Physical Resources / 111The classification may be powerful, but there are attractions in calling Mollisols ‘prairie soils’ (or
chernozems), Histosols ‘peat’ or ‘muck’, and, given the widespread environmentalist concern over
the degradation of some tropical soils, calling Oxisols ‘lateritic soils’, which are the names by which
they used to be known. There is also a Canadian classification system that divides soils into two
orders, Brunisolic comprising 4 Great Groups and Chernozemic with 3 Great Groups and a total of
42 Subgroups (burgundy.uwaterloo.ca/bio1446/chap2bsm.htm).
27. Transport by water and wind
Once it has developed, soil does not necessarily remain in the place where it formed. It can be transported,
sometimes over very long distances, by wind and water, and over short distances by gravity.
Occasionally, rain in northern Europe leaves everything exposed to it coated by a thin layer of reddish
dust. It is Saharan dust, lifted from the desert, carried some 2500 km by air movements, then washed to
the ground. Some of that dust remains to form a very minor constituent of European soil. Fine soil
particles lofted by wind in the North American Great Plains during the dust-bowl years of the 1930s fell
as dust in New York and discoloured the Atlantic for hundreds of kilometres from the American coast.
The Dust BowlAn area of about 390000 km^2 in south-western Kansas, south-eastern Colorado,
north-eastern New Mexico, and parts of Oklahoma and Texas that was originally
prairie. The climate is semi-arid and prolonged droughts are common. After
1918, US grain prices rose steeply, encouraging farmers to plough the prairie,
and for several years they produced satisfactory yields. A drought began in
1933 and lasted until 1939, being especially severe in 1934 and 1935. It was
in those two years that topsoil, exposed by ploughing and reduced by aridity to
a fine dust, blew away, the lighter particles forming clouds 8 km high. Farmers,
most of whom were already poor, were ruined and thousands of them migrated
to seek other work.
In 1935 the US Department of Agriculture founded the Soil Conservation Service
to promote sound soil conservation practices throughout the country, and with
the return of the rains some farming resumed, although much of the area was
returned to grass. Drought has continued to afflict the region at intervals of
about 20–22 years.Where aeolian (wind-blown) deposits accumulate they make a soil known as ‘loess’. Obviously, the
smaller the particles the further the air will carry them, and so the material is graded, becoming finer
with increasing distance from its source. Loess soils are extensive, covering much of the central
United States, where in places they are several metres thick (HUNT, 1972, p. 138). There are also
extensive deposits in Argentina, various parts of Europe, and in China; in the northern and eastern
highlands they are believed to be 300 m thick (DONAHUE ET AL., 1958, pp. 24–25), with the
underlying rock projecting through it locally as hill and mountain ranges.
Although deserts are a source of wind-blown dust, modern dust storms make only a minor contribution
to such deposits; most are ancient, dating from past ice ages. When glaciers thawed, they released