Geology Reference
In-Depth Information
Figure 7.13.
Ice-wedge deformation patterns (A-C) and mode of polygon deformation pattern (D)
according to Mackay (1990a) and Kokelj and Burn (2004). Reproduced by permission of John Wiley
& Sons Ltd.
terrain, the creep of near-surface permafrost means that wedges will deform in the down-
slope direction.
Ice wedges can be classifi ed in several ways. Based upon the frequency of occurrence of
cracking, they can be active, inactive, or relict. They may also be classifi ed in terms of their
infi ll composition. This can range from pure ice to icy silt and sand. To complicate matters
further, pseudomorphs and casts of Pleistocene-age wedges can be classifi ed according to
whether the wedge infi ll is either primary or secondary in nature, or if it is composite
(see Chapter 12). When discussing actively-forming wedges in present-day permafrost
environments, as in this chapter, a useful approach is to consider the direction of wedge
growth relative to the land surface. Following J. R. Mackay (1990a, 2000), ice wedges can
then be classifi ed as being epigenetic, syngenetic, or anti-syngenetic (Figure 7.14).
7.5.1. Epigenetic Wedges
Epigenetic ice wedges grow in pre-existing permafrost and are younger than the host
sediment. With repeated cracking and infi lling, the ice wedge grows progressively wider
but not deeper (Figure 7.14A). At the same time, there is a net outward movement of the
active layer from the polygon center towards the bordering ice-wedge trough. The end
result is the highly distinctive “double-ridge” ice-wedge polygon topography. The top of
an actively-forming epigenetic wedge commonly shows a small ice nipple extending
towards the base of the active layer. Where there has been recent formation of near-surface
organic material, the active layer will thin and wedge “rejuvenation” will occur (Figure
7.15A). In a sense, this is the beginnings of syngenetic growth. Sometimes several episodes
of rejuvenation can be related to vegetation-succession sequences and the associated thin-
ning of the active layer (Figure 7.15B). Epigenetic ice wedges typically occur on fl at sur-
faces that are experiencing neither erosion nor sediment accretion. In poorly-drained
