Page 301 - Fundamentals of Geomorphology

284 PROCESS AND FORM

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