Environmental Engineering Reference
In-Depth Information
5. Solifluction fans or lobes - crudely bedded gravelly or bouldery deposits, thickening
downslope, grading into slopewash beneath the lower slope;
6. Mudflow scar underlain by low-angle shear surface;
7. Mantle of solifluction debris (S) intertongued with gravels (G) deposited by
periglacial braided stream, and overlain by peats and silts (M) forming the floodplain
of the modern meandering stream;
8. Till (T) in the buried valley and in pockets in the rockhead;
9. Polygonal patterned ground and fossil casts of ice-wedged cracks;
10. Frost-shattered bedrock;
11. Doughnut-shaped degraded ramparts left by former pingos;
12. Soil mantle resulting from mechanical churning (frost heaving) of pedological soils
and bedrock. The underlying rockhead is often highly irregular;
13. Involutions and contortions in the soil caused by high pore pressures developed dur-
ing re-freezing of thaw-soaked soil;
14. Faulted soil zones caused by ground contraction during freezing and/or collapse into
voids following melting of buried ice;
15. Patterned ground - polygonal nets and stripes;
16. Blockfield;
17. Alluvial fans;
18. Dry Valleys;
19. Terraced gravels (G) deposited by braided periglacial streams. The bedrock surface
below may contain scoured channels and hollows;
20. Loess.
Eyles and Paul (1985) point out that most of the features indicated on this diagram may
also develop (more slowly) in warmer climates. The authors agree and in particular con-
sider it likely that the “Deep-seated disturbances” Features 1 and 2 would have been
developed to a large extent as a result of destressing (as described in Chapter 2) before any
cold-climate effects.The following explanation of the main cold-climate processes and fea-
tures indicated on Figure 3.55 is based also on Eyles and Paul (1985).
Solifluction (7) is the slow flowage or creep of a water-soaked mass of soil and rock
debris, either as a true flow or as a slide where most movement occurs over a basal shear
surface. Typical flow rates range from 10 mm to 60 mm per year. Solifluction is caused by
the generation of excess pore pressures during the thaw. Eyles and Paul (1985) provide a
summary of theoretical studies of the process by Nixon and Morgenstern (1973) and
McRoberts and Morgenstern (1974).
Another solifluction mechanism is the downslope displacement of soil particles by
needle-ice. The ice needles grow normally to the ground surface and so during each freeze-
thaw cycle the supported particles are displaced slightly downslope.
Frost-shatter (10) is the mechanical disruption of rock masses by the expansion on
freezing of groundwaters.
Cyroturbation (12), (13) and (15) is the churning or mixing of soils which occurs in
permafrost conditions at the end of the melt season due mainly to high pore pressures set
up towards the base of the thawed layer as the soil re-freezes from the surface down.
Expansion due to freezing of porewaters also contributes, as does the upward heave of
cobbles and boulders resulting from their greater thermal conductivity. The “involutions”
(13) are pseudo-intrusive structures similar to flame structures in sediments and to gilgai
in clays. Patterned ground (15) is ground showing a regular hummocky pattern (polygons
or circles). It is a surface expression of cyroturbation.
Ice-wedging (9) refers to cracks which occur in soils during intense cold widened by
wedging action when water freezes in them. The cracks are preserved as casts by soil which
migrates into them.
Search WWH ::




Custom Search