Geology Reference
In-Depth Information
and Central Europe, it is possible to recognize a broad zone,
200-300 km wide, where
permafrost probably existed at different times during the Pleistocene (see Figure 11.12).
To the east, the zone broadened to over 500 km in the Russian Plain. Distribution of the
same phenomena in the mid-latitudes of North America suggests a similar, but much nar-
rower, permafrost zone existed along the maximum southern limit of the ice (see Figures
11.6; 11.13). The uneven quality of the information means that we are unable to determine
the variability in extent of permafrost in the different cold periods; Figures 11.12 and 11.13
refl ect mainly the extent of Late-Pleistocene permafrost. On both continents, the presence
of casts and pseudomorphs in areas within the maximum limit of last glaciation means
that the Late-Pleistocene periglacial zones followed the retreating ice. In Central Asia
and northeastern China, sand-wedge casts are the predominant line of evidence that is
used to map the former extent of Pleistocene permafrost (see Figure 11.14).
∼
12.2.3. Frost-Mound Remnants
Pingo remnants provide incontrovertible proof of the former existence of permafrost.
However, as shown in Chapter 6, a range of frost mounds exist (see Figure 6.11), some of
which merely require cold-climate conditions, not permafrost, for their formation. There-
fore, the identifi cation of pingo remnants needs careful consideration. Moreover, it must
be stressed that pingos are not common features of the periglacial landscape; they are
abundant in just a few geographic areas and, for the most part, are relatively small and
isolated features.
In present permafrost environments, collapsed pingos possess a raised rim or rampart,
formed by mass wasting down their sides, that encloses a central depression where the ice
core has melted (see Figure 6.13). The rampart is signifi cant because this distinguishes a
depression caused by the previous growth of a frost mound from one of simple thermokarst
(thaw lake) origin. If modern collapsed pingos are a guide, the ramparts vary from as little
as 0.5 m to over 5.0 m in height, and the diameter of the depressions within the ramparts
may be as great as 200-300 m. Some pingos are known to exist for several thousands of
years. Finally, it must be remembered that pingos are hydrologic phenomena, and there
are two major types, hydraulic and hydrostatic (see Chapter 6).
It is important to transfer this knowledge selectively to the possible recognition of
former pingos in the now-unfrozen mid-latitude landscapes. Presumably, the same hydro-
logic conditions that favor pingo growth in present-day permafrost environments must
also have existed if pingos were to have formed. It should also be stressed that pingos are
aggradational permafrost landforms. This is important for two reasons. First, they are
progressively destroyed by the thaw of the ice core during any subsequent permafrost
degradation. Such localized thaw-modifi cation, accentuated by on-site groundwater fl ow
if the feature is of hydraulic origin, makes pingo - remnant recognition, on both morpho-
logical and sedimentological grounds, extremely diffi cult. Second, it follows that Pleis-
tocene pingo remnants should be found at locations that either possessed the appropriate
hydrological conditions (e.g. groundwater discharge, if hydraulic in nature) or allowed the
localized freezing of a talik (e.g. drained lake, abandoned river channel, if hydrostatic in
nature). Such specifi c requirements should allow one to eliminate a pingo hypothesis in
the early stages of any Pleistocene paleo-reconstruction investigation. There is also the
necessity of eliminating non-permafrost explanations. For example, seasonal-frost mounds
do not necessarily require permafrost for their formation; deep seasonal frost at a site of
groundwater discharge together with an underlying impermeable layer is an equally suit-
able environment for their occurrence (see Chapter 6). In previously-glaciated terrain, a
kettle origin (i.e. a partial melt-out or ablation feature) must also be considered.
