pieces of rock that are quite angular in all dimensions and act quite a bit differently
than the smaller pieces known as clay and larger pieces known as sand (seeSoils).
Land concentrations of silt are plentiful because of the Pleistocene glaciations that
covered much upper middle latitude continental surface ground bedrock into those
sizes. Barren of vegetation after the major melting commenced about 18,000 years
ago, landscapes dried out and the silts were moved by the wind. The result is a
widespread silt covering called lo ̈ss (from the Swiss German meaning “loose”).
Lo ̈ss can also have origins out of arid regions and volcanic material. It probably
covers a tenth of the world’s land surfaces.
Lo ̈ss is light brown in color and is capable of producing very fertile soils that
are, unfortunately, very easily eroded via agricultural activity. Prominently, the
Huang He (Yellow) River drainage basin of China has a thick lo ̈ss covering caus-
ing the river and the adjoining sea to be yellow. Unlike sand, the angular pieces
of lo ̈ss can maintain vertical slopes. The bluffs along parts of the Mississippi River
valley of the United States represent lo ̈ss many tens of meters thick.
HillsofsandarecommononEarthand are known as sand dunes. These are
formed of materials 0.1 mm across up to 2.0 mm across and can be composed of
weathered quartz, gypsum, calcium carbonate, or other materials. The world’s
largest dunes (in Peru) exceed 1,100 m from base to summit. It is tempting to
believe that deserts are covered with ergs (from the Arabic for “dune field”). Sand
is relatively plentiful in arid regions in that it is the result of incomplete chemical
weathering. Aridity greatly slows weathering and largersand particles are
common rather than the clays in more humid environments. It should be noted that
dunes occur in non-arid regions. For instance, major dune formations can be found
along the shores ofoceansand large lakes. Here, processes within the water
present a plentiful supply of sand to the coast and then wind builds the dunes.
The expansive Sand Hills of north central Nebraska are vegetation-covered relicts
of the sand transported on the ending of the Pleistocene ice age.
There are three major dune types and other combinations and subtypes. The
most prevalent type is the barchan, which results when wind-blown sand is slowed
by a small obstruction. The sand builds up in an arc with the tips and steeper side
faced away from the wind. This dune type is indicative of a small supply of sand
with a consistent wind direction. Martian barchans have been observed by satel-
lite, thus providing someremote sensingknowledge of conditions on Mars.
A second type of dune is the seif, from the Arabic word for “sword.” The seif is
also called a longitudinal dune. The form is that of a ridge with two slopes that
average 33°inclination. The result is a long thin form that is about six times as
wide as it is high. Seifs are formed in places with modest supplies of sand and sea-
sonally alternating wind directions. Seifs are found in the Sahara and in Saudi Ara-
bia and can be as many as 400 km long.366 Wind Erosion and Deposition