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Geomorphology 67 (

Sandstone weathering: a century of research and innovation

Alice V. Turkingtona,*, Thomas R. Paradiseb

aDepartment of Geography, University of Kentucky, Lexington, KY 40506, USAbDepartment of Geosciences and the King Fahd Center for Middle East and Islamic Studies, University of Arkansas,

Fayetteville, AR 72701, USA

Received 11 November 2003; received in revised form 2 July 2004; accepted 27 September 2004

Available online 9 December 2004

Abstract

A review of sandstone weathering research, particularly in the past 100 years, reveals a trajectory of enquiry from early

description and classification of features, to development of process-based explanations, to decreasing scales of investigation,

and a disparity between understanding of process(es) and explanations of the genesis of sandstone weathering features.

Developments in expositions on mesoscale weathering features on sandstone surfaces are discussed, demonstrating a range of

approaches to weathering phenomena—field-based and laboratory-based—that must be linked to provide an explanation of

observed features on a landform scale. Throughout the twentieth century, a thematic chronology highlights certain trends in

research: description of forms, often in arid and semi-arid environments; single process–form models; an emphasis on

experimentation; difficulties in measuring weathering rates; and a persistent emphasis on physical causes of breakdown. A new

research agenda is promoted in which biodeterioration and chemical processes gain parity, a holistic approach based on

conceptual modeling of weathering systems gains prominence, and scale issues are addressed more rigorously.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sandstone; Weathering; Weathering geomorphology

1. Introduction

Dramatically displayed in the Grand Canyon and

the brose-redb cliffs of Petra, sandstone represents a

distinctive and often spectacular component of land-

0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2004.09.028

* Corresponding author. Tel.: +1 859 257 9682; fax: +1 859 323

1969.

E-mail addresses: alicet@uky.edu (A.V. Turkington)8

paradise@uark.edu (T.R. Paradise).

scapes. Used in historic structures, such as the ancient

temples of the Nile or Boston’s Trinity Church,

sandstone has also proved to be a useful and beautiful

building stone, combining ease of crafting with

aesthetic appeal. While rock weathering and stone

decay research has tended to be focused on granite

and limestone, sandstone remains a relatively over-

looked landscape element, building stone, and study

focus. Sandstone occupies a similar proportion of the

Earth’s surface as granite or limestone (Meybeck,

1987) and is an important architectural stone in many

2005) 229–253

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253230

parts of the world. The disparity between the spatial

significance of sandstone and the insignificant

research conducted on the weathering of this rock

type was recognized by Young and Young (1992),

who suggested it may be due to the widespread belief

that weathering of sandstone is a relatively straight-

forward case of physical breakdown controlled by

simple lithological and structural conditions.

On sandstone landscapes, tafoni are the most

common small-scale weathering forms on near-verti-

cal bare rock surfaces, with gently dipping surfaces

displaying a variety of pits, hollows, and boxwork

patterns of indurated joints. At a larger scale, weath-

ering forms on massive sandstones include cliffs,

arches, columns, towers, and domes (Koons, 1955;

Bradley, 1963; Robinson, 1970). Studies of larger

scale landforms have principally invoked observations

of morphology and structure, commonly relying on

knowledge of geological history and rock response to

stress and strain. More recent work in this area has

built upon an understanding of rock mechanics (Yatsu,

1966; Cruickshank and Aydin, 1994) and increasingly

made use of methods of assessing rock strength,

porosity–permeability, cementation, and fracture

intensity (Nicholas and Dixon, 1986). In this paper

we focus on small-scale sandstone features, which are

produced by physical, chemical, and biological

processes operating on a granular scale rather than

larger landscape elements, which are often created in

part by fluvial and hillslope processes.

Some of the earliest Western references to sand-

stone and its particular characteristics may be found in

the classical writings of Herodotus (c. 450 BCE),

Aristotle (c. 340 BCE), Strabo (c. CE 25), and Pliny

(c. CE 50), where the unusual bpittedb and borganicbshapes and weathering features were attributed to

influences by the divine, running water or rainfall,

snow or freezing, rock integrity (crack, fractures),

plant attachment, or human modification. However,

these early observations are relatively rare. Early

English language observations on sandstone weath-

ering were made by Stephens (1837) and Burton

(1879), whose extensive travels throughout Central

America, Europe, the Mediterranean, East Africa, and

the Levant were well-documented and publicized

throughout the West’s Victorian Period. Stephens

and Burton described sandstone cliff faces and

pinnacles as bpocked with intricate niches derived

from running water and windb and features in the

desert (i.e., yardangs) as blooking like indurated

caravans of packed camelsb and btowering rock

sentinels of timeb. Some of the first references to

differential weathering influenced by rock composi-

tion were made by Dana (1849), Darwin (1859), and

Hall (1882), who stated that similar rocks displayed

varied berosional workingsb which may be attributed

to bdifferent internal ingredientsb. In these writings,

we find some of the first and most accurate expla-

nations for rock weathering (including sandstone)

affected by uniform bioclimatic influences.

Weathering studies have always played an integral

role in geomorphological research, as rock weathering

may be considered to be the starting point of many

dynamic systems and is also a dominant force in

shaping many landscapes. Before the shift toward

reductionism and quantification in physical geography

around 40 years ago, much investigation into sand-

stone weathering focused on the response of land-

forms and landscapes. More recent research into

sandstone weathering has followed the general shift

within geomorphology toward process studies. Tradi-

tional structural theory (which held that regional

geology dictates landform development) and climatic

theory (which held that mesoscale climatic variability

is the major control on geographical variability in

weathering) are acknowledged to be oversimplifica-

tions. Weathering is recognized to be the result of the

operation of a wide range of processes, which may

operate sequentially or simultaneously, and emphasis

is placed on intensive study of relationships between

process(es), weathering form(s), rock properties, and

environmental conditions. Traditional methodologies

of field observations, measurement, and character-

ization of weathering features and products have been

augmented by increasingly sophisticated assessment

techniques, which allow investigation of morphology;

rock structural, chemical and mineralogical properties;

and environmental conditions at smaller, more

detailed, scales.

