Discussion

This acceptance of freeze-thaw as the dominant weathering mechanism has led

to its use in almost any discussion regarding the origin of cold region landforms or

sediments. Thus, in the absence of any empirical testing, a number of criteria

have evolved that are now considered indicative of the past or present action of

freeze-thaw weathering. Such criteria include, for a cold region present or past,

the finding of angular clasts or attributes to a landform, the origin of any non-glacial

feature that requires weathering, and the causative mechanism for the breakdown

of rock (bedrock or transported).

Thorn (1988,1992) provides extensive, reasoned accounts regarding the definition and historical context of the freeze-thaw concept and its application with regard to a number of landforms. As much of what he writes underpins the rationale for this thesis, an extensive presentation will be made of his thoughts. Thorn (1992, p.10) synthesizes the perceived problem where he states, "From its inception, periglacial geomorphology has been dominated by the concept of frost wedging, a synonym for weathering by freezing and thawing. The story is one of casual empiricism gathering respectability by repetition until it attained the stature of an article of faith." Historically, the concept that (Thorn, 1992, p.11) ".. .freeze-thaw weathering dominates cold regions gained respectability long before there was the ability to test it in the field." As Thorn (1992, p.11) then explains, "..the most common argument to substantiate the importance of freeze-thaw weathering is both circumstantial and circular." Thus, the angular rock fragments found in the field "...were assumed to be the product of the dominant process, namely freeze- thaw weathering. Today, it is common to assume that angular rock fragments are definitive evidence of frost weathering." As a brief aside, texts concerning processes in hot deserts (e.g. Abrahams and Parsons, 1993) also show highly angular rock and debate the relationship of process to landform. The discussions in these texts are almost identical to that found in cold region texts. Thus, if a geomorphologist working in a hot desert is plagued by almost identical questions, how can a periglacial geomorphologist simply assume that his/her angular c1asts must be the product of frost action when the desert geomorphologist may well be considering clasts with an identical form the product of thermal stress fatigue or salt weathering? The whole problem justifies Thorn's (1992, p.11) assertion that,

"...what periglacial geomorphologists need more than any other single item is a way to determine in the field whether or not bedrock fragments have been frost weathered." To date, no such test exists. Everything is based on assumption.

The application of laboratory studies to the concept of freeze-thaw weathering is

not without its problems. "Foremost among these problems has been uncertainty concerning the thermal and moisture regimes which actually prevail within natural bedrock and regolith fragments. Consequently, freeze-thaw cycles used in laboratory samples mayor may not reflect natural temperature ranges. A similar problem has overshadowed the moisture issue, and most laboratory experiments have embraced very crude approaches to moisture conditions and supply" (Thorn, 1992, p.10). This issue was outlined in papers by McGreevy and Whalley (1982) regarding temperature, and McGreevy and Whalley (1985) dealing with moisture.

These authors observed that thermal conditions used in experiments rarely ever reflect rock conditions but are usually related to temperature variability monitored in the air (McGreevy and Whalley, 1982). As Thorn (1988, p. 13) states, "In this context the entire range of published papers that use meteorological screen temperatures as a surrogate for bedrock and/or regolith temperatures is irrelevant, since the variables that intervene between air and surface temperatures are too numerous and too complex in their interaction to permit reliable extrapolation. This deficiency places all laboratory research in jeopardy, as the supply of field data from bedrock sites is entirely inadequate to reliably validate laboratory results".

With regard to moisture conditions McGreevy and Whalley (1985) make the point that, at the time of writing, almost no data regarding rock moisture content or rock moisture chemistry (which will affect the freeZing point) were available. Thus, given "..almost total ignorance of both field conditions and applicable theory, the experiments have not even precluded the possibility of other mechanisms.."

(Thorn, 1992, p.11). The key to all of the problems surrounding laboratory experiments regarding freeze-thaw was, in a similar context, stated by Warren (1914a, p.413) who warned that, it would be unsound "..to assume that the results of a certain experiment must also be produced by natural agencies, without evidence that similar conditions exist in Nature to those employed in the experiments". Sadly, hardly any laboratory experiments have taken heed of this;

they have assumed conditions that may really have no relationship to either the

thermal or moisture conditions the rocks under experimentation ever experience(d) in Nature. Key issues in this are such factors as the amount of water, the chemistry of the water, the rock temperatures and, particularly, the rate of change of temperature (boT/t), and the thermal gradient. Without such data, it cannot be known whether the laboratory experiments replicate the field situation or not.

Thus, "Field corroboration is something of a misplaced concept with respect to

frost weathering. At present there is no adequate criterion to establish that

bedrock weathering or further comminution of rock fragments has been dominated

by freeze-thaw weathering. Nevertheless, it is clear that the majority of periglacial

researchers believe that freeze-thaw weathering of bedrock is an established fact,

and that it is an acceptable premise upon which to base many secondary concepts

(e.g. cryoplanation)" (Thorn, 1992, p. 11). The problems cited above with regard

to laboratory experiments (Le. the need for data on rock temperatures, moisture

content, etc.) apply equally to the use of the freeze-thaw concept in the generation

of landforms. Rather than deduction based on empirical data, it is usually the

observation of 'angular clasts' that provides the assumption of 'freeze-thaw'

weathering as factor in landform origin. Thus, the invocation of freeze-thaw as a

central tenet of nivation, cryoplanation, blockfields, tors, etc. is without empirical

foundation (Thorn, 1988, 1992).