Geology Reference
In-Depth Information
disintegration of quartzite and granite. There are also other considerations. First, it must
be proven that these blocky accumulations are not forming under present environmental
conditions. In the Appalachian Mountains of the eastern United States, both actively
forming and relict blockfi elds exist (Hack and Goodlett, 1960, pp. 31-32; Rapp, 1967).
Second, many of the kurums and boulder fi elds in northern environments, such as Trans-
baikal, northern Ural and the northern Yukon mountains (French, 1987; Perov, 1969;
Romanovskii, 1985; Tyurin et al., 1982), appear largely inactive. One must question,
therefore, their validity as modern analogues. Third, blockfi elds may form under glacial,
as opposed to periglacial, conditions (Dahl, 1966; Ives, 1966). It has been suggested that
blockfi elds, and entire periglacial landscapes, can be preserved beneath protective cold-
based ice covers (Kleman and Borgström, 1990). It follows that blockfi elds, although
certainly cold-climate phenomena, cannot be uncritically regarded as diagnostic of either
intense frost action or permafrost conditions.
Equally ambiguous are the bedrock outcrops, known generally as tors, which exist on
many hillslopes and upper summits of the mid-latitude landscapes (Figure 13.3B). They
have attracted considerable attention (Caine, 1967; Czudek, 1964; Demek, 1964; Fahey,
1981; Jahn, 1962; Linton, 1955, 1964; Palmer and Nielson, 1962; Palmer and Radley, 1961;
Te Punga, 1956; Wood, 1969) but their climatic signifi cance is uncertain. If of periglacial
(i.e. frost action) signifi cance, tors refl ect the remnants of frost-shattered bedrock that is
now surrounded by a low-angled surface across which frost-shattered debris has been
transported by mass wasting. The alternative interpretation is that tors result from differ-
ential deep weathering during Tertiary times and have subsequently experienced regolith
removal by mass wasting during the Quaternary to expose intact bedrock.
Both interpretations have periglacial merit since they stress the importance of either
frost action or mass wasting. It is signifi cant that tors of both origins have both been
described from within the same areas (Demek, 1964; Jahn, 1962; Linton, 1955; Palmer
and Nielson, 1962). On the other hand, it is unwise to uncritically accept a simple frost-
action explanation. This is because tors in high latitudes are just a further example of the
problem of cryogenic weathering discussed in Chapter 4. Moreover, tors are also explained
in terms of chemical weathering (Derbyshire, 1972) and wind abrasion (St-Onge, 1965).
Therefore, while they may be regarded as indicative of previous cold-climate conditions,
their exact signifi cance is still imprecise. Closely related to tors are so-called “cryoplana-
tion terraces.” While the existence of these surfaces in northern latitudes is unquestiona-
ble (see Chapter 9), no study has yet demonstrated their active formation under cold-climate
conditions. Some, initially interpreted as cryoplanation (altiplanation) forms, have been
shown to be raised marine benches (Péwé et al., 1982, pp. 2533). Thus, it seems more
appropriate to stress the role of cold-climate mass wasting rather than frost action in any
interpretation of tors and associated bedrock surfaces.
13.2.3. Stratifi ed Slope Deposits
Intense freeze-thaw action, together with slopewash activity, is generally believed to result
in rhythmically-stratifi ed slope-waste deposits (Figure 13.4). First described from the
Charente region of western France, these deposits were termed “grèzes litées” or “éboulis
ordonnés” (deWolf, 1988; Guillien, 1951; Malaurie and Guillien, 1953). Other stratifi ed
slope deposits were subsequently described from central Poland (Dylik, 1960, 1969a) and
southern England (Kerney et al., 1964).
The sediments involved are essentially frost-shattered debris, the nature of which
depends upon the lithology concerned. The deposits, which may attain a thickness in
excess of 10m, possess coarse bedding and a degree of sorting. Typically, they mantle
