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(A)
(B)
Figure 8.7. Schematic diagram showing how thermokarst may modify syngenetic permafrost. (A)
Undisturbed syngenetic permafrost with large ice wedges. The permafrost is shown to contain
micro-lenticular cryostructures. (B) Thermokarst modifi cation. The three ice wedges have been
thaw-modifi ed to (i) an ice-wedge pseudomorph (left), (ii) a partially-thawed (truncated) ice wedge
(centre), and (iii) an ice pseudomorph (thermokarst-cave ice) that fi lls a tunnel in the wedge (right).
The extent of secondary (thaw-modifi ed) deposits is indicated schematically. The expanded image
shows the reticulate-chaotic cryostructure that would form adjacent to the ice pseudomorph. From
Bray et al. (2006). Reproduced by permission of John Wiley & Sons Ltd.
rims are the result of lateral thermal expansion within the active layer moving material
from the polygon center to the periphery, as explained earlier (see Chapter 6). Low-
centered polygons are well suited, therefore, for the initiation of self-developing
thermokarst. Ground subsidence in the polygon center is associated with standing water
and the melt of pore and segregated ice. At the same time, standing water in the ice-wedge
trough accelerates thawing along the line of the wedge. The most favored location for thaw
is at the junction of two or more wedges, and small, deep pools of standing water may
persist in such localities throughout the summer months. If integrated drainage occurs in
such terrain, it can assume a “beaded” pattern.
An evolutionary sequence of low-centered polygon morphologies can be recognized,
with the end phase being characterized by the formation of so-called “fortress,” or
thermokarst, polygons (Root, 1975) (Figure 8.8). The near-vertical walls of such extreme
low-centered polygons may be as much as 1.0-1.5 m high. They have been little studied
 
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