Geology Reference
In-Depth Information
“head” deposit, and J. Prestwich (1892) documented “rubble-drift” deposits elsewhere in
southern England. Subsequently, the mapping of “head” became standard practice by both
the Geological Survey and the Soil Survey of Great Britain (Avery, 1964; Dines et al.,
1940). On the European Continent too, the signifi cance of “pseudo-glacial” sediments
had been appreciated at an early date and the recognition of frost-derived sediments in
the lowlands of western and central Europe quickly became commonplace (Büdel, 1944;
Edelman et al., 1936; Troll, 1944). In North America, the mapping of “surfi cial” materials
quickly became standard practice, although much surfi cial material is clearly glacial
rather than periglacial in origin. Today, “head” deposits are synonymous with solifl uction
deposits.
Head deposits vary considerable in their composition. Their cold-climate origin is
based upon two characteristics. First, they consist of predominantly poorly-sorted, unstrat-
ifi ed, angular debris of local derivation with larger elongate clasts often aligned downslope
and tilted upwards. These are characteristics of modern solifl uction deposits (see Chapter
9). Second, some head deposits contain faunal remains (mollusca, coleoptera) indicative
of cold-climate conditions. Today, slopewash, soil creep, and eolian deposition account
for much of the head that is forming in temperate mid-latitudes today and which is gener-
ally termed “colluvium.”
As general indicators of cold-climate conditions, head deposits represent an important
line of evidence. However, as with much of the evidence already discussed, their inter-
pretation in terms of frost action is not always easy. A fundamental problem lies in our
inability, even in present-day periglacial environments, to differentiate between a solifl uc-
tion deposit that is primarily the result of frost creep and retrograde movement, and one
primarily the result of gelifl uction. The former implies seasonal frost and the latter implies
permafrost. A second problem is that not all solifl uction deposits contain readily identifi -
able cold-climate faunal remains. A third is that solifl uction is sometimes diffi cult to dis-
tinguish from till. For example, where a slope deposit has been derived from glacial
sediments, as in ablation till, it is impossible to distinguish till from solifl uction because,
to all intents and purposes, they are the same.
13.2.5. Frost-Disturbed Soils and Structures (Involutions)
Disturbed, distorted, and deformed structures occurring in unconsolidated Quaternary
sediments have been described frequently from mid-latitudes. In the European literature,
these str uctu res a re usua l ly refer red to as “c r yotu rbations” (E del ma n et a l., 19 3 6) , “brodel-
boden” (Gripp, 1926; Troll, 1944), or “pseudo-solle” (Maarleveld and van den Toorn,
1955). In the English-language literature, the term “involution” is preferred (Denny, 1936;
Sharp, 1942b).
Use of the term “involution” requires clarifi cation because it is possible to identify large
structures that form during permafrost degradation and smaller ones that form within
the active layer. Therefore, one must distinguish between (i) “thermokarst involutions,”
formed in water-saturated sediments by loading during thaw-degradation of an ice-rich
permafrost body, and (ii) “periglacial involutions,” formed by repeated frost action (cryo-
turbation) within a seasonally-frozen layer. Almost by defi nition, periglacial involutions
are usually less than 1.0-2.0m in vertical extent and are restricted to either a paleo-
permafrost active layer or, less likely, to a paleo-zone of deep seasonal frost. It is the
latter type of involution that is of concern in this section.
Most periglacial involutions are more regular in form and occurrence than thermokarst
involutions. In spite of this, the diversity of forms is high (Figure 13.5A), ranging between
