Geology Reference
In-Depth Information
Many of the frost-mound remnants that occur in mid-latitudes are, almost certainly, a
legacy from the most recent period of cold-climate conditions. The signifi cance of any sort
of open-system interpretation lies in the fact that this does not require continuous perma-
frost. Instead, their irregular distribution probably refl ects the limiting hydrologic condi-
tions necessary for growth. The discharge of water fl owing beneath or within permafrost
must be relatively small in amount and close to 0 °C in temperature, otherwise either a
perennial spring will result if the temperature is considerably in excess of 0 °C, or the
conduit will be sealed and the mound will cease to develop if the temperature drops sig-
nifi cantly below
3 °C. The possibility that icings and related features formed at the
site of these frost-mound features is also likely yet direct morphological and stratigraphic
evidence for such occurrences is diffi cult to fi nd.
Frost-mound remnants have not been reported with the same frequency from the mid-
latitudes of North America and elsewhere. The reason for this is not clear. The depression
infi ll from one so-called “pingo” locality in central Pennsylvania (Marsh, 1987) also indi-
cates a late-glacial age for sediment deposition. Subsequent investigations concluded that
this depression was a “ground-ice scar” (seasonal-frost mound?) that formed between two
wind-aligned ridges at a groundwater seepage zone (Clark et al., 1992; Marsh, 1998, 1999,
pp. 24-29). Elsewhere, the DeKalb Mounds of north-central Illinois (Flemal et al., 1973)
and the “prairie mounds” of southern Alberta (Bik, 1969) were initially interpreted as
remnants of large Pleistocene pingo “fi elds”. However, it seems best to regards these fea-
tures as ablation-till phenomena (Stalker, 1960).
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2 to
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12.3. PAST PERMAFROST DEGRADATION
The most widespread evidence for the former existence of permafrost comes from the
mass-wasting and landscape modifi cation that must have accompanied the thaw and even-
tual disappearance of permafrost. This modifi cation would have been especially drastic
in areas where the permafrost was ice-rich and had developed in soft bedrock or uncon-
solidated materials. In fact, one might argue that the identifi cation of past thermokarst
activity should constitute the vast majority of Pleistocene periglacial paleo-environmental
studies.
Unfortunately, the “self-destroying” nature of much thermokarst activity hinders such
study. By defi nition, thermokarst sediments are heterogeneous diamicts. They often defy
description. Landforms that result from thermokarst activity assume a variety of shapes
and sizes, and many have experienced repeated modifi cation. There are few diagnostic
sedimentary structures associated with thermokarst-derived sediments, a notable excep-
tion being thermokarst lake-basin sedimentation (Murton, 1996b). Finally, once perma-
frost has disappeared, non-periglacial processes act to further modify, or degrade, whatever
form the evidence takes.
12.3.1. Thermokarst Depressions
Pits, ponds, and shallow depressions that lack surrounding ramparts are sometimes given
a thermokarst interpretation. Present-day analogues are, presumably, thaw lakes and
depressions (see Chapter 8). In the northwest European lowlands, inferred thermokarst
depressions were fi rst referred to as “mares,” “mardelles,” or “solle” (Cailleux, 1956, 1957;
Troll, 1962). In the Beauce and Brie areas of northern France their density reaches 35/km
2
.
A. Pissart (1958, 1960) was the fi rst to explicitly interpret these depressions as thermokarstic
