessarily synchronous with full glacial
conditions only, because it continued to form during early deglacial times, which were
still cold and windy and saw limited plant cover. Luminescence dating of loess has
enabled the rates of loess accumulation to be determined with increasing accuracy
and precision. For example, the rate of accumulation of loess in what is now semi-
arid Nebraska was very high between 18 and 14 ka (Roberts et al.,
2003
). The high
atmospheric dust loading over that area may have caused the climate there to remain
colder than present for several thousand years, despite higher-than-present summer
insolation values (Roberts et al.,
2003
). Away from the ice margins, in the deserts of
the south-west, wind-blown dust derived from bare alluvial surfaces and small patches
of sand dunes contributed fine material in the form of clay and calcium carbonate to
the soils, which began to form as the climate became less arid and plants colonised the
previously bare surfaces of alluvial fans and river flood plains. Much as in central Asia,
the loess deposits of the Great Plains consist of alternating units of loess and fossil
soils (see
Chapter 9
). As a general rule, the loess and desert dust accumulated during
colder, drier, windier episodes when the glacial outwash and desert source areas were
more extensive and frost action was pronounced across the landscape (Pewe,
1981
;
Pye,
1987
; Maher et al.,
2010
). The soils developed on the loess reflect a warmer,
wetter climate and an established plant cover. This applies equally to the alluvial fans
and pediments of the Basin and Range Province, where the late Pleistocene soils are
relatively thick and have well-developed soil horizons (see
Chapter 15
), in contrast to
the much thinner and less-developed Holocene soils (McFadden et al.,
1986
; Wells
et al.,
1987
; Dohrenwend et al.,
1991
). Soils overlying volcanic rock often contain
eolian quartz indicating that the parent material was blown in during dust-storms.
In the Chihuahuan, Sonoran and Mojave deserts, the area covered by desert dunes
is quite limited. The gypsum dunes in White Sands National Monument in the Chi-
huahuan Desert were blown from gypseous playa deposits, and often form a series
of crescentic dunes reminiscent of the gypseous lunettes of Algeria and southern
Australia. In the Mojave Desert, many of the sand ramps deposited against the foot of
the mountains are polygenic and contain fossil soils as well as colluvial and alluvial
deposits (Tchakerian,
2009
). The
Gran Desierto del Altar
sand sea (5,700 km
2
)inthe
north-west Sonoran Desert is at the distal end of the Colorado River and has been sub-
ject to repeated changes in local climate, tectonic events and changes in the location
of the Colorado River delta, with the result that the dunes have varied orientations
and often cross older, underlying dunes (Warren,
2013
,
fig. 8.12
).
The Nebraska Sand Hills (51,000 km
2
) are the most extensive of the former sand
seas on the High Plains, and are continued to the west and south by smaller dune fields
in Wyoming, Colorado, Kansas, New Mexico, Oklahoma and Texas. The majority
of these dunes are now vegetated and stable, although in extreme drought years they
