Geology Reference
In-Depth Information
12.3.6. Discussion
Thermokarst continues to be neglected in many Pleistocene periglacial investigations.
This was fi rst highlighted over 40 years ago by J. Dylik (1964b) and remains true today.
The reality is that evidence for the degradation (thaw) of permafrost should be far more
widespread and much better preserved than evidence for the earlier growth of permafrost.
However, the literature indicates an over-emphasis upon the recognition of features and
structures associated with aggrading permafrost (pingo remnants, frost fi ssures) and a
relative neglect of thermokarst sediments and structures.
A classic illustration is provided by the Pleistocene freeze-thaw phenomena (cryotur-
bations) that are discussed in Chapter 13. Some were fi rst interpreted within the context
of either seasonal freezing and thawing, or a deep active layer, rather than in the context
of degrading permafrost (Edelman and Tavernier, 1940; Sharp, 1942b; Wolfe, 1953). Yet
interpretations such as these need careful re-evaluation in light of modern understanding
of thermokarst processes. For example, the “frost kettles” or “barrel-shaped macropoly-
gons” that are described from central Europe (Pecsi, 1964, p. 287; Sekyra, 1956, p. 358)
are better interpreted as “sediment pots” (Figure 12.11). Likewise, many of the deformed
structures and large-scale patterned ground (stripes) that characterize the chalky
landscapes of southern and southeastern England (Te Punga, 1957; Watts et al., 1966;
Williams, 1964) are now interpreted in the context of active-layer deepening through ice-
rich permafrost that occurred during a shoft-lived climatic oscillation approximately 21 ka
ago (Murton et al., 1995, 2003). For example, on Thanet, Kent, pebbly silt was involuted
into underlying chalk diamicton during an episode of soft-sediment deformation, and
sandy stripes were subsequently formed by cell-like circulation within the deepened active
layer (Murton et al., 2003). Reference is made to the inferred movements associated with
earth hummocks that appear characteristics of the modern active layer in present perma-
frost environments (see Chapter 6).
A fi nal example comes from several stratigraphic and geotechnical investigations in
southern England that indicate the previous instability of slopes developed in lithologies
that, under current conditions, would be regarded as stable (see Chapter 9, Table 9.7). For
example, the degraded form of a slope developed in London Clay beneath an old cliff-line
at Hadley, Essex, can best be understood in terms of thaw-consolidation theory rather
than by conventional geotechnical analyses (Hutchinson and Gostelow, 1976). It must be
assumed that many slopes in many mid-latitude areas refl ect, to varying degrees, the
imprint of Pleistocene thermokarst activity.
12.4. SUMMARY
The evidence for Pleistocene thermokarst in the mid-latitudes is widespread but the
complex nature of thermokarst activity, in both bedrock and unconsolidated sediments,
makes that evidence diffi cult to recognize. The most reliable information for the former
existence of permafrost comes from various casts and pseudomorphs that refl ect thermal-
contraction cracking, a process that clearly demands permafrost for its operation. Even
here, the complexity of form makes recognition diffi cult. Likewise, evidence for the aggra-
dation of permafrost, such as the paleo-permafrost table, and various types of frost
mounds, is also diffi cult because much will have been destroyed by subsequent thermokarst
activity. Finally, at the warm limits of the distribution of past permafrost, the distinction
between perennial and seasonal frost, and continuous and discontinuous permafrost con-
ditions, may be virtually impossible to determine.
