Geology Reference
In-Depth Information
Plate 11.1
Thermokarst thaw lakes, Mackenzie Delta, Northwest Territories, Canada.
(
Photograph by Tony Waltham Geophotos
)
The origin of patterned ground is not fully clear. Three
sets of processes seem important - sorting processes,
slope processes, and patterning processes (Figure 11.5).
The main patterning processes are cracking, either by
thermal contraction (frost cracking), drying (desicca-
tion cracking), or heaving (dilation cracking), of which
only frost cracking is confined to periglacial environ-
ments. Patterning may also result from frost heaving and
mass displacement. Frost heaving is also an important
source of sorting, helping to segregate the large stones
by shifting them upwards and outwards leaving a fine-
grained centre. As many forms of patterned ground are
so regular, some geomorphologists have suggested that
convective cells form in the active layer. The cells would
develop because water is at its densest at 4
◦
C. Water at
the thawing front is therefore less dense than the over-
lying, slightly warmer water and rises. Relatively warm
descending limbs of the convective cells would cause
undulations in the interface between frozen and unfrozen
soil that might be echoed in the ground surface topog-
raphy. How the echoing takes place is uncertain, but
frost heaving is one of several possible mechanisms. Stripe
forms would, by this argument, result from a downslope
distortion of the convective cells. Another possibility
is that convective cells develop in the soil itself, and
evidence for a cell-like soil circulation has been found.
But the processes involved in patterned ground forma-
tion are complex, and all the more so because similar
kinds of patterned ground appear to be created by dif-
ferent processes (an example of equifinality - see p. 25),
and the same processes can produce different kinds of
patterned ground. For instance, patterned ground occurs
in deserts.
Periglacial slopes
Periglacial slopes are much like slopes formed in other
climatic regimes, but some differences arise owing to
frost action, a lack of vegetation, and the presence of
frozen ground. Frost-creep and gelifluction are important
periglacial processes and form sheets, lobes, and terraces.
Gelifluction sheets, which occur mainly in the High Arc-
tic, where vegetation is absent, tend to produce smooth
terrain with low slope gradients (1
◦
to 3
◦
). Tongue-like
lobes are more common in the tundra and forest tun-
dra, where some vegetation patches occur (Plate 11.6).
Solifluction lobes
tend to form below snow patches.
Terraces
are common on lower slopes of valleys
(Colour Plate 15, inserted between pages 208 and 209).
Ploughing boulders
or
ploughing blocks
move down
