Geography Reference
In-Depth Information
and in soil this is usually parallel to the ground surface. Frost
heaving moves masses of soil and may even push stones upwards to
the surface. This may happen as the stone and surrounding soil gets
pushed up during freezing in the active layer. When melting occurs
in the summer, the finer sediments settle back down filling in the
gap below the stone and supporting it. Additionally, soil water
flowing around the stone may flow into the pore spaces below it
and when this water freezes push the stone up once more. This
leaves the stone a little higher in the soil profile each year. Over
long periods this results in a net movement of stones to the surface.
The types of mass movement described in the section earlier in
this chapter on weathering and erosion occur in periglacial areas.
However,
frost creep
and
soliluction
are important in these
environments. Frost creep occurs when sediment is pushed
upwards on a slope during freezing, as part of frost heave, but then
gravity pulls the sediment in a downslope direction when it melts
and lowers so that over many years there is a net movement
downslope. Frost creep will operate in conjunction with solifluc-
tion. Solifluction is the slow downslope movement of saturated soil
in a very slow flow. It is more exaggerated in periglacial environ-
ments where this process in known as
gelifluction.
.
Water can either freeze within pore spaces between solid parti-
cles of soil or sediment, or it can migrate to form discrete masses of
ice known as
segregated ice
.
Coarse gravels and sands are highly
permeable but because pore spaces are large there is little 'suction'
potential (see section on soils in Chapter 3) and so they do not
retain much water. Finer soils such as clay have low permeability
but high water retention potential. This means that soils with inter-
mediate grain sizes, such as silt, have the greatest potential to form
segregated ice within the ground. Segregated ice may form lenses
or form bands. Where the bands are thick, sometimes up to several
metres, they are known as
massive ice
. A large body of experi-
mental research has shown that liquid films can coat ice surfaces
even when the temperature is below the pressure-melting point.
These films provide fluid conduits that supply the growth of segre-
gated ice. Hence there is slow movement of very thin films (a few
hundreds of a thousandth of a metre in thickness) of water that
coats the ice and enlarges it. The movement of these thin films
occurs from the sediment adjacent to the ice through several quite
