Geomorphology 149–150 (2012) 1–10

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Review

On the persistence of ‘weathering’ Kevin Hall a,⁎, Colin Thorn b, Paul Sumner a a b

Department of Geography, Geoinformatics and Meteorology, University of Pretoria, South Africa Department of Geography, University of Illinois, Urbana-Champaign, IL, USA

a r t i c l e

i n f o

Article history: Received 4 July 2011 Received in revised form 7 December 2011 Accepted 11 December 2011 Available online 22 December 2011 Keywords: Weathering Climate Rock properties Rock decay Energy transfer

a b s t r a c t The term ‘weathering’ has been in use for a very long time but it has come to mean different things to different people and hence, as scientific short-hand, it no longer functions. Here we question the tenets underpinning the most common usage of the term and note that the climate-process linkage implicit to the term is often missing and amounts to misdirection. Rather than climate as the primary driver behind specific weathering processes, it is argued that rock properties constitute the dominant control. Further, a case is made for reconsideration of our present bipartite (mechanical/chemical) division of weathering processes and of the weathering processes currently deemed to be ‘those that occur’. As process studies become evermore reductionist in nature, so the functionality of the term comes more and more into question. The linkage between process and landform, the scaling-up attribute, is seen as a current weakness and one that will become more confusing as reductionist approaches continue. As a ‘way forward’ it is suggested that weathering, stripped of specific preconceived notions of specific processes, be envisaged as a function of energy transfer and be investigated in that light. Identification of new processes as well as restructuring of known processes, particularly when considering weathering on other planets, is a potential outcome of such an approach. With a process foundation rooted in energy transfer, ‘rock decay’ provides a better umbrella term and liberates researchers from the inescapable conceptual baggage implicit to the term ‘weathering’. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The premise of this paper is that there is an ongoing and flexible relationship between scientific terminology and the impact that terminology has on the view of science held by researchers, and beyond that on the research they can envisage and undertake. At the grand scale such ideas are crystallized in the relativist view put forward by Kuhn (1970), but there are reverberations at virtually all scales in science. Here we simply discuss the repercussions, shorn of any philosophical overtones, of such circumstances on the central geomorphic topic of rock ‘weathering’. Our objectives are to demonstrate some of the inconsistencies presently embedded in the prevailing terminology, plus the impact of the terminology on researchers of the topic, and hence upon progress in weathering studies. As a discipline geomorphology is focused upon explanation of the surface of the Earth, although other planetary surfaces have also been garnering attention (Viles et al., 2010). The Earth's landforms and landscapes (assemblages of landforms) at any moment in time express the integration of the balance between internal (endogenetic) and external (exogenetic) forces influencing that surface, as modulated by both a unique local history and the surface material. The balance is clearly a dynamic one. As a matter of record, rather than ⁎ Corresponding author at: Geography Programme, University of Northern British Columbia, 3333 University way, Prince George, BC, Canada V2N 4Z9. E-mail address: [email protected] (K. Hall). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.12.024

necessity, geomorphologists have been consumers, rather than producers, of research on endogenetic processes, but preoccupied as a disciplinary group with researching exogenetic processes. Within this limited framework few topics are more fundamental than ‘weathering’. Indeed, a reasonable case can be made that weathering is the initial or primary expression of the transition from domination of material by internal forces to modification by external forces. The term itself is venerable and sits amidst similarly established terminology, e.g. denudation and erosion. However, the majority of such terms were established in an era of ‘eye-ball’ science, more particularly in the case of geomorphology in an era dominated by field studies. Given the burgeoning complexity in the discipline and the pursuit of some topics such as weathering at ever-more reductionist scales much of the older terminology needs reconsideration. A failure to undertake the task of scrutinizing existing terminology with some regularity condemns geomorphologists to several constraints, among the most severe of which is a flawed terminology that inhibits communication while simultaneously restricting and/or steering the avenues of research. This paper attempts such a review and is therefore, by its very nature, incremental and transient. Geomorphologists, both the producers and consumers of weathering concepts and data, would benefit from a substantive reexamination of the term ‘weathering’. It is an area of research that has surged forward in recent decades, primarily as the result of a sharp decrease in the scale of research as ever more fundamental answers are sought — the phenomenon usually labeled reductionist

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science. Two of the most obvious by-products of such changes are that researchers active in the area have generated a new set of concepts and terminology while geomorphologists pursuing other topics are generally happy to use familiar, but outdated terminology. This is not an uncommon situation in all sciences where established scientific terminology (‘jargon’) inevitably lags conceptual development if the discipline is healthy and growing. A third problem is not directly related to terminological issues and that is that contemporary reductionist research on weathering is difficult to scale up to a landform, let alone a landscape, scale. Characterization of surficial material modification as ‘weathering’ (see below for a consideration of formal definitions) clearly reflects an emphasis on the external part of the relationship between the atmosphere and surficial materials, and it is probably reasonable to suggest that this view received considerable impetus from the flowering of the paradigm of climatic geomorphology (e.g. Davis, 1909; Peltier, 1950; Tricart and Cailleux, 1972; Büdel, 1977), although it must be admitted that the term ‘weathering’ itself was in use long before climatic geomorphology was formally articulated early in the 20th century (see Büdel (1982) and Tricart and Cailleux (1972) for comprehensive descriptions of the two dominant approaches to climatic geomorphology). Implicit to such an approach is the notion that a change in climate produces a change in the nature of weathering. Here we use the word ‘nature’ to mean identifiably different chemical and/or physical (mechanical) reactions of surficial materials. Conceptually, such a view seems not only to minimize the contribution to the weathering process(es) of the material being changed, but also to render a similar impact on the contribution(s) by lower fauna and flora life-forms. Variability in the rate of change of weathering processes between climatic zones has also tended to be overshadowed by the emphasis on change in the nature of weathering. This does not mean that there are not many studies of the weathering of individual rock types, but rather that the primary input to the process, the host material, often plays a secondary role to an emphasis on climatic inputs which is accompanied by a subtle inference that weathering in kind, not just rate, varies with climatic variability. Before proceeding further we would note an important caveat. Herein we consider only weathering of bedrock and coarse fragments of all sizes; specifically, we exclude all consideration of the weathering of fines and pedogenesis. Clearly, weathering is absolutely central to pedogenesis and many of the processes are identical or closely akin to those discussed here. However, pedogenesis contains many additional concepts that are above and beyond weathering per se but it is certainly worth noting en passant that a significant number of the points we raise also apply to pedology. As an illustration we would note the work of Mohr et al. (1972) who in a review of tropical soils explicitly note that attempts to relate climate to soil type fails. They identify such issues as rock properties (notably chemistry/mineralogy) and permeability, not climate, as the key parameters. This is an important point in so much as most geomorphologists would certainly identify tropical climates as the most intense weathering regimes, and characteristics appearing in such environments are likely to occur elsewhere, but are perhaps much harder to detect. We address the topic from a number of different perspectives. First, weathering is examined in terms of its external relationships within the general lexicon of the discipline. Second, internal subdivisions within the overall topic of weathering are examined. This is followed by two sections, one focused upon the external portion of weathering (the weather or more accurately ‘climate’) that may effectively be considered the ‘drivers’; the second focused upon the role of the host (bedrock and/or coarse debris). We then examine briefly the relationship between weathering and landforms. The paper concludes with an attempt to outline in a preliminary fashion a refocusing of weathering studies that truly address the central issues and how they would benefit from a recasting of the relevant concept.

