Geology Reference
In-Depth Information
Figure 6.1. Oblique view of Sachs River Lowlands, southern Banks Island, Canada, showing high-
and low-centered ice-wedge polygons and themokarst lakes.
volumetric expansion refl ect the varying mineral composition of rocks. In consolidated
sedimentary rocks, expanded bedrock joints are often the result (see Figure 7.6D).
6.2.2. Ice, Sand, and Soil Wedges
Thermal-contraction cracks can be fi lled with either ice, mineral soil, or a combination of
both.
Ice wedges are wedge-shaped bodies of ice, composed of foliated or vertically-banded
ice. They form when hoar frost develops in the open crack in winter and when water from
melting snow penetrates the crack in the early spring. Ice wedges are best observed in
unconsolidated sediments but they may also occur in bedrock and on sloping terrain.
Favored environments for their formation are poorly-drained tundra lowlands underlain
by continuous permafrost. In more arid polar deserts, such as the High Arctic islands,
Antarctica, and the Qinghai-Xizang (Tibet) Plateau, ice wedges are not so well developed,
undoubtedly refl ecting lack of moisture. Many large ice wedges known to exist in central
Alaska, northern Canada, and Siberia are probably Pleistocene in age and inactive (Péwé,
1966b). Ice wedges are discussed further in Chapter 7 in the context of ground ice.
The term “sand wedge” is somewhat of a misnomer, especially in the Pleistocene
context, because the typical mineral infi ll of these wedges ranges from loess through to
medium and coarse sand, and even to more locally-derived sandy infi ll. Usually, stratifi ca-
tion is minimal although there may be a tendency for particle size to decrease in size with
increasing depth. Most actively-forming sand wedges and sand veins that have been
described are from the extremely cold, arid, and ice-free areas of Antarctica (Black, 1973;
Péwé, 1959; Sletten et al., 2003) and Greenland (Djikmans, 1989; Péwé, 1974). Almost
certainly, sand wedges also form in the polar deserts of the Canadian High Arctic
(Hodgson, 1982; Pissart, 1968). In all the regions mentioned above, strong winds and a
relative absence of moisture allow fractures to be fi lled with wind-blown sediment and
other material (Figure 6.2A, B).
A literature review (Murton et al., 2000) suggests that many sand wedges are either
relict (inactive) features or Pleistocene-age pseudomorphs (casts). Both are examined in
more detail in Part III. Here, only actively-forming sand wedges are discussed.
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