Geology Reference
In-Depth Information
Figure 11.11.
Cave drawings from the Dordogne region, southern France, indicate the cold-
climate fauna, now extinct, that once lived in the mid-lattitude Pleistocene periglacial zone. (A)
Woolly mammoth, Cave of Les Combarelles; (B) woolly rhinoceros, Cave of Font de Gaume.
Reproduced in Sutcliffe (1985), Archambeau and Archambeau (1989), and Delluc et al. (2001).
From a geomorphological viewpoint, this distinction is also useful. For example, one
must distinguish between seasonal-frost cracks (ground veins or “soil wedges” in the
Russian literature), and perennial-frost cracks (see Chapters 6 and 12). The former
are limited to deep seasonal frost, the latter penetrate permafrost. A second example
relates to frost mounds. It is necessary to distinguish between seasonal- and perennial-
frost mounds because only the latter are unambiguous indicators of permafrost (see
Chapter 6).
In terms of cryostratigraphy, it is useful if one can identify the boundary between the
base of former seasonal-frost penetration and the top of former permafrost. In the cur-
rently non-frozen sediments of the mid-latitudes, this is diffi cult although it has been
attempted in a number of ways. For example, G. C. Maarleveld (1976) used the depth to
which involutions occur to infer the depth of the active layer but thermokarst involutions
must be differentiated from active-layer cryoturbation features, as outlined in Chapter 6.
Another method is to identify the boundary based on micromorphological characteristics
of the sediments (Harris, 1985; Van Vliet-Lanoë, 1982, 1985). However, specifi c differ-
ences may refl ect the lithological and mineralogical (weathering) complexes involved. The
ancient permafrost table can also be inferred from pedological studies that involve the
recognition of fragipans and other weathering characteristics thought associated with
the former existence of permafrost (Nikiforoff, 1955; Vogt et al., 1995) (see Chapter 4).
The most reliable method probably involves the recognition of mineralogical changes
above and below an inferred thaw unconformity (Burn et al., 1986; Xing et al., 1980).