2. Weathering — the conceptual demarcations There is no clear-cut starting point for a discussion of weathering because there is no single definition of the term. Below we list two from widely distributed sources: Weathering The destructive process or group of processes by which earthy and rocky materials on exposure to atmospheric agents at or near the Earth's surface are changed in color, texture, composition, firmness, or form, with little or no transport of the loosened or altered material; specif. the physical disintegration and chemical decomposition of rock that produce an in-situ mantle of waste and prepare sediments for transportation. Most weathering occurs at the surface, but it take place at considerable depths, as in welljointed rocks that permit easy penetration of atmospheric oxygen and circulating surface waters. Some authors restrict weathering to the destructive processes of surface waters occurring below 100 °C and 1 kb; others broaden the term to include biologic changes and the corrasive action of wind, water, and ice. Obsolete syn: demorphism; clastation. Glossary of Geology, Fifth Edition (2005), page 718. Weathering represents the response of minerals which were in equilibrium at a variety of depths within the lithosphere to conditions at or near the earth-atmosphere interface. Here they are in contact with the atmosphere, hydrosphere and biosphere giving rise to their largely irreversible change to a more clastic or plastic state, involving increases in bulk, decreases in density and particle size, and in the production of new minerals which are more stable under the interface conditions. Obviously, landform evolution depends to a large extent upon these weathering processes. Chorley et al. (1984), page 203. We believe that weathering seems best captured by the definition proposed by Chorley et al. (1984, p. 203). It might be modified by inclusion of the limits (b100 °C for temperature and b1 kb for pressure) proposed in the alternate definition cited (Glossary of Geology, Fifth Edition, 2005, p. 718), but the terms ‘atmospheric’ and ‘hydrosphere’ are more flexible and thus more appropriate to a general definition (at least on Earth but maybe not on other planetary surfaces). Precise boundaries are always appealing, but just as surely they are also more inhibiting and subject to subversion. Some will always question the ‘transport’ (‘movement’) concept that is usually invoked to constrain weathering, i.e., that only very limited movement is permitted beneath the rubric of weathering. Again this concept needs flexibility within a general definition as at some scale there will always be movement if any form of transformation occurs. Consequently, some minimal movement, defined at the scale of the weathering process invoked, must be accepted. Such a movement is likely to vary in magnitude between, for example, the processes of chemical modification and mechanical crack widening by tree roots, but there seems to be little point in debating this in absolute terms. Perhaps the only functional separation of weathering and mass movement (wasting) rests with the focus of attention. Where attention is on chemical change or mechanical reduction movement is considered incidental. Where emphasis is upon transport mechanisms further change or breakdown is accepted as inevitable, but regarded as incidental. The notion proposed in the Glossary of Geology definition that ‘corrasive’ action be included in the definition of weathering is indicative of the definitional problems that have (e.g., Dove, 1997), and continue, to beset geomorphology. In the Encyclopedia and of Geomorphology (1968, pp. 202–203) corrasion incorporates both vertical down wearing by mechanical means and the subsequent transportation of any material loosened thereby. This cuts across

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what seems like a logical set of conceptual boundaries separating breakdown in situ (weathering), entrainment, and transport. The Glossary's definition confronts virtually identical issues and also stirs the hornet's nest of whether weathering should be subdivided in a bipartite (mechanical [physical] versus chemical) or tripartite (mechanical [physical], chemical, biological) manner. The crux of the matter would seem to be that ultimately all biological processes are mechanical or chemical, therefore a bipartite subdivision is appropriate with biological inputs being recognized as one of, if not the, primary agent(s) by which they are accomplished. The counter argument is that some biological actions, particularly chemical ones, are uniquely biological and therefore merit a separate category. We favor the bipartite subdivision because it seems more logical while not seeking in any way to minimize the role played by biological agents in weathering which is clearly enormous, perhaps even paramount in many environments. If we accept Chorley et al.'s (1984) definition of weathering and subdivide it in a bipartite manner, while accepting the central role of biological processes in both mechanical and chemical weathering, we have a workable definition. We would also argue that for a generalized definition we are better off accepting boundaries stated in a qualitative manner rather than precise absolute limits that will inevitably crumble in the face of increasing instrumental resolution. The question then becomes “how do we fit weathering into the established framework of adjoining and/or abutting concepts such as denudation and erosion?” Approached systematically from the perspective of process geomorphology it is an enormous task. Like the term weathering itself many of the abutting concepts are old, deeply entrenched, overlapping, conflicting, inconsistently defined and generally out of touch with prevailing scientific standards in process geomorphology. They certainly do not resonate with the scale or standards prevailing in current weathering investigations. While we raise these issues we will not address them here — they clearly merit attention, but represent an enormous undertaking that is not the focus of our attention which is the more constrained task of addressing the term ‘weathering’ and its definition in isolation. Such texts as those referred to above (encyclopedias, handbooks, basic texts) tend to be consensual, and hence conservative. If we broaden our perspective to embrace individual research papers the range of definitions becomes even wider, and consequently in some, but not all, cases even sharper discrepancies emerge. We consider only some of the most diverse here. Historically, terminological confusion can be very great when scrutinized in terms of modern usage. Examination of Gregory (1911) and Bissell (1921) provide not only individually curious demarcations among terms such as weathering, erosion, and denudation, but also sharply conflicting ones when their terminologies are compared. While such papers may be dismissed as dated, it is possible to move forward into much more recent times and still confront highly unusual definitions. Harris (1992) distinguishes among ‘weathering’ which he applies to naturally-occurring rock, ‘corrosion’ which he applies to building stone, and ‘rusting’ which he applies to iron. He then suggests that the terms may be used synonymously and that frost action on building stone is ‘corrosion’ by physical processes (Harris, 1992, 237). There are comparable inconsistencies when authors attempt to distinguish between weathering and erosion. Ollier (1984, 26) includes abrasion as a component of physical weathering, while Easterbrook (1999, 21) considers the impact of falling blocks which eject other blocks a form of mechanical weathering. The wearing away of rock by water and/or wind (Jones et al., 1991) or by volcanic eruptions or rainwater (Doerr and Coling, 1993, 180) have also been characterized as weathering. Finally, we could also choose to include ‘tree throw’ as weathering according to Gabet and Mudd (2010). The issue here is not so much a matter of right or wrong, but one of consistency. In other words, when you use the term ‘weathering’ is what you mean the same as I think it means?

