112 / Basics of Environmental Science
meltwater that flowed as rivers and flooded low-lying areas. The waters carried suspended particles,
which settled as mud. Then, as the temperature fell, the meltwater flow ceased, the flooded areas
dried, and wind blew the dust far away from the valley bottoms. The process was repeated at intervals,
some of them long enough for new soil to begin to form above the loess before being buried beneath
a later deposit. Loess soils are usually yellowish in colour and when young they are rich in mineral
plant nutrients and calcium. This makes them inherently fertile, although their later history may
exhaust that fertility. They are fine-grained, and rivers cutting through loess commonly have very
steep, almost vertical, banks. Much of south-east England was once blanketed by such loessic soils.
As you will know if you have walked over a dry, sandy beach in a strong wind, sand grains also blow.
Being much larger than the silt-sized particles that comprise loess, they are not carried far, but
repeated lifting and dropping by a prevailing wind can transport them a considerable distance. Where
they accumulate, dunes form, sometimes with a characteristic shape from which the wind direction
can be determined. Crescent-shaped (called ‘barchan’) dunes, for example, are aligned across the
direction of the prevailing wind, the convex side facing the wind behind a long ‘tail’ of sand being
blown little by little up the tail to the top, where the dune collapses on the sheltered side to produce
the face; strong, steady winds erode valley-like troughs and linear dunes; dunes forming straight
ridges may be parallel (seif dunes) or at right angles to the wind direction (aklé dunes); and changing
winds produce star dunes, of radiating ridges.
Blown sand is unsuitable for cultivation, but it can be stabilized if hardy plants become established
on it. In temperate climates marram grass (Ammophila arenaria) is often used for initial colonization
in coastal areas. Its underground stems (rhizomes) form networks that help hold the sand in place
and allow other plants to obtain purchase. As a diverse plant community develops the marram grass
is unable to compete and disappears. Then soil will form above the sand, burying it. Unstabilized
sand dunes migrate slowly and can bury fertile land downwind.
Water is a much more powerful agent of transport than wind. Gravel and small stones cannot be
carried far by even the strongest wind, but a river can move them long distances. A soil that is
mineralogically unrelated to the bedrock beneath it and that contains sand mixed with stones of
various sizes has been placed in its present position by water. It may be a marine deposit, formed on
the bed of a sea that has long since disappeared. If the particles are sorted by size into layers, the
deposit is more likely to be lacustrine, marking the location of a former lake. Many lacustrine deposits
contain a high proportion of clay particles; if the material is more than 50 per cent clay it will be
almost impermeable to water and thus prone to waterlogging or flooding.
Glaciers also transport material, but they seldom carry it very far. Their action is mainly to mix the
soil already formed beneath them and to transport large pieces of rock that become frozen into the
ice. Then, when the glacier retreats by melting at its lower end, these stones join the mixed soil to
make ‘till’ (which was formerly known as ‘boulder clay’). Till deposits cover substantial areas in
Europe and North America. Although glaciers rarely carried the till more than 10 km, large stones
entrained in the ice were sometimes carried much further and deposited as ‘erratics’. Their
orientation and that of stones in the till itself can be used to determine the direction in which the
ice was moving. Because glaciers filled or made broad, flat-bottomed valleys, glacial till often
occurs in gently rolling ‘till plains’. Material pushed to the sides of a glacier and ahead of its
upward-curved front, or ‘snout’, was left as a moraine, now visible as ridges or hills that are often
too rocky to be easily cultivable.
Permafrost occurs in the vicinity of glaciers and ice sheets. This is ground where the temperature
below the surface remains below freezing throughout the year. In summer the surface layers may
thaw, and if the soil is on a slope greater than about 2°, the resulting mud will flow, carrying with it