Geology Reference
In-Depth Information
Geo-Focus
The Dust Bowl—An American Tragedy
The stock market crash of 1929 ushered
in the Great Depression, a time when
millions of people were unemployed
and many had no means to acquire
food and shelter. Urban areas were
affected most severely by the depression,
but rural areas suffered as well, espe-
cially during the great drought of the
1930s. Prior to the 1930s, farmers had
enjoyed a degree of success unparalleled
in U.S. history. During World War I
(1914-1918), the price of wheat soared,
and after the war, when Europe was
recovering, the government subsidized
wheat prices. High prices and mecha-
nized farming resulted in more and
more land being tilled. Even the weather
cooperated, and land in the western
United States that would otherwise have
been marginally productive was plowed.
Deep-rooted prairie grasses that held
the soil in place were replaced by
shallow-rooted wheat.
Beginning about 1930, drought pre-
vailed throughout the country. Drought
conditions varied from moderate to se-
vere, but the consequences were particu-
larly severe in the southern Great Plains.
And because the land, even marginal
land, had been tilled, the native vegeta-
tion was no longer present to keep the
topsoil from blowing away. And blow
away it did—in huge quantities.
A large region in the southern Great
Plains that was particularly hard hit by
drought, dust storms, and soil erosion
came to be known as the Dust Bowl.
Although its boundaries were not well
defi ned, it included parts of Kansas, Col-
orado, and New Mexico, as well as the
panhandles of Oklahoma and Texas
(
otherwise might have been productive
soils. Agricultural production fell pre-
cipitously, farmers could not meet their
mortgage payments, and by 1935, tens
of thousands were homeless, on relief, or
leaving (Figure 1c). Many went west to
California and became the migrant farm
workers immortalized in John Stein-
beck's novel
The Grapes of Wrath.
The Dust Bowl was an economic
disaster of great magnitude. Droughts had
stricken the southern Great Plains before
and have done so since, but the drought
of the 1930s was especially severe. Politi-
cal and economic factors contributed to
the disaster. Due in part to the artifi cially
infl ated wheat prices, many farmers were
deeply in debt—mostly because they
had purchased farm machinery in order
to produce more and benefi t from the
high prices. Feeling economic pressure
because of their huge debts, they tilled
marginal land and employed few, if any,
soil conservation measures.
If the Dust Bowl has a bright side,
it is that the government, farmers, and
the public in general no longer take soil
for granted or regard it as a substance
that needs no nurturing. In addition, a
number of soil conservation methods
developed then have now become
standard practices.
Figure 1a); together the Dust Bowl
and its less affected fringe area covered
more than 400,000 km
2
!
Dust storms were common during
the 1930s, and some reached phenom-
enal sizes (Figure 1b). One of the largest
storms occurred in 1934 and covered
more than 3.5 million km
2
. It lifted dust
nearly 5 km into the air, obscured the sky
over large parts of six states, and blew
hundreds of millions of tons of soil east-
ward where it settled on several eastern
cities, as well as on ships 480 km out in
the Atlantic Ocean. The Soil Conserva-
tion Service reported dust storms of
regional extent on 140 occasions during
1936 and 1937. Dust was everywhere. It
seeped into houses, suffocated wild ani-
mals and livestock, and adversely affected
human health.
The dust was, of course, the topsoil
from the tilled lands. Blowing dust
was not the only problem; sand piled
up along fences, drifted against houses
and farm machinery, and covered what
◗
two meanings. One is a size designation, but the term also
refers to certain types of sheet silicates known as
clay miner-
als.
However, most clay minerals are also clay sized.
Weathering is fundamental to the origin of sediment and
sedimentary rocks, and so are erosion and
deposition
—
that is, the movement of sediment by natural processes
and its accumulation in some area. Because glaciers
are moving solids, they can carry sediment of any size,
whereas wind transports only sand and smaller sedi-
ment. Waves and marine currents transport sediment
along shorelines, but running water is by far the most
common way to transport sediment from its source to
other locations.
During transport,
abrasion
reduces the size of particles, and
the sharp corners and edges are worn smooth, a process known
as
rounding
, as pieces of sand and gravel collide with one another
(
◗
Figure 6.16a, b). Transport and processes that operate where
sediment accumulates also result in
sorting
, which refers to the
particle-size distribution in a sedimentary deposit. Sediment is
characterized as well sorted if all particles are about the same
size, and poorly sorted if a wide range of particle sizes is present
(Figure 6.16c). Both rounding and sorting have important im-
plications for other aspects of sediment and sedimentary rocks,
such as how readily fl uids move through them, and they also
help geologists decipher the history of a deposit.
Regardless of how sediment is transported, it is even-
tually deposited in some geographic area known as a
depositional environment
. Deposition might take place on a
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