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Historical terminological issues also include the use of synonyms for weathering which are both plentiful and informative. Among the most prominent are: rock decay (Chalmers, 1898); rock crumbling (Blackwelder, 1927); rock decomposition (Goldrich, 1938); rock rotting (Linton, 1955); rock disintegration (Sperling and Cooke, 1985); rock disaggregation (Fahey, 1986); rock breakdown (Matsuoka, 1990). Clearly, there is a universal emphasis on the rock, or more specifically the impact on the rock with this suite of terms — a topic to which we will return. 3. The role of weather in weathering While weather and weathering are homophones the most highly developed conceptual link between the atmosphere and geomorphology historically has been between climate and geomorphology. This claim is clearly true if only because of the extensively developed concept of climatic geomorphology (e.g., Tricart and Cailleux, 1972; Büdel, 1982) has claimed so much research effort, especially in continental Europe. For the purposes of this paper it is sufficient to accept the conventional distinction between weather and climate whereby weather is short-term atmospheric variability and climate a longterm averaging of weather integrated with seasonality (itself an intermediate averaging of annual atmospheric variability). The essential implication for geomorphologists is that variation in climate as a driving force will produce systematic variation in response by surficial materials (bedrock and coarse debris in this paper, but soil as well in other studies). Intuitively, freeze-thaw weathering might appear to be readily correlated with climatic drivers (e.g. Peltier, 1950); such processes as weathering by thermal stresses, salt weathering, weathering by wetting and drying appear to (perhaps) be less obviously linked; while unloading (dilatation) is simply unrelated. A conceptually slippery slope quickly emerges. If chemical processes are considered closely it is not possible to discern any direct link to the climate. Oxidation requires the presence of a metal, carbonation requires the presence of carbonate — no climate can produce these processes in the absence of the specified materials. Approached somewhat differently it might be argued that hydrolysis always requires water which is inherently associated with climate, but once again appropriate minerals in an appropriate state must be present. Adding biotic elements such as lichens to the weathering scenario changes the argument little or not at all. Lichens, in hot or cold environments, are just lichens performing remarkable similar feats such as hydrolysis, in all climes. In short, systematic variability in atmospheric conditions, over the short-term (meteorology) or the long-term (climatology) does not provide systematic prediction of weathering type. While we may sharpen the argument by taking up the commonly overlooked truth that it is ground, not air, climate that should be the subject of our attention (Thorn, 1982), this represents merely better definition of the driver and does nothing to reduce the central role of the materials involved. Any apparent climate linkage is primarily associated with climatic geomorphology concepts, but those linkages are false. Relating mean annual (air) temperature to rock temperatures directly (e.g. Peltier, 1950) has been shown to be a meaningless task (Thorn, 1982; Sumner et al., 2004; Hall, 2007). Indeed Hall (2007) demonstrated that in northern Canada, despite sub-zero air temperatures, given the presence of water and appropriate orientations, rocks could experience chemical weathering throughout the winter. This reality was amply demonstrated on a number of occasions when, during nighttime sub-zero rock temperatures, exotherms occurred that indicated the presence of unfrozen water during the daytime (when rock temperatures were in the +20's °C, despite sub-zero air temperatures) and hence, despite the low air temperatures, chemical weathering could take place. The graph presented by Peltier (1950) relating the importance of weathering to climate suggests that, in the presence of moisture, temperatures in northern Canada, and those in

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Antarctica too (Balke et al., 1991), would permit ‘moderate’ or ‘strong’ chemical weathering. The limitation is not temperature but moisture and available data now indicate that in many situations some moisture may well be present in what are widely designated as cold environments. Misinterpretation of cold region rock surface conditions are not limited to Peltier's (1950) consideration of temperature. Ollier (1984, p. 130) suggests that thermal stresses should be high where temperatures are high but precipitation is low. However, Ollier's suggestion also fails because, as Hall (2007) has shown, high rock temperatures (due to solar radiation) in the presence of cold air conditions can result in rapid cooling of the rock when, for example, clouds obscure the Sun — such a situation (following Newton's Law of Cooling) is highly conducive to large tensile forces in the rock that can cause thermal fracturing (Hall, 1999). Consequently, even in cold regions, where the relationship between weather and weathering appears to be its most intuitively obvious strength it is, in fact, anywhere from tenuous to outright misleading. Indeed, the only clear connection between the atmosphere as a driver and weathering is with the former as a forcing agent of process timing and intensity or, indirectly, as a factor in determining the nature and extent of organisms present. The basic suite of mechanical and chemical weathering processes (e.g., Bland and Rolls, 1998; Gregory, 2010) have been with us, essentially unchanged, for a century or more. While nomenclature may vary, the mechanical processes of dilatation, wetting and drying, salt weathering, freeze thaw and insolation weathering, as well as chemical solution, carbonation, oxidation, hydrolysis and hydration continue to be used to explain both products and landforms. During this time there have been, with few exceptions (e.g. Hall, 1989) no ‘new’ processes forthcoming. Rather, emphasis has been on attempts to define at increasingly reductionist scales, or in purely theoretical terms, the long-accepted processes or to apply them to new concepts. The hypothetical model proposed by Hallet (1983) to explain freezethaw weathering is an example of the former, while Murton et al.'s (2006) presentation of the weathering impact of two-sided freezing in permafrost is an example of the latter. We might unpack many, if not all, weathering processes in a similar fashion, insolation weathering (weathering due to thermal stresses) springs to mind as a ready example. However, it seems unlikely that we have, in fact, identified all weathering processes. Nevertheless, the suite of known processes remains the framework within which all data, both old and new, must be fitted. There is, of course, a true intellectual dilemma and/or conundrum here because we do not need to throw out the existing suite but, rather, expand it and probably subdivide some elements of it afresh. ‘What we see depends on mainly what we look for’ (Lubbock, 1895), but by the same token how do we ‘see’ what we are unfamiliar with? This problem is central to all science, not just weathering studies. Our current suite of known processes is effectively a set of inviolate boxes, but we see real complexity in the field. Our usual response to this conundrum is to envisage the boxes working ‘in series’ (sequentially in time) or ‘in parallel’ (simultaneously in space) yet it seems likely that reality must include synergistic relationships at the very least, as well as not yet recognized processes. The concept of ‘process boxes’ may be readily illustrated using the so-called freeze-thaw process. For freeze-thaw weathering to work the rock/crack must be wet, the temperature becomes cold enough for the water to freeze (i.e., have lower kinetic energy) and exerts a stress, then there must warming that causes the water to thaw (thereby relieving the stress). In addition, some degree of drying of the rock occurs during the warming phase. This is not a ‘singularity’ (as is suggested by the term) but rather (within current thought) a series of separate processes each of which must occur in a linked sequence to produce ‘freeze-thaw weathering’ as commonly identified. Unpacked in this fashion it is easy to see that the chain of events could also cause weathering by: wetting and drying; thermal stress (given

