Geology Reference
In-Depth Information
hummock,” “earth hummock,” “slope hummock,” “thufur,” and “pounus” have been used
to describe these features (Beschel, 1966; Corte, 1971; Lewkowicz and Gudjonsson, 1992;
Raup, 1966; Seppälä 1998; Schunke, 1977; Van Vliet-Lanoë and Seppälä, 2002).
A polygenetic origin is most likely. Certainly, not all are of the same origin. Desiccation
or seasonal-frost cracking, or a combination of the two, is probably the initial cause. Desic-
cation can occur for a number of reasons: wind may promote evaporation from the ground,
the ground may be subjected to a drainage change, and desiccation occurs during freezing
and ice segregation. Some, such as thufur and pounus, occur in areas of seasonal rather
than perennial frost (Luoto and Seppälä, 2002). Where small polygons form distinct nets
or hummocks, it is likely that other processes, such as differential frost heave and snowmelt
erosion, are also important (Lewkowicz and Gudjonsson, 1992; Seppälä, 2005).
Stripes occur on sloping terrain. Most non-sorted stripes consist of bare areas, com-
monly 0.3-1.0 m wide (Figure 6.23A), that are separated by parallel lines of vegetation-
covered ground. In all probability, they refl ect the same processes of cryoturbation that
form circles but modifi ed by the effects of slopewash and other mass-wasting processes
(see Chapter 9). For example, the vegetation lines often merge, suggesting seepage and/or
surface wash. Sorted stripes are less common and show a marked differentiation between
lines of coarser and fi ner material oriented parallel to the gradient (Figure 6.23B). Usually,
the coarser stripes are narrower than the fi ne stripes, and stones and boulders are com-
monly on edge with their long axes parallel to the line of movement. As with non-sorted
stripes, it is possible that wash, by preferentially removing fi nes, may be a factor in their
development.
ADVANCED READING
Haeberli, W. (1985). Creep of mountain permafrost: internal structure and fl ow of alpine rock
glaciers.
Mittgeilugen der Versuchsanstalt fur Wassenbau Hydrologie und Glaziologie
, no. 77,
142 pp.
Mackay, J. R. (1998). Pingo growth and collapse, Tuktoyaktuk Peninsula area, Western Arctic
Coast, Canada.
Géographie physique et Quaternaire
,
52
, 271-323.
Mackay, J. R. (2000). Thermally-induced movements in ice-wedge polygons, Western Arctic Coast:
a long-term study.
Géographie physique et Quaternaire
,
54
, 41-68.
Washburn, A. L. (1997). Plugs and plug circles: a basic form of patterned ground, Cornwallis Island,
Arctic Canada - origin and implications.
Geological Society of America
, memoir 190, 87 pp.
DISCUSSION TOPICS
1. What surface features are typical of permafrost terrain?
2. What are the unique hydrological characteristics of permafrost?
3. Differentiate between the various forms of frost mounds.
4. What are the controls over thermal-contraction cracking in ice-rich sediments?
