Page 302 - Fundamentals of Geomorphology

PERIGLACIAL LANDSCAPES 285

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 evidence for a cell-like soil circulation has been found.
sets of processes seem important – sorting processes, But the processes involved in patterned ground forma-
slope processes, and patterning processes (Figure 11.5). tion are complex, and all the more so because similar
The main patterning processes are cracking, either by kinds of patterned ground appear to be created by dif-
thermal contraction (frost cracking), drying (desicca- ferent processes (an example of equifinality – see p. 25),
tion cracking), or heaving (dilation cracking), of which and the same processes can produce different kinds of
only frost cracking is confined to periglacial environ- patterned ground. For instance, patterned ground occurs
ments. Patterning may also result from frost heaving and in deserts.
mass displacement. Frost heaving is also an important
source of sorting, helping to segregate the large stones Periglacial slopes
by shifting them upwards and outwards leaving a fine-
grained centre. As many forms of patterned ground are Periglacial slopes are much like slopes formed in other
so regular, some geomorphologists have suggested that climatic regimes, but some differences arise owing to
convective cells form in the active layer. The cells would frost action, a lack of vegetation, and the presence of
develop because water is at its densest at 4 C. Water at frozen ground. Frost-creep and gelifluction are important
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the thawing front is therefore less dense than the over- periglacial processes and form sheets, lobes, and terraces.
lying, slightly warmer water and rises. Relatively warm Gelifluction sheets, which occur mainly in the High Arc-
descending limbs of the convective cells would cause tic, where vegetation is absent, tend to produce smooth
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undulations in the interface between frozen and unfrozen terrain with low slope gradients (1 to 3 ). Tongue-like
soil that might be echoed in the ground surface topog- lobes are more common in the tundra and forest tun-
raphy. How the echoing takes place is uncertain, but dra, where some vegetation patches occur (Plate 11.6).
frost heaving is one of several possible mechanisms. Stripe Solifluction lobes tend to form below snow patches.
forms would, by this argument, result from a downslope Terraces are common on lower slopes of valleys
distortion of the convective cells. Another possibility (Colour Plate 15, inserted between pages 208 and 209).
is that convective cells develop in the soil itself, and Ploughing boulders or ploughing blocks move down