adequate rates); chemical processes (dependent upon impurities, temperature, and rock type); and salt specifically. However, conventionally the freezing and thawing of rock is interpreted exclusively in terms of the freeze-thaw process (Hall and Thorn, 2011). Two important questions emerge, 1) How should the total weathering achieved be apportioned among the suite of processes actually involved? 2) Why are we forced to characterize what is clearly an interactive suite of processes as a single process or entity? These questions might be redefined as “are not the components of the process a singularity rather than a synergy of connected entities?”

4. The role of parent material The emphasis in weathering studies seems historically to have been on the processes involved with a strong secondary emphasis on the material (rock) involved. The latter is readily seen in any number of studies focused upon individual rock types, e.g., Twidale (1982) and Ford and Williams (1989) being two textbook-scale examples. Today, there appears to be something of a shift toward emphasis on the host material. Such a shift has a marked impact on the data that are considered relevant (e.g. King, 1962; Small, 1970). Not only is chemical weathering seen as fundamentally controlled by the composition of the material, for example carbonation simply cannot occur unless carbonates are present and Fe-rich olivines have the Fe that is required for oxidation/reduction to take place, but this is now clearly understood also to have secondary ramifications. Biological weathering by lichens is directly controlled by the lichens, but the presence or absence of the specific lichens is influenced by the chemistry of the host material (Banfield et al., 1999). Similarly, in the purely physical realm, rock properties are increasingly emphasized as controls on: porosity and permeability (e.g. Kieffer et al., 1999; Benavente et al., 2007), pre-existing joints, bedding planes (Small, 1970), internal stresses and lines of weakness (Yershov, 1998), mineral connectivity and properties (albedo and thermal conductivity, capacity and emissivity) (Hall et al., 2007), etc. Despite the growing awareness of the significance of such rock properties they probably merit even greater attention. Texts such as those by Yatsu (1988) and Selby (1985) provide some discussion of rock properties but the reader is often left with the sense that this information is presented as a foundation for understanding weathering processes (e.g. the detailed discussions by Yatsu, 1988, 401–485) rather than as a set of constraints and opportunities that truly control the very nature of the weathering regime. At smaller scales, both temporal and spatial, it would seem reasonable at first sight to take the inputs or drivers of weathering, namely the climate, as a constant, yet we still encounter major differences in weathering even given what intuitively seems to be a constant rock type. Some of this variability stems directly from microclimate, e.g., see Hall et al. (2005, 2008) for the impact of aspect; while other variations stem from micro-variability within the host material (Hall et al., 2008). Granites provide splendid illustrations of this point. The classic granite tor is one example, while core stones embedded in grus are even smaller examples. Blair (1975) provides an elegant explanation of such features in Colorado, demonstrating that fracturing, microfracturing, and biotite content all play critical roles. It is clearly the nature of the rock that controls the weathering in kind, for example whether biotite is absent or present, abundant or sparse. Biotite also controls the rate of weathering not only by constraining the kind of weathering but also by controlling processes that are identical in kind from place to place but very variable in rate in response to water availability. Nor are rock characteristics limiting of only chemical processes. Hudec (1973) has demonstrated that freeze-thaw weathering is not only limited exclusively by temperature and moisture but also by rock properties; some rocks simply do not exhibit appropriate permeability and/or porosity

