Geology Reference
In-Depth Information
problem is to identify the degree to which the effects of the Pleistocene cold climates have
been eliminated. Often lithology is the important variable controlling the degree of pres-
ervation of periglacial landforms. Careful fi eld investigation is required before the nature
and magnitude of the periglacial legacy can be established.
It is useful at this point to distinguish between the terms “proglacial” and “paraglacial,”
and to relate the conditions implied by these terms to landscape modifi cation. Paraglacial
refers to the disequilibrium that occurs as one geomorphic environment moves from one
equilibrium condition to another (Church and Ryder, 1972). In the case of the periglacial
environment, the transition is usually to, or from, a glacial or temperate environment, the
so-called glacial and interglacial periods of the Pleistocene. The proglacial environment,
which refers specifi cally to ice-marginal conditions, is a periglacial environment in the
original sense of Lozinski.
If we bear these two concepts in mind, one can illustrate the temporal relationships
that exist between the glacial, periglacial, and temperate landscapes (Figure 2.1). Whether
or not it is correct to assume that glacial landscapes are relatively high energy and temper-
ate latitude landscape are relatively low energy is open to question. However, this is not
of fundamental concern at this point. What is more relevant to our discussion is the
assumption that periglacial landscapes refl ect a constant energy condition, or equilibrium.
Equally debatable is whether or not such equilibrium characterizes periglacial environ-
ments today. For example, the classic periglacial terrain that was examined by participants
on the XI International Geological Congress fi eld trip to Svalbard in 1910 constitutes the
recently ice-free margin of a heavily-glaciated and mountainous island that is currently
undergoing isostatic rebound. In short, they were visiting either a paraglacial or proglacial
environment, rather than the periglacial environment as defi ned by Lozinski.
One perspective to this dilemma may be found in the cold regions of the world that
have remained unglaciated throughout much, if not all, of the Quaternary. Such environ-
ments may reasonably be regarded as being in geomorphic near-equilibrium, although the
rate and effi cacy of the processes involved may not have remained constant. The possibil-
ity exists that certain landforms may be inherited from earlier, warmer times.
Figure
2.1.
Temporal relationship between glacial, periglacial, and temperate landscapes under
changing energy conditions. A to B: glaciation; B to C: deglaciation; C to D: periglacial equilibrium;
D to E: climatic amelioration. Modifi ed from Thorn and Loewenhertz (1987) with permission from
Cambridge University Press.
