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