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to make them vulnerable to the formation of ice crystals as a result of water being unable to enter the rock. Not all of the above ideas are by any means ‘new’, for Merrill (1906, p. 227) begins: “Rock-weathering manifests itself in a variety of ways, much depending on the climate, though naturally the controlling factor is that of the mineral composition”. Elsewhere Merrill (1906, p. 253) states “That all minerals of a rock mass are not equally acted upon by atmospheric agencies has been sufficiently noted” and so it has certainly long been realized that whilst the climate plays a role, it is clear that rock properties exert the dominant control, a point considered at some length by Small (1970). Some of these earlier attempts to relate rock type directly to weathering responses (e.g. Ollier, 1984) are fairly nebulous by today's research standards. For example Ollier (1984, p. 87) states: “Schists: These split easily along the ‘schistosity’ and this is very important in weathering. They contain some very resistant minerals but weathering is moderately easy. Frost weathering can rapidly break up schist”. Clearly, such comments are of limited utility. Personal experience (KH) has shown schists to exhibit extensive chemical weathering in an Arctic situation and primarily mechanical weathering in the maritime Antarctic (Hall, 1987). In the first case the schists had extensive, open schistosity while in the latter they were highly compact. Frost action must be dependent upon moisture availability, rock permeability and freezing temperatures each of which is independently limiting. Given a disparate set of ‘filters’ the entire process is highly constrained in operation (Hall et al., 2002; Hall and Thorn, 2011). In warm to hot climate simulations schists have been shown to be impacted mechanically by salts (e.g., Wells et al., 2006) or by rapid chemical weathering and solution (Sharmeen and Willgoose, 2006) due to high rock temperatures (70 °C) combined with a high annual rainfall (1480 mm). Thus, the reality is that climate exerts an influence on what processes might be able to take place (i.e. it influences temperatures and moisture supply) but the rock determines which processes do take place. 5. Weathering rate versus weathering kind (nature) Faced with strong, even sharp, differences in the extent or amount of weathering achieved within a seemingly homogeneous rock type, a researcher may invoke several possible explanations. This is true at any and all scales. Important possible explanations are: 1) a difference in weathering process or combination of processes; 2) a difference in process intensity due to variability in the rock or in the inputs which drive the weathering process; 3) a difference in the time available for the weathering process to act (i.e., duration of rock exposure, commonly phrased simply as the ‘age’ of the surficial material); 4) a change in climate. These possibilities may, of course, overlap or interact, but generally have been investigated discretely. The first point has seemingly consumed most of our investigative energies to this time. The matter of localized variability in rock properties has already been addressed (e.g., Hudec, 1973; Blair, 1975), while differences in the degree of weathering stemming from differing times of exposure to surficial weathering processes have been extensively exploited in Quaternary studies (e.g., Porter, 1975; Thorn et al., 2007). Attention here is focused upon variability in weathering intensity stemming from variations in the intensity of the driving forces at small and large scales, not upon the variability in weathering dependent upon the nature or kind of weathering process. Because of our own experience the examples are drawn primarily from cold region research; however, we believe the principles discussed are universal. As shown in the preceding sections the kind or nature of weathering possible is determined primarily by the properties of the rock. It seems that only freeze-thaw weathering is also significantly dependent upon climate directly. Even this claim needs to be parsed carefully because although some level of rock cooling (thereby allowing water to freeze) is necessary, the precise intensity (rate of cooling) remains poorly understood. Furthermore, Hudec's (1973) research

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clearly shows that while cooling is ‘necessary’ it is not ‘sufficient’ because some rocks lack the appropriate physical properties (e.g. permeability, porosity) to be frost susceptible. It is possible that climate may determine the nature of weathering indirectly if it determines the presence or absence of flora or fauna which produce unique weathering mechanisms. However, the topic remains a largely uninvestigated area. It is certainly true that many lichens for example do not appear to accept all rock types as suitable hosts. So even here we are confronted by the classic ‘necessary versus (not) sufficient’ dilemma. In contrast, the rate of weathering is primarily determined by moisture availability and then by temperature and, in some circumstances, the presence of biological agents. As is widely recognized, warmer temperatures increase most rates of chemical activity, while temperatures well below freezing may arrest chemical activity on a seasonal basis. While we have sound evidence that cold temperatures alone do not preclude chemical weathering (Thorn et al., 2001), we have little or no evidence of truly seasonal presence or absence, or intensity variation, of chemical weathering. The oft-cited spring-time release of debris supposedly weathered by freeze-thaw processes (e.g., Matsuoka, et al., 1997) really only tells us about the mass wasting, not weathering, mechanism. As indicated above, temperatures close to freezing are likely to produce complex situations at or near the ground surface. It is, of course, true that in all circumstances geomorphologists need to be focused upon ground climate and not air climate. Unfortunately, and especially in cold region geomorphology, geomorphologists interested in weathering have been far too preoccupied with temperature, even worse normally air temperature, and not nearly interested enough in ground moisture regimes. In fact, there are some truly important questions here that have been rarely considered, for example if seasonal snowfall produces a dry season (winter) followed by a truly wet season (spring melt), what are the relative efficiencies of a fixed amount of precipitation delivered more evenly as rainfall versus the profoundly uneven ‘effective’ delivery of moisture as seasonal snowfall? A prima facie interpretation would suggest that mm for mm rainfall should be more efficient than snowfall, but that snowfall should produce much more varied spatial weathering of the landscape due to its redistribution prior to melt and, hence, sharp spatial contrasts in total absolute weathering as opposed to efficiency in weathering (see Thorn (1975) for an examination of some associated data). Indeed, the speed of springtime runoff may reduce weathering efficiency by virtue of the reduced residence time of meltwater. This may be offset, but does not have to be, by large increases in absolute water volume derived from winter snowpack concentration. Other kinds of ‘rate versus kind’ issues may be quite obscure. Present-day debates concerning the origins of autochthonous mountain-top blockfields (e.g., Boelhouwers, 2004; Whalley et al., 2004; Ballantyne, 2010) strongly suggest that climate change has played a role in their origins, but is it one where weathering kind is important? Our argument above suggests that the essential nature in which a particular rock type weathers cannot be changed. It seems more likely that the rate of weathering and the relationship between it and the removal of fines are critical. Virtually, all modern research on autochthonous blockfields suggest that they originated as coarse material within a matrix and that the matrix has been removed. In general there is now convincing evidence that many periglacial environments where such features occur have experienced very little weathering since the last deglaciation (e.g., André, 2002; Nicholson, 2009) and, albeit a slow process, weathering today in such environments is largely chemical and driven mostly by organic agents. What appears to have happened in these contexts is that the matrix of fines has been largely removed and while the agency of this is uncertain it may well be associated with the paraglacial phase. If such a scenario is valid, autochthonous blockfields are an illustration of a residual landform that is currently experiencing a much reduced rate, rather than kind, of weathering and is largely

