Geology Reference
In-Depth Information
“periglacial” (Dylik, 1964a; Dylikowa, 1962; Hamelin, 1964). This raised criticism
because the term was used to refer to both processes and areas. The apparent misuse
of the term led to the suggestion that it be replaced by more specifi c terms such as
permafrost, ground-ice, or soil-ice environment (Linton, 1969). More recently, cryologi-
cal terminology has been reintroduced (ACGR, 1988) in order to accommodate a major
semantic problem posed by the thermal and physical states of water. The problem
appears simple. Unfrozen water can exist in soil or rock at temperatures below 0 °C due
to the presence of mineral salts, pressure, or other causes. Because of this, “perennially-
frozen ground” (i.e. ground at a temperature
0 °C, or “cryotic” in nature) may not, in
fact, be “frozen”. Thus, cryotic ground can be either “frozen” or “unfrozen” depending
upon the amount of unfrozen water present. It follows that not all permafrost is neces-
sarily frozen!
These terminological problems persist today. An obvious example is in the defi nition
of the active layer proposed by the ACGR (1988). Traditionally, the active layer was
defi ned as the near-surface layer above permafrost which thaws during summer (see
Chapter 5). The ACGR defi nition would include the uppermost part of permafrost in
those situations where either salinity or clay content allows permafrost to thaw and
refreeze annually, even though the material remains cryotic (below 0 °C). Under this defi -
nition, it is argued that the active layer cannot be determined consistently in time and
space (Burn, 1998b) and the term is impossible to use effectively in the fi eld. A partial
solution has been to recognize a “transient layer” at the top of permafrost and at the base
of the active layer (Shur et al., 2005). The active layer and the transient layer are discussed
in detail in Chapter 5.
Today, the utility of the periglacial concept requires careful assessment. Lozinski
referred to a specifi c and limiting environment that is not typical of the vast majority of
periglacial environments today. In fact, few, if any, modern-day analogues can be identi-
fi ed (French, 2000). Lozinski also used the term to refer primarily to areas or regions.
However, one may argue that typical “periglacial regions” do not exist, and, if they do,
lack well-defi ned boundaries. Thus, André (1999) explicitly questions the effectiveness of
freeze-thaw in fashioning the periglacial landscape and refers to the “smokescreen of the
periglacial scenery.” In a similar vein, French and Thorn (2006) suggest that periglacial
areas are cold-climate “zones” in which seasonal and perennial frost, snow, and normal
azonal processes are present to greater or lesser degree. The reality is that many perigla-
cial landscapes inherit the imprint, in varying degrees, of either glacial or non-cold-
climate conditions.
A further complication is that so-called “periglacial” conditions often extend south of
the latitudinal treeline and below the altitudinal timberline (Figure 1.2). This is because
many areas of northern boreal forest are underlain by relict permafrost and, in alpine
regions, glaciers may extend below timberline and into the forest zone. Finally, the
treeline is a zone rather than a line and may extend over a latitudinal distance of
∼
>
100-150 km.
To summarize, therefore, modern usage of the term “periglacial” refers to a range of
cold, non-glacial processes. We can defi ne periglacial geomorphology as the sub-discipline
of geomorphology concerned with cold non-glacial landforms. Because permafrost is a
central, but not defi ning, element, much of the core of periglacial geomorphology is also
a component of geocryology. However, periglacial geomorphology also includes the impact
of seasonal freezing and the roles of seasonal snow, and of ice of a fl uvial, lacustrine, and
marine nature. Finally, periglacial geomorphology must embrace the azonal processes
such as running water, wind, and waves that exhibit distinct characteristics in cold-climate
environments.
