Geology Reference
In-Depth Information
2
Periglacial Landscapes?
This chapter critically examines the concept of a “periglacial landscape.” Many such
landscapes do not contain typical periglacial landforms because they have only recently
emerged from beneath Quaternary ice sheets. As such, periglacial processes are merely
modifying already-existing glacial landscapes.
Two cold-climate landscapes are described from the western Canadian Arctic. Both
were never covered by moving continental ice sheets during the Quaternary and can
be regarded, therefore, as models to which landscapes might evolve under prolonged
periglacial conditions. Two landscapes are also described from now-temperate mid-
latitude regions where there is also no glacial legacy to consider. These are regarded
as largely relict periglacial landscapes because, for special reasons, each has experi-
enced relatively little subsequent modifi cation.
2.1. INTRODUCTION
It is commonly assumed that periglacial landscapes are distinct. This is because most
periglacial landforms are usually associated with permafrost. The most widespread are
tundra polygons. Ice-cored hills, or pingos, are a less widespread but equally “classic”
periglacial landform. Thermokarst features, such as ground-ice slumps, thaw lakes, and
irregular depressions, are also permafrost-related. Other phenomena result from frost
wedging and/or cryogenic weathering of exposed bedrock. For example, coarse, angular
rock debris occurs widely over large areas of the polar deserts and semi-deserts, and
angular frost-shattered bedrock protuberances (tors) refl ect more resistant bedrock.
Superimposed upon these “typical” landscape features is the occurrence of patterned
ground. A fi nal assumption relates to the overall smoothing of slopes thought typical of
periglacial regions. This is generally attributed to mass wasting and “cryoplanation.”
It is important that we establish the accuracy of these widely held impressions. Unfor-
tunately, the manner and speed of landscape evolution under periglacial conditions is still
unclear. The assumption of rapid change, implicit in many early periglacial investigations,
may merely refl ect either the relatively rapid adjustment from glacial to non-glacial condi-
tions or the susceptibility of certain lithologies to intense frost action.
2.2. PROGLACIAL, PARAGLACIAL OR PERIGLACIAL?
Given the fl uctuating climates of the Pleistocene, and the fact that many areas of current
periglacial conditions have only recently emerged from beneath continental ice sheets,
relatively few areas of the periglacial domain can be regarded as being in geomorphic
equilibrium. In mid-latitudes, which currently experience temperate climates, the major