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immobile because of a change in mass wasting regime rather than anything to do with weathering. Of course, the contrast between the weathering of a matrix versus that of included residual boulders and/or stones is open to both a ‘rate’ or ‘kind’ argument. However, the classification of such features as autochthonous would seem to suggest variability in localized weathering rates produced by rock variability (perhaps akin to granite corestones) rather than localized variability in weathering kind. 6. Landforms As suggested by the scenario outlined above and discussed elsewhere (Hall, In Press) the relationship of weathering to landforms is often a tenuous one. This reality may be interpreted in more than one way, for example variations in granitic weathering (e.g., Twidale, 1986) may produce very different landforms (e.g., etch plains and tors) while grus may be produced in multiple rock types and in multiple contexts or azonally (Mignon and Thomas, 2002). While many landforms are related to weathering the reality is no landforms are due solely to weathering — the possible exception that proves the rule being thermal contraction cracks in bedrock (e.g., Eppes et al., 2010). This is important because many landform names incorporate an epithet suggestive of an origin derived solely from weathering; e.g., ‘weathering pits’ (e.g. Dahl, 1966), ‘weathering hollows’ (e.g. Matsukura and Tanaka, 2000), ‘honeycomb weathering’ (French, 2007) or ‘cavernous weathering’ (e.g. Dahl, 1966). Such weatheringbased nomenclature belies the reality of the necessity of an agent (or agents) of removal. In many instances (frequently in the case of tafoni) while there is some understanding of the weathering involved (e.g. Strini et al., 2008) there is very little understanding of the removal processes. Yet, surely, the removal process is as vital to the landform creation as is the weathering, perhaps more so as it may exert further erosional or form-related effects? Even the frequent linkage of some forms with specific climates (e.g. tafoni with cold regions: Calkin and Cailleux, 1962) can be misleading as the forms are clearly azonal in occurrence. Further, in a number of environments the association of certain forms with that environment is now being questioned with regard to whether the form is a product of that environment or merely preserved within it (see blockfields above); and hence a product of a former environment (André et al., 2008). Thus, the linkage of weathering to either form or climate, through landform, is a highly contentious one and should be viewed with great caution. Implicit to a weathering landform that is truly climate-derived would seem to be the implication that weathering differs in kind from climate to climate in a manner that has varying spatial expression either as a result of varying weathering kind and/or rate. It seems much more plausible to argue for variation in rate than kind (see seasonal snowpack argument above) and to argue that any ‘weathering landform’ is actually an expression of inherent variation in the rock resulting in differential exploitation. This would also permit similar landforms (e.g., tafoni) to develop in different climates, perhaps at different rates, but not necessarily with different forms except in so much as the host rock varies, but azonally. It seems that, at the present time, by far the biggest intellectual problem in weathering studies, albeit a very unfashionable one, is the need to scale-up a vastly increased reductionist knowledge of weathering mechanisms to address our understanding of landforms. Such a comment brings to mind Chorley's (1978, p. 10) analogy of the different skills demanded of a car mechanic (small-scale engineering) and a traffic engineer (large-scale knowledge of group behavior). While we recognized that we are primarily addressing the secondary topic of ‘car mechanics’ in this paper we cannot afford to lose sight of the reality that ‘traffic engineering’ (landforms individually and collectively) is the ultimate objective of geomorphologists. Successful resolution of our ideas of weathering is likely to require reconsideration of the topic across all scales.

7. Reconsideration Data collection and analysis in weathering studies generally evaluate the timing, role, and effect of the different component processes (Smith, 1994). While individual components may be permitted to behave synergistically they, themselves, remain inviolate and continue to be assigned their own discrete controls, operational boundaries, and effects. It is true that increased effectiveness is implicit to the concept of synergy, an example would be Williams and Robinson's (2001) consideration of freeze-thaw weathering in the presence of various salts. However, it is also true that the prevailing subdivision of weathering represents only one possible way of subdividing reality — perhaps one that merits reconsideration. We suggest that indeed the present subdivision itself represents one fundamental hindrance to further meaningful development of theory. So what happens if we discard this line of thinking and begin anew? Rather than considering the present processes as our definitive list of discrete entities, perhaps we need to reconsider the problem from two viewpoints: first, one of creating new knowledge, that is identifying unknown processes; second, one of abandoning our existing, occasionally synergistic, individual processes for a new set wherein there are different combinations of processes that operate as single, discrete entities. The first option becomes more and more exciting as better and more sophisticated technology becomes available that facilitates the consideration of processes at finer and finer scales (Fig. 1). New developments within weathering at the nano-level (e.g. Pope, 1995; Pope et al., 1995; Butenuth, 2001; Hochella, 2002) and on other planets (e.g. Dombard et al., 2010; Viles et al., 2010) open avenues for the contemplation of new processes. Indeed, consideration of weathering on other planets must surely open the door to new processes (and new or, at least, different kinetics) and require relinquishment of Earth-bound concepts operating in Earthconstrained environmental conditions. The second approach may seem little more than semantics, a reapportioning of attributes, but this is not so. If such an approach is taken, it should lead to a new way of monitoring processes, for new protocols would be required. Now we would need to monitor this new ‘grouping’ holistically rather than simply saying (assuming) we need to monitor several discrete entities the attributes of which are thought to be known. We would have to rethink quite how we would monitor a process, a single process, that is a combination of, say, thermal conditions, moisture amounts, and moisture chemistry, all mediated by rock properties. Whilst, in some instances monitoring might actually not change, data evaluation certainly would, as would quite probably our assessment of landscape development. For example, using the freeze-thaw synergy noted earlier, perhaps it is the physico-chemical stresses of unfrozen water at the crack tip (Wiederhorn and Johnson, 1973) that produces crack extension rather than any stresses associated with the water to ice phase change. Even if such suggestions are impractical at the present time in some instances then the conceptualization alone may foster experimental development and also heighten awareness of our present intellectual limitations. Indeed, consideration of some Russian approaches to weathering (e.g., Konishev, 1982; Yershov, 1998) indicate that it is the unfrozen water film that plays a dominant role in freeze-thaw weathering and that this, in turn, is linked to the mineral assemblage (French, 2007). Yershov (1998, p. 222) refers to many of the weathering mechanisms as “physico-chemical” such that, again, they would not neatly fit into our present simplistic suite of weathering processes. 8. Weathering: a new conceptual framework Although a renaming of weathering is probably ‘a bridge too far’ it would, in truth, be beneficial. A new term needs to: 1) retain the sound conceptual content we already have; 2) shed the present restraining conceptual content; 3) provide a meaningful framework

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Fig. 1. Visualization of nanoscale weathering placed within broader spatial scales of weathering phenomenon. Examples presented from nano to landscape scales: (A) nanoscale silica spheroids a few tens of nm across from a silica glaze in Tibet (HRTEM image); (B) micron-scale silt formation from quartz weathering in Arizona (BSE image); (C) millimeter- to centimeter-scale rock coatings and weathering rinds from Wyoming (BSE image); (D) Death Valley (case hardened rock shelter); (E) meter-scale weathering form: a “mushroom rock”, Arizona; (F) limestone karst stone forest, Kunming; (G) varnish-coated alluvial fan, western China.All images and information, except for (H) are courtesy of Dr Ron Dorn; (H) is copyright of NASA.

for future development. In fact, creation of a new term per se would be superfluous. Of the many available synonyms for weathering “rock decay” (as used, for example, by Chalmers, 1898; Blackwelder, 1927; Lobeck, 1939; Linton, 1955; White, 1976) certainly puts the emphasis where it needs to be and has been used to describe the processes(es) in environments ranging from tropical to polar. Indeed, Chalmers (1898, p. 280) identifies the agencies causing rock decay to be “mechanical and chemical”, and so the use of this term appears to create no untoward problems — keeping firmly within the spirit of the meaning of ‘weathering’. Further, the term ‘decay’ (of an object) implies a break down to (its) constituent parts and can encompass any and all breakdown mechanisms without, by its definition, referencing climatic parameters. The term decay also, inherently, implies an activity in situ — as, for example, in “tooth decay” wherein the tooth is broken down by mechanical, biological and/or chemical processes operating on and/or within the tooth (e.g. Featherstone et al., 1979). It also has, within its accepted meaning, the notion of a decrease in size (as in distance decay) and/or activity (as in radioactive decay). This latter attribute, ‘activity’, fits well with the argument below that rock decay is related to energy transfer. Currently, chemists identify reactions, some of which we appropriate within ‘chemical weathering’, as a function of “energy transfer”. Is there any reason why such a term would not equally apply to mechanical processes? Surely any form of ‘weathering’ relates to “energy transfer” in the sense that whatever process takes place it is, ultimately, related to energy gain or energy loss. Energy can be released at a variable rate from very slowly to rapidly, and “the time rate of liberation of energy, in J s − 1, is termed power” (Leeder and Pérez-Arlucea, 2006, p. 44). Indeed, in terms of mechanical processes

for the non-reversible cracking of rock due to non-chemical reactions then “a crack can only extend if the energy released by this process is at least large enough to supply the surface energy required to form the new surface area of the crack faces” (Jaeger et al., 2007, p. 311): in other words, a form of energy transfer. Indeed, the theory of linear elastic fracture mechanics is dependent on this energy release during cracking (Jaeger, et al., 2007). Even in biological processes, microorganisms exchange electrons and energy for the benefit of the organism, for example Acidthiobacilus ferroxidans derive their energy from the oxidation (i.e. weathering) of iron (Espejo et al., 1988). In fact, oxidation of pyrite by organisms is 34-fold faster than pure abiotic reactions (Olson, 1991); all being energy transfer. Once away from geomorphology texts, work on weathering (e.g., Butenuth, 2001) is all related to energy (surface, interfacial, and internal) and energy transfer. In discussions on this topic some have mooted the idea that perhaps weathering is better considered as related to ‘entropy’. However, at least in terms of rock fracture mechanics, no texts (e.g. Rossmanith, 1983; Goodman, 1989; Butenuth, 2001; Leeder and Pérez-Arlucea, 2006, or Jaeger et al., 2007) mention entropy — but all discuss the role of energy. Entropy relates to the Second Law of Thermodynamics and is a measure of the energy not available for work. Entropy is hence a measure of the tendency of a process, such as a chemical reaction, to proceed in a particular direction. In terms of the thermal energy it determines that it will flow from regions of higher temperature to regions of lower temperature in the form of heat and, in so doing, reduce the state of order of the initial system. Thus, entropy is an expression of ‘disorder’ or ‘randomness’. Obviously, then, the entropy concept is linked with energy but, in simple

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terms, it defines the direction of progress and hence (perhaps?) is a predictor of the end product; it expresses the amount of wasted energy in a dynamical energy transformation from one state to another. Thus, as seductive as entropy may appear, it may not be as clear cut and useful as first viewing may suggest — hence, at this stage, we suggest proceeding with the (already existent) idea of ‘energy transfer’. A good example of weathering as a result of the interaction of strain energy with fracture surface energy due to chemical reactions is the work of Fletcher et al. (2006). This work clearly illustrates the sort of weathering conundrum (in terms of weathering ‘processes’) we have discussed in that they (Fletcher et al., 2006, p. 455) “present a model for spheroidal weathering that explicitly couples the physical processes of fracturing that leads to disaggregation ….to the chemical processes during water-rock interaction” (our italics). Once unpacked the questions are (1) what has this to do with climate (‘weathering’) as spheroidal weathering is found in many climatic regimes, and (2) it is a physico-chemical reaction rather than a process that fits neatly into our “chemical weathering” or “mechanical weathering” boxes? As Fletcher et al. (2006, p. 444) state: “Formation of each fracture is postulated to occur when the strain energy in a layer equals the fracture surface energy. The rate of spheroidal weathering is thus a function of the concentration of reactants, the reaction rate, the rate of transport, and the mechanical properties of the rock”. Replacing the term ‘weathering’ with ‘decay’ and evaluating it within the framework of Rock decay ¼ f energy transfer; rock properties facilitates a bottom-up approach that does not attempt to define process per se but inherently requires a study of all energy transfer components and their interaction with the rock properties for that given situation. Accepting the notion of ‘energy transfer’ as underpinning what is the cause of rock decay, we can thus suggest that rock decay is a synergistic function of energy transfer and rock properties. Given such an approach, so the resulting landforms (the macro-expression of the energy transfer) become a synergistic function of energy transfer, rock properties and product removal (erosion? — which is also the necessary precursor to any subsequent deposition and its associated landforms): Landforms ¼ f energy transfer; rock properties; product removal: Such a formulation side steps the role of weathering in aggradational landforms. However, issues connected with identification and naming of process(es) per se are negated and the whole becomes a holistic approach in terms of measuring attributes (thermal, moisture, etc.) and how they facilitate decay. Further, erosion will influence the kinetics of energy transfer when considered over longer time frames insofar as erosion removes products thus facilitating further (faster?) energy transfer. Conversely, in the absence of product removal so, ultimately, rock decay will come to a halt due to the inability for any further energy transfer to occur, so erosion must influence the energy transfer potential and thus the landform is a macro-expression of that energy transfer. Thus, ultimately, any attribute of currently known weathering processes can be explained in terms of energy transfer — the results being decomposition and/or disaggregation. All mechanical processes, just as in chemical processes, can be viewed as relating to energy transfer — the gain or loss of heat in thermal stresses, the loss of latent heat required to turn water to ice, the strain energy release in dilation, and so on. It is not just the gain or loss of energy but also the rate at which this may occur that can affect/determine the process (e.g. thermal shock requires a very high rate of energy transfer while thermal fatigue does not). Any arguments that such notions are inherent to the term ‘weathering’ are spurious. Furthermore, continued use of the term ‘weathering’ seems to offer little beyond ever

