Geology Reference
In-Depth Information
Numerous shallow ridges and ramparts that sometimes enclose indistinct depressions
occur widely in the lowlands of western Europe and eastern North America (Figure 12.6).
Frequently, they have been interpreted as the remnants of Pleistocene frost mounds,
especially pingos (de Gans, 1988; de Gans and Sohl, 1981; de Groot et al., 1987; Flemal,
1976; Marsh, 1987; Pissart, 1963; Weigand, 1965). While a frost-mound interpretation is
probably correct in general terms, it is likely that few are remnants of either hydrostatic-
or hydraulic-system pingos. Most are inconsistent with the hydrologic conditions neces-
sary for pingo growth. Instead, the ramparts are often irregular in plan and “mutually
interfering,” giving rise to a complex alignment and distribution. Some are semi-circular
and open in an upslope direction while others are elongate in the direction of slope. These
morphological characteristics are more typical of seasonal-frost mounds in which new
generations are repeatedly born at the same site by the continual movement of ground-
water to the surface. Many occur in typical open-system localities, such as on lower valley-
side slopes or at spring-line locations.
Some depressions are interpreted as the remnants of palsas or mineral palsas (“lithal-
sas”). However, palsas usually refl ect icy permafrost bodies lying beneath localized peaty
sediments (see Chapter 6). Their degradation does not take place in the same manner as
a pingo (where thaw is induced along dilation cracks followed by collapse of the summit)
but through slumping around the base of the feature. Thus, no ramparts develop and, in
the absence of a large ice core, there is no central depression. In this context, features that
have attracted considerable attention over the years are the circular, rampart-surrounded
depressions known as “viviers” (fi sh ponds) in the Hautes Fagnes uplands of Belgium. First
described and interpreted by A. Pissart (1956) as pingo remnants, they were subsequently
reinterpreted as remnants of mineral palsas and, most recently, as lithalsas. According to
Pissart (2000, 2002), they are analogous to small frost mounds that occur today in areas
of discontinuous permafrost where mineral soil overlies impermeable bedrock. Several of
these frost-mound remnants have been investigated by stratigraphic and palynologic studies
of the material within the central depression. Pollen analyses suggest that they formed
during the Late Dryas (10 500-11 000 years BP) (Mullenders and Gullentops, 1969). In the
UK, apparently similar features formed and quickly degraded during and at the end of
Zone III of the Late Glacial (Sparks et al.,
1972; Watson, 1977).
Figure 12.6.
Oblique air view of part of Walton Common, Norfolk, eastern England, showing
ramparts and enclosed depressions associated with the formation of Late-Pleistocene frost
mounds, probably seasonal in nature. Photograph is reproduced by permission of the University of
Cambridge Photo Collection.
