Periglacial landforms
207
The oriented lakes are unusual and the best example is located in the Arctic coastal plain
of Alaska. The length of these elliptical or rectangular lakes reaches 5 km, being two or
thee times larger than the width and may be up to 6 m deep (Black and Barksdale, 1949).
Complex shapes due to the coalescence of several lakes are also recognised. Their
uncertain origin has been related to differential thawing of the permafrost under the
influence of prevailing winds that accumulate sediments in the shores perpendicular to
the wind direction. The deposits insulate these shores from thawing and from wave
action as they reach a very gentle profile. This situation favours the development of drift
currents with high erosion capacity towards the extremes of the lakes leading to their
enlargement. The mean retreat rates are very high, reaching 25 m/yr (Carson and
Hussey, 1962).
Collapsed pingos may host crater-like enclosed subsidence depressions surrounded by
an uplifted edge whose sediments show centrifugal dips. These ponds are filled with peat
and other lacustrine-paludal deposits (De Gans, 1988) (Figure 8.37). These landforms
have been identified in areas from Europe and Asia that were under periglacial conditions
in the past. They may be very difficult to differentiate from other thermokarst depressions
or some fluvioglacial kettles.
Where the frozen ground is exposed to the surface, it may thaw and give rise to thaw
collapses. These are a series of linked and curved depressions opened on the downslope
side that form a festooned scarp. As the uppermost part of the permafrost thaws, it
becomes saturated in water and flows, thus generating a head scarp several meters high.
This is one of the most rapid erosion processes of the periglacial regions as the retreat of
these scars may reach 7 to 10 m/yr (French and Egginton, 1973).
The term alas derives from central Yakutia (Siberia) (Soloviev, 1962, 1963 in Jahn,
1975; Czudek and Demek, 1970b; Soloviev, 1973) and designates closed depressions
with steep margins and a flat floor commonly covered with grass vegetation. They are
rounded or elliptical in shape and in some cases host shallow lakes. Their length ranges
from 0.1 to 15 km and are 3 to 40 m deep. They form from long-term localised melting of
the permafrost and the coalescence of several depressions may generate large linear
throughs known as alas valleys tens of kilometres long. As the thawing progresses several
landforms develop (Figure 8.38). The growth of ice wedges produces linear depressions
separated by domed cells called baydjarakhs, 3 to 4 m high and 3 to 20 m in diameter
(stage I). The progressive melting of the ice produces closed depressions with conical
mounds called duyodas (stage II). The generation of the alas (stage III) takes place when
the depression has scarped edges and a flat bottom with grass vegetation and is sometimes
covered with a shallow lake. If the permafrost reactivates, ice veins and pingos may be
developed in the bottom of the depression (khonu) that may finally collapse (stage IV).
In Yakutia, 40 to 50% of the original surface has been modified by alas systems that are
several thousand years old and have been related to the Climatic Optimum (2500 to
9000 BP). Currently, more than 10% of the territory has active permafrost.
207
The oriented lakes are unusual and the best example is located in the Arctic coastal plain
of Alaska. The length of these elliptical or rectangular lakes reaches 5 km, being two or
thee times larger than the width and may be up to 6 m deep (Black and Barksdale, 1949).
Complex shapes due to the coalescence of several lakes are also recognised. Their
uncertain origin has been related to differential thawing of the permafrost under the
influence of prevailing winds that accumulate sediments in the shores perpendicular to
the wind direction. The deposits insulate these shores from thawing and from wave
action as they reach a very gentle profile. This situation favours the development of drift
currents with high erosion capacity towards the extremes of the lakes leading to their
enlargement. The mean retreat rates are very high, reaching 25 m/yr (Carson and
Hussey, 1962).
Collapsed pingos may host crater-like enclosed subsidence depressions surrounded by
an uplifted edge whose sediments show centrifugal dips. These ponds are filled with peat
and other lacustrine-paludal deposits (De Gans, 1988) (Figure 8.37). These landforms
have been identified in areas from Europe and Asia that were under periglacial conditions
in the past. They may be very difficult to differentiate from other thermokarst depressions
or some fluvioglacial kettles.
Where the frozen ground is exposed to the surface, it may thaw and give rise to thaw
collapses. These are a series of linked and curved depressions opened on the downslope
side that form a festooned scarp. As the uppermost part of the permafrost thaws, it
becomes saturated in water and flows, thus generating a head scarp several meters high.
This is one of the most rapid erosion processes of the periglacial regions as the retreat of
these scars may reach 7 to 10 m/yr (French and Egginton, 1973).
The term alas derives from central Yakutia (Siberia) (Soloviev, 1962, 1963 in Jahn,
1975; Czudek and Demek, 1970b; Soloviev, 1973) and designates closed depressions
with steep margins and a flat floor commonly covered with grass vegetation. They are
rounded or elliptical in shape and in some cases host shallow lakes. Their length ranges
from 0.1 to 15 km and are 3 to 40 m deep. They form from long-term localised melting of
the permafrost and the coalescence of several depressions may generate large linear
throughs known as alas valleys tens of kilometres long. As the thawing progresses several
landforms develop (Figure 8.38). The growth of ice wedges produces linear depressions
separated by domed cells called baydjarakhs, 3 to 4 m high and 3 to 20 m in diameter
(stage I). The progressive melting of the ice produces closed depressions with conical
mounds called duyodas (stage II). The generation of the alas (stage III) takes place when
the depression has scarped edges and a flat bottom with grass vegetation and is sometimes
covered with a shallow lake. If the permafrost reactivates, ice veins and pingos may be
developed in the bottom of the depression (khonu) that may finally collapse (stage IV).
In Yakutia, 40 to 50% of the original surface has been modified by alas systems that are
several thousand years old and have been related to the Climatic Optimum (2500 to
9000 BP). Currently, more than 10% of the territory has active permafrost.