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result from the downward propagation of the cold and warm seasonal temperature waves.
The gradual transition from clear to bubble-rich ice refl ects the change from winter to
summer, and the abrupt transition from bubble-rich to clear ice refl ects the change from
summer to winter.
In summary, hydrostatic (closed) system pingo growth is an example, albeit dramatic,
of lake-bottom heave and aggrading permafrost.
6.5.4. Other Perennial-Frost Mounds
Pingo-like mounds occur in other geomorphic settings. For example, groups of pingo-like
mounds have been described from the Canadian Arctic islands (Balkwill et al., 1974;
Pissart, 1967a). On Prince Patrick Island, shallow mounds, between 1 m and 13 m in height
and with average dimensions of about 60 m, are formed within thick sand formations of
Tertiary age that rest discordantly upon impervious Paleozoic-age rocks. None appears
to be growing today. Their location appears to coincide with deep-seated geological dis-
continuities that suggest the mounds are related to ancient groundwater movement along
faults in underlying bedrock. The pingos probably developed when permafrost fi rst began
to form sometime during the early or middle Pleistocene. Those on Amund Ringnes
Island are equally problematic.
A second geomorphic setting is provided by elongate and partially-collapsed
mounds of varying sizes and shapes that occur in river valleys and on low fl uvial terraces
of Banks Island (French, 1975b, 1976c; French and Dutkiewicz, 1976). Most are less than
3 m in height and all appear relict. Especially puzzling are remnants of small, mutually-
interfering mounds that occur on the broad fl uvial surfaces of Central Banks Island. It is
hypothesized that these may be seasonal-frost-mound remnants of Late-Pleistocene age
that formed in shallow sections of braided channel systems.
6.5.5. Seasonal-Frost Mounds
Where freezing of the active layer restricts perennial discharge from intrapermafrost or
subpermafrost aquifers, a variety of seasonal-frost mounds (frost blisters, icing blisters)
may develop at the site of groundwater discharge. These have been described from north-
ern Canada, Alaska, northern Scandinavia, Tibet, and Siberia. Typically, seasonal-frost
mounds range between 1.0 m and 4.0 m in height. They form by the upheaval of seasonally-
frozen ground brought about by the subsurface accumulation of water under high hydrau-
lic potential. This occurs during progressive freezing of the active layer. Figure 6.16
illustrates the formation of frost blisters, one of the most common types of seasonal-frost
mounds.
Seasonal-frost mounds are sometimes confused with palsas. The basic difference is that
the former result from ice injection while the latter result from ice segregation. Thus, the
interior of a frost blister, for example, is usually characterized by a core of pure ice with
ice crystals aligned in a vertical columnar fashion that refl ects the freezing of free water
(Pollard and French, 1983, 1984, 1985).
Most seasonal-frost mounds are destroyed completely by thawing and collapse during
the fi rst summer after their formation. However, others may be preserved through one
or more summers, depending on the insulating quality of their soil cover. As a result,
they may assume the morphology and time duration of a palsa. It is also possible that
some mounds combine both palsa and seasonal-frost-mound growth mechanisms. For
example, palsa-like mounds have been described from a number of damp valley-bottom
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