Geology Reference
In-Depth Information
Figure 7.12.
Large epigenetic ice wedges, near King Point, Yukon Coast, Canada.
150 cm, 100 cm, 50 cm, 30-50 cm, and 25-50 cm, respectively. When the maximum depth
of the thaw unconformity observed at Pullen Island is used in the Stefan equation, the
resulting thawing index value (approximately 1800) is similar to that currently experi-
enced by Whitehorse. Thus, during the early Holocene, the summer climate at the
location of the present Arctic coast was probably typical of that which exists today near
Whitehorse, in central Yukon Territory. Likewise, the thaw unconformity that occurs
on Melville Island corresponds to a summer climate experienced by Tuktoyaktuk and
Sachs Harbour (Ikaahuk).
7.5. ICE WEDGES
Their widespread occurrence and distinctive surface manifestation makes ice wedges
some of the most characteristic features of the periglacial landscape (see Figure 6.1). After
pore and segregated ice, ice-wedge ice constitutes the third most important ground-ice
type in terms of volume (Figure 7.12). Some Russian authorities regard wedge ice as the
dominant type of underground massive ice in central and northern Siberia (Dostovalov
and Popov, 1966; Popov, 1962; Shumskii and Vtyurin, 1966). More typically, ice-wedge
ice constitutes between 20% and 35% of the total ice volume in the upper 5-10 m of per-
mafrost (see Table 7.2A).
Ice wedges are best developed in unconsolidated sediments but they also occur in
bedrock and on sloping terrain. The most favorable environments for their formation are
poorly-drained tundra lowlands underlain by continuous permafrost.
For several reasons, the growth of an ice wedge must involve deformation of both the
ice and adjacent ground. First, the ice must deform because the length of the initial crack
depth is always less than the length of the side of the wedge (Figure 7.13A). Second,
because wedge ice is less dense than the surrounding frozen ground, there is a tendency
for ice wedges to move upwards in diapiric fashion (Figure 7.13B). Third, the top of an
actively-forming ice wedge may be affected by thaw as the wedge rises into the base of
the seasonally-thawed zone (the active layer) (Figure 7.13C). In like fashion, because of
the volume addition to the growing ice wedge, the enclosing sediments adjacent to the
wedge typically exhibit upward bending, and there is a tendency for deformation to occur
upwards and away from the wedge (Figure 7.13D). Finally, if ice wedges form on sloping
Search WWH ::




Custom Search