greater divergence between terminology and the reality of contemporary research. The failure of older, untested assumptions regarding process is sharply illustrated by an inability to address or grasp the inter-relationships inherent within physico-chemical breakdown which really finds no place within our present framework. Any suggestion that the invocation of synergistic ‘weathering’ boxes (acting in parallel or in series) is adequate will only further delay a longoverdue modernization of terminology that is presently characterizing the state-of-research poorly — indeed stultifying it to some degree. Many traditionalists will doubtless view our argument as iconoclasm for its own sake. However, we would argue that both laboratory experiments and field studies in ‘weathering’ have simply outgrown the existing terminology. Furthermore, they will advance much more rapidly and incisively with a contemporary, rather than archaic, vocabulary because it is surely our instinctive use and response to our own technical jargon on a daily basis that represents the bedrock of inter-, and more especially intra-, disciplinary communication. 9. Conclusions Since its inception and development in a discipline dominated by fieldwork and eye-ball science, weathering has been a central concept within geomorphology, although its essential role as a component of the rock cycle remains under-examined. During its development weathering has been enriched by both a broadening and deepening of our understanding of the topic. This has frequently been via importation of ideas from more mature disciplines such as chemistry, as well as the burgeoning growth of essentially derivative disciplines such as geochemistry, biochemistry, and geobiology. Today, increasing knowledge of, and interest in, fields such as the study of building materials and building conservation are also important sources of information and stimulation. The net result is a massive reduction in the scale of research, as well as a huge increase in information. We would argue that timely progress requires more appropriate terminology to better reflect and steer contemporary research and serve as a stronger foundation for further insights. Indeed, while it is nearly always reasonable for scientists to invoke the need for more data it is much more difficult to create scientific excitement over the need to reconsider accepted disciplinary organization. Nevertheless, the conceptual or intellectual framework within which the scientists of a particular discipline scrutinize the available information may serve to stimulate or stultify disciplinary progress. We suggest that weathering studies are presently facing a situation where old ideas developed without the benefit of modern research findings, and the sheer scope of those findings, puts weathering studies in a position where the benefits of reconsideration afford the potential for considerable gain. Reconsideration of the organization of the data presently at hand certainly does not guarantee progress; however, it offers the prospect of at least as much, probably more, progress than continuing to generate unquestioningly more of the same kind of information. The ever-greater precision and nicety with which we generate weathering data will perhaps slow in the future, but probably only slowly. It is limited really only by instrumental constraints and the money supply. However, it seems improbable that 21st century data are truly accurately depicted by 19th century concepts and organization. While laboratory research requires isolation to identify individual processes this task has been an elusive one for researchers of natural weathering processes. As knowledge grows at ever-finer scales, true isolation of any individual mechanism or process seems to always be illusory. For those who wish their laboratory experiments to mimic, rather than artificially investigate, real world situations the task is equally frustrating because it is impossible to understand fully the synergies present in the real world. This means there is a huge disconnect between laboratory research and natural

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weathering. Rather than just seeing presently-accepted processes as acting sequentially and/or ‘in parallel’ and seeking to pile up ever larger mountains of data, it would seem profitable also to question the established boundaries among processes. One clear-cut illustration of scientific organization steering our thinking is the influence the term ‘weathering’ has had on the standing of rock type in weathering studies. It is not that there is not an abundance of research on the weathering of individual rock types, but rather this primary input is viewed as secondary by the very implications of the word weathering. Second, there is the subtle but powerful implication that the weathering of any rock type will vary in kind from climate to climate. From first principles, it seems much more likely that the weathering of a particular material will be constant in kind, given a fixed constitution, but variable in rate, given variability in the driving force (climate). If, which seems likely to be a very big if, there is variability in kind from climate to climate it is just as likely to stem from variable synergies as from the introduction of fundamentally new processes. This would seem to be yet another reason for reconsidering process boundaries. While we consider the case we present to merit serious consideration we also believe weathering studies to be in need of another element not considered here. Weathering studies urgently need to move up the investigative scale as well as down. The present link between contemporary weathering studies and landforms per se is painfully underdeveloped. We do no more than note that shortcoming, but it merits considerably more attention than we can give it herein. In summary, we believe that weathering studies have reached a point where we need better questions rather than better answers. The issues that confront weathering researchers are centered upon the behavior of rock materials and much less upon climate variability. The former dictate what will happen, the latter, at most, the rate at which it will happen. Can we persist with ‘weathering’ within its current framework? In all likelihood no, given the inadequacies entrenched within the term and the current explosion in techniques and data availability, we need a term that reflects the reality of what is happening more accurately. Our choice would be ‘rock decay’ evaluated with the notion of energy transfer as the basis for considering process. Acknowledgments Over time many have influenced our thinking on the topic of ‘weathering’ and much of what is here, but we would like to highlight the following as exerting (positive) influences: Josolito Arocena, John Dixon, Ron Dorn, Jean-Pierre Lautridou, Jim McGreevy, Ian Meiklejohn, Bernie Smith, David Walton, and Brian Whalley. The muddled thinking, though, remains entirely ours. PDS acknowledges funding from the National Research Foundation but any opinions expressed are those of the authors and therefore the NRF does not accept any liability in regard thereto. We would also like to thank the two referees and the Editor who helped with better presentation of our ideas and also in making us better focus our own thoughts. References André, M.-F., 2002. Rates of postglacial rock weathering of granite roches moutonnées in northern Scandinavia (Abisko-Riksgrånsen area, 68°N). Geografiska Annaler 64A, 139–150. André, M.-F., Hall, K., Bertran, P., Arocena, J., 2008. Stone runs in the Falklands: periglacial or tropical? Geomorphology 95, 524–543. Balke, J., Haendel, D., Krüger, W., 1991. Contribution to the weathering-controlled removal of chemical elements from the active debris layer of the Schirmacher Oasis, East Antarctica. Zeitschrift für Geologische Wissenschaften 19, 153–158. Ballantyne, C.K., 2010. A general model of autochthonous blockfield evolution. Permafrost and Periglacial Processes 21, 289–300. Banfield, J.F., Barker, W.W., Welch, S.A., Taunton, A., 1999. Biological impact on mineral dissolution: application of the lichen model to understanding mineral weathering

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