2. Weathering research in the nineteenth century

The establishment of the Ordnance Survey in

Britain in 1795 and the U.S. Geological Survey in

1801 heralded an era of surveying and exploration

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 231

that provided a wealth of data that would prove

invaluable to the development of many important

concepts in physical geography. During the nineteenth

century, notable landscape features were observed,

documented, and measured, with an emphasis on

description of new or unusual features. A fascination

with unusual recessional features on rock surfaces,

such as tafoni, became widespread. Conventionally

accepted as small alveolar or cavernous weathering

features often found in granite, sandstone and grey-

wacke, btafonib was originally the site-specific term

for the occurrence of dramatic weathered pockets on

the island of Corsica (Bourcart, 1930). These features

that were initially called bhoneycombb by Darwin

(1839), bpocketsb by Dana (1849), later termed

bstonelaceb by Bryan (1928) and bnichesb by Black-

welder (1929), are now widely termed tafoni. Orig-

inally, Charles Darwin (1839) misattributed the

occurrence of tafoni to the effects of tree root

penetration, while James Dana (1849) credited its

occurrence to the intersticial movement of water

redepositing hardening agents, a factor often cited as

important today (e.g., Conca and Rossman, 1982).

Not all field-based research during this imperial

period was concerned with mere data collection and

description, with researchers such as Hutton (1795),

Lyell (1830), and Gilbert (1877) setting the stage for

the later development of process studies in geo-

morphology. In rock weathering research, attempts to

elucidate the operation of weathering processes and to

quantify their effects were introduced early in the

nineteenth century. Brard (1828) recognized that salt

damage and frost damage to stone are similar in

nature, and he proposed a salt crystallization test as a

method of assessing the frost resistance of stone. This

test involved boiling 5-cm cubes of stone in saturated

sodium sulphate solution, drying the samples at room

temperature until efflorescence appeared, followed by

re-immersion in the salt solution. The cycles of

treatment were continued for 5 days, when detached

fragments were weighed. This experiment forms the

foundation for salt crystallization tests conducted

today, in that salt solutions are added to stone

samples, which experience alternate wetting and

drying cycles. Procedural differences have evolved

in more recent tests, however, with regard to such

parameters as specimen size, soaking and drying time,

and concentration of salt solutions. The salt crystal-

lization test used in Britain, which was developed by

the Building Research Establishment, based on

modifications introduced by Schaffer (1932), is used

for testing the resistance to salt damage of limestones

in particular, but also sandstones and other permeable

rock (Ross and Butlin, 1989). This test is also the

progenitor of later weathering simulation experiments

conducted by geomorphologists (Evans, 1970).

3. Weathering research in the twentieth century

In the twentieth century, we begin to see con-

ceptual development in sandstone weathering studies

(Fig. 1). Bryan (1922, 1928), Brown (1924) and

Blackwelder (1929, 1933) discussed many of the

processes responsible for weathering. These works

were among the first to address the possible processes

responsible for sandstone weathering and not just the

simple description of recessional features (i.e., tafoni,

stonelace, alveolar weathering). Controversy arose

during this period when Bryan (1922, 1928) found the

effects of water through sapping to be a significant

sandstone weathering influence in arid regions. This

was contrary to the conventional theories of aeolian

abrasion espoused by Blackwelder (1929) and com-

mon during that period. Bryan’s research was revolu-

tionary in that it proposed the chemical dissolution of

matrix cement as the predominant agent in the

disaggregation of sandstone.

Sandstone weathering studies remained focused

on physical breakdown, however, with a particular

interest in the role of insolation-induced temperature

changes on rock surfaces. Although Dana (1849)

was among the first to postulate that sunlight can

heat and crack rock, others had promoted the

significance of insolation-induced weathering in

sandstone until Blackwelder (1933) challenged this

theory and, on the basis of laboratory experimenta-

tion, concluded that the role of insolation in weath-

ering was uncertain. Schaffer’s (1932) laboratory

work found gentle heating to be important in

intensifying particle disintegration by increasing the

frequency of wetting and drying. Griggs (1936)

found temperature fluctuation to be insignificant

until the introduction of moisture, which drastically

accelerated the rate of weathering. Since Griggs’

landmark work, research on insolation weathering

Fig.1.Bibliographical

sketch.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253232

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 233

has been roughly divided into insolation effects

versus moisture availability. Sandstone may show

significant moisture expansion because of active

porosities of up to 35% by volume and potentially

high saturation coefficients; thermal expansion may

also be significant in causing sandstone decay as

quartz, for example, has a coefficient of volumetric

expansion of 0.36 between 20 and 100 8C.The debate over the efficacy of insolation weath-

ering, which has continued for many decades,

exemplifies the changing emphasis within weathering

studies, and the advancement in collection of data and

information. Laboratory simulations by Blackwelder

(1933) and Griggs (1936) failed to produce appreci-

able rock breakdown when heating and cooling cycles

were conducted on dry specimens. Ollier (1963)

contended that cracked and split boulders in central

Australia indicated the effectiveness of differential

thermal expansion in causing rock breakdown. Fol-

lowