In arctic regions the subsoil is permanently frozen
(permafrost). Ice accumulatesin lenses or wedges, which grow due to
the thermal gradient. Thus patternedground develops with hummocks
and trenches, often in polygonal forms.
In other cases low-centred polygons develop, which flood during
summer withmelting water from snow and the upper part of the frozen
ground.
Stones in the soil are pushed upwards to the surface by ice
accumulatingunderneath the stones. On gentle slopes long stripes of
stone lines developalso under the influence of solifluction, slow
soil creep over the permafrost.
Hummocky ground, caused by frost heaving, with typical tundra
vegetation, north Canada.
Cross-section through a hummock.
Large ice lens in peat. Such ice lenses may grow huge,
eventually forming a "pingo".
Characteristic for Cryosols is the layer of cryoturbation.
Through the actionof freezing and thawing, soil horizons get
disrupted and swirl-like patternsdevelop.
Permafrost at shallow depth under an earth hummock.
Ice layer in a Histic Cryosol.
Cryosols often have a Histic surfacehorizon, which develops
under the wetconditions caused by water stagnatingon the
permafrost.
Small ice lenses in the subsoil of a Cryosol.
Ice wedge in a Histic Cryosol.
Vegetation moves with the movements in the soil caused by
cryogenic processes.Oblique and fallen trees are evidence of these
movements and give rise to localnames such as "drunken forest".
Large ice lenses may eventually develop into a pingo
(dome-shaped mound intundra regions with a core of ice; the name
stems from Eskimo).
When the earth layer on top of the pingo breaks, the ice core
starts to melt. Theearth material will slide down over the surface
of the melting ice and is depositedin a wall around it. The ice
core will eventually disappear completely, leaving thewall around a
lake behind.
Such morphological features are also found in formerly
periglacial regionssuch as The Netherlands. The small lake in the
centre-right of the photograph issuch a "pingo ruin", evidence of
the arctic conditions that once prevailed here.
