Geology Reference
In-Depth Information
diameter of 10-50 m. Palsas have a core of frozen peat or
silt (or both), small ice crystals, and a multitude of thin
ice lenses and partings. They often form islands within
bogs.
Peat plateaux
are larger landforms formed by the
coalescence of palsas.
String bogs
, also called
patterned fens
, occur in
muskeg. They are alternations of thin, string-like strips
or ridges of peat, mainly
Sphagnum
moss, which may
contain ice for at least part of the year and may include
true palsas, and vegetation with shallow, linear depres-
sions and ponds. The ridges stand some 1.5 m high,
are 1-3 m wide, and are tens of metres long. The linear
features often lie at right-angles to the regional slope.
It is not certain how string bogs form. Possible formative
processes include gelifluction, frost thrusting of ridges
from adjacent ponds, differential frost heaving, ice-lens
growth, and differential thawing of permafrost, and may
involve hydrological and botanical factors.
Thermokarst and oriented lakes
Thermokarst
is irregular terrain characterized by topo-
graphic depressions with hummocks between them.
It results mainly from the thawing of ground ice, mate-
rial collapsing into the spaces formerly occupied by ice.
Thermokarst features may also be fashioned by flow-
ing water released as the ice thaws. The thawed water
is relatively warm and causes thermal and mechanical
erosion of ice masses exposed along cliffs or in stream
banks. The term thermokarst reflects the resulting land-
form's likeness to a karst landscape in limestone regions.
Thermokarst features may result from climatic warming,
but they are often part of the natural variability in the
periglacial environment. Any modification of surface
conditions can give rise to them, including vegetation
disturbance, cliff retreat, and river-course changes.
Thaw lakes
are prevalent in thermokarst landscapes
(Plate 11.1). Many thaw lakes are elliptical in plan, with
their long axes pointing in the same direction, at right-
angles to the prevailing wind during periods of open
water. The alignment may relate to zones of maximum
current, littoral drift, and erosion, but its causes are far
from fully studied.
Oriented thaw lakes
are common
in permafrost regions, but oriented lakes occur in other
environments, too.
Frost blisters
Smaller mounds than palsas contain ice cores or ice lenses.
Seasonal frost blisters, common in Arctic and subarctic
regions, may grow a few metres high and a few to around
70 m long during winter freeze-back, when spring water
under high pressure freezes and uplifts soil and organic
sediments. They are similar to palsas but form in a differ-
ent way, grow at a faster rate, and tend to occur in groups
as opposed to singly.
Patterned ground
In the periglacial zone, the ground surface commonly
bears a variety of cells, mounds, and ridges that create
a regular geometric pattern. Such ground patterning
occurs in other environments, but it is especially com-
mon in periglacial regions, where the patterns tend to be
more prominent. The main forms are circles, polygons,
nets, steps, and stripes (Washburn 1979, 122-56). All
these may occur in sorted or non-sorted forms. In sorted
forms, coarser material is separated from finer material,
whereas in non-sorted forms there is no segregation of
particles by size and the patterns are disclosed by micro-
topography or vegetation or both. The various forms
usually connect, with a transition from polygons, circles,
and nets on flattish surfaces grading into steps and then
stripes as slopes become steeper and mass movements
become important (Box 11.2).
Icing mounds and icing blisters
Icings
or
ice mounds
are sheet-like masses of ice formed
during winter by the freezing of successive flows of water
seeping from the ground, flowing from springs, or emerg-
ing through fractures in river ice. They may grow up to
13 m thick. They store water above ground until it is
released in spring and summer, when they boost runoff
enormously. Icings in stream valleys block spring runoff,
promoting lateral erosion by the re-routed flow. By so
widening the main channel, they encourage braiding.
Icing blisters
are ice mounds created by groundwater
injected at high pressure between icing layers.
