Page 298 - Fundamentals of Geomorphology

PERIGLACIAL LANDSCAPES 281

water as ice for much of the year, and the low levels and grooved bedrock surfaces, deflation hollows in
of biological activity. However, studies on compara- unconsolidated sediments, and ventifacts (p. 301). Wind
tive rates of chemical and mechanical weathering in is also responsible for loess accumulation (p. 296).
periglacial environments are few. One study from north-
ern Sweden indicated that material released by chem-
ical weathering and removed in solution by streams PERIGLACIAL LANDFORMS
accounted for about half of the denudational loss of
all material (Rapp 1986). Later studies suggest that, Many periglacial landforms originate from the presence
where water is available, chemical weathering can be of ice in the soil. The chief such landforms are ice and
a major component of the weathering regime in cold sand wedges, frost mounds of sundry kinds, thermokarst
environments (e.g. Hall et al. 2002). Geomorphic pro- and oriented lakes, patterned ground, periglacial
cesses characteristic of periglacial conditions include frost slopes, and cryoplanation terraces and cryopediments.
action, mass movement, nivation, fluvial activity, and
aeolian activity.
Ice and sand wedges
Fluvial action Ice wedges are V-shaped masses of ground ice that pen-
etrate the active layer and run down in the permafrost
Geomorphologists once deemed fluvial activity a rel- (Figure 11.3). In North America, they are typically 2–3 m
atively inconsequential process in periglacial environ- wide, 3– 4 m deep, and formed in pre-existing sediments.
ments due to the long period of freezing, during which Some in the Siberian lowlands are more than 5 m wide,
running water is unavailable, and to the low annual pre- 40–50 m long, and formed in aggrading alluvial deposits.
cipitation. However, periglacial landscapes look similar In North America, active ice wedges are associated with
to fluvial landscapes elsewhere and the role of fluvial continuous permafrost; relict wedges are found in the
activity in their creation has been re-evaluated. To be discontinuous permafrost zone. Sand wedges are formed
sure, river regimes are highly seasonal with high dis- where thawing and erosion of an ice wedge produces an
charges sustained by the spring thaw. This high spring empty trough, which becomes filled with loess or sand.
discharge makes fluvial action in periglacial climates a
more potent force than the low precipitation levels might Frost mounds
suggest, and even small streams are capable of conveying
coarse debris and high sediment loads. In Arctic Canada, The expansion of water during freezing, plus hydrostatic
the River Mechan is fed by an annual precipitation of or hydraulic water pressures (or both), creates a host of
135 mm, half of which falls as snow. Some 80–90 per multifariouslandformscollectivelycalled‘frost mounds’
cent of its annual flow occurs in a 10-day period, during (see French 1996, 101–8). Hydrolaccoliths or cryolac-
which peak velocities reach up to 4 m/s and the whole coliths are frost mounds with ice cores that resemble
river bed may be in motion. a laccolith in cross-section (p. 119). The chief long-
lived mounds are pingos, palsas, and peat plateaux, while
Aeolian action short-lived mounds include earth hummocks (p. 286),
frost blisters, and icing mounds and icing blisters.
Dry periglacial environments are prone to wind erosion,
as witnessed by currently arid parts of the periglacial Pingos
environments and by areas marginal to the Northern
Hemisphere ice sheets during the Pleistocene epoch. Pingos are large, perennial, conical, ice-cored mounds
Strong winds, freeze-dried sediments, low precipitation, that are common in some low-lying permafrost areas
low temperatures, and scant vegetation cover promote dominated by fine-grained sediments (Box 11.1). Their
much aeolian activity. Erosional forms include faceted name is the Inuit word for a hill. Relict or inactive pingos