Geology Reference
In-Depth Information
Figure 7.17.
Photo showing the surface expression of an anti-syngenetic ice wedge on sloping
terrain near Illisarvik, Pleistocene Mackenzie Delta, Canada. Excavation indicated the wedge width
to exceed 8.5 m and an adjacent wedge to exceed 6.5 m. There was no proof of winter cracking. The
individuals are standing on either side of the wedge. Photo and caption information supplied cour-
tesy of Professor J. R. Mackay.
tion keep pace with removal of surface material, the ice veins penetrate progressively to
greater depths. Therefore, anti-syngenetic wedges grow downwards normal to the ground
surface and not upwards. The top of an anti-syngenetic wedge continues to be truncated
by thaw as the active layer descends in response to surface lowering. These wedges were
fi rst described from the Mackenzie Delta region, Canada (Mackay, 1990a, 1995b).
Favorable places for the growth of anti-syngenetic wedges include convex sites near
hilltops where the rate of downslope sediment movement is high, on actively receding or
fl uvially-undercut slopes, and in badland thermokarst terrain. However, their surface
expression is masked by gravity-controlled mass movements in the active layer, such as
gelifl uction. As such, they are rarely recognizable in the landscape (Figure 7.17). Their
identifi cation is further complicated by the lack of natural exposures on slopes far removed
from actively eroding stream banks or coastal bluffs. It can be theorized that the shape
of anti-syngenetic wedges will depend largely upon the rate of surface lowering and the
time over which the wedge has formed. One must suspect that anti-syngenetic wedges are
a most common type of ice wedge.
7.6. MASSIVE ICE AND MASSIVE-ICY BODIES
Thick, often bedded, and sometimes deformed layers of massive ground ice and icy sedi-
ments are the most spectacular of ground-ice forms (Figure 7.18). These icy bodies are
important not only because of their origin, and the light this may throw upon permafrost
histories, but also because of the thaw-settlement properties of terrain underlain by such
material.
7.6.1. Nature and Extent
Bodies of massive ground ice and icy sediments exist in parts of Western Siberia
(Astakhov, 1992; Astakhov and Isayeva, 1988; Astakhov et al., 1996; Dubikov, 1982, 2002;
