Physical Resources / 113stones released by frost-shattering from adjacent rock. These stones, most of very irregular shape,
will tend to become aligned with their long axes indicating the direction of slope and parallel to the
surface. At the base of the slope the flowing material will collect as ‘head’, often forming a thick
deposit. The sliding of material down a slope over ground that is frozen a little way below the surface
is called ‘gelifluction’ or ‘congelifluction’. It is the cold-climate variety of solifluction, the downslope
creep of material lubricated by water.
Glacial meltwaters often flowed with great force. They carried huge volumes of water, sometimes
through confined spaces so they were under pressure, and they carried particles of all sizes, including
quite large stones. As the flow slowed, the heaviest particles were deposited first and such ‘outwash’
material is usually coarse-grained and sorted by grain size into beds. The finer-grained material
travelled further and settled as mud on land that dried as the water flow ceased or on the bottom of
glacial lakes, many of which have since disappeared. Glacial lacustrine deposits consist mainly of
clay of little agricultural value, but in some places used for brick-making.
Soils derived from river-borne sediment are called ‘alluvial’ and occur on land that is, or was,
repeatedly flooded. Many rivers burst their banks occasionally, but for the flooding to affect soil
formation they must do so often, and this is most likely if their waters are periodically augmented by
drainage following very heavy rain or seasonally by the melting of deep snow. Charged with a
greater volume of water to transport, they flow faster and this gives them the energy to transport
more material. When the river overflows its banks, the water escapes to the sides and the pressure on
it is greatly reduced. It loses energy and deposits its load, the heaviest particles first. These may
collect close to the point where the overflow occurred, after many floodings forming a distinctive
raised bank, called a ‘levee’. This may become covered with soil, and small particles will be trapped
within it, but basically it is made from gravel and larger stones, and water drains through it freely.
Silt and clay are precipitated beyond the levee. They pack tightly together and drain poorly, but are
rich in plant nutrients and the flood plain they produce as they fill natural depressions and make a
level surface is usually very fertile.
Downstream, where the river, fed by many tributaries, flows as a wide stream across land with
a very low gradient, it may form meanders. A system of meanders may also create a flood plain,
but by a quite different mechanism that does not require the river to burst its banks. On the
outside of each meander bend, the stream flows against the bank. This increases its turbulence
and also its speed, since it has slightly further to travel, and material is drawn into the water
from the bank, eroding it. Some of this material may flow across the stream, near the bed, where
it enters water flowing against the inside bank. Here the flow is slower and calmer. The river
loses energy and deposits some of its load, extending the bank into the channel. Figure 3.11
illustrates the process.
At the same time, the entire meander tends to migrate in a downstream direction and since this
movement affects all the meanders, the entire system migrates downstream. The land behind a
migrating meander is covered by river-bed material, so an alluvial plain forms that is the same width
as the widest meander. Meanders migrate slowly, so the fertile flood plain can be used, although the
ground may be wet for most of the time.
Till and alluvium, as well as loess, are commonly buried beneath soils developed since they were
deposited, so they may not be visible at the surface. They will be detectable, however, because they
form the parent material for the soil covering them. This soil may not resemble its parent, but will
betray its origin by being unrelated to the underlying bedrock.
A section through the soil will reveal its character, of course, and a river may cut a suitable section.
As Figure 3.12 shows, a stream channel quickly penetrates the surface material to expose the