PERIGLACIAL LANDSCAPE EVOLUTION

The evolution of landscape under the control of cold-climate rock weathering and associated mass wasting of the regolith. Typical slope evolution is thought to involve a progressive and sequential reduction of relief with the passage of time. This progression, termed cryoplanation, consists of slope replacement from below, with the formation of Richter denudation slopes that are ultimately replaced by low-angle pediments (cryopediments). Richter slopes are rectilinear, debrisveneered, bedrock surfaces on which the rate of production of weathered debris is equal to, or less than, the ability of gravity-controlled transport processes to remove it.

There are several reasons why it is difficult to generalise about the evolution of periglacial landforms and processes. Firstly, certain lithologies are more prone to frost weathering than others; equally, some are more capable of preserving distinct periglacial slope morphology, once formed. Secondly, the variety of periglacial climates (see periglacial environments) existing today means that periglacial landform assemblages may also vary. For example, depending upon the degree of aridity, running water may, or may not, be an important landscape-modifying process. Thirdly, many areas experiencing periglacial conditions today have only recently emerged from beneath continental ice sheets. Therefore, these paraglacial landscapes cannot be regarded as being in true geomorphological equilibrium. Finally, there are relatively few studies that detail, in quantitative terms, the manner and speed of periglacial slope evolution.

The typical slope forms found in periglacial regions today can be summarised as follows: (1) rectilinear debris-mantled (Richter) slopes, (2) free-face (i.e. exposed bedrock) and associated debris (i.e. talus) slope profiles, (3) smooth convexo-concave debrismantled slopes, (4) stepped profiles and (5) pedimentlike forms. Whilst (2) is frequently associated with glacially oversteepened valleys, as in Svalbard and northern Scandinavia, the other slope forms fit the cryoplanation concept reasonably well.

Richter denudation slopes are best developed in very arid periglacial regions, such as Antarctica, central Siberia, north-central Alaska/interior northern Yukon Territory and in the Canadian high arctic. They indicate a landscape evolution model that involves slow frost weathering of bedrock combined with gravity-controlled free-face retreat and slope replacement from below. Cryopediments are the end result. In more humid environments, especially those recently deglaciated, not only is the rate of debris production greater due to the increased efficacy of frost weathering on exposed and oversteepened rock walls but solifluction, slopewash, snow avalanches and debris flows are more common. As a result, slope forms are more varied.

It is sometimes assumed, though not proven, that slopes evolve more rapidly under periglacial conditions than under non-periglacial conditions. The few available data suggest that this applies only to relatively humid and recently deglaciated regions. In the equally extensive arid environments, there is no evidence that landscape evolution is faster than elsewhere. Instead, evidence points to a similarity of forms with hot deserts and the semi-arid regions of the world. It is probable that many slope forms in the temperate mid-latitudes of North America and Europe are essentially relict periglacial forms dating from the pleistocene.

[See alsoequilibrium concepts in geomorphological and landscape contexts, slope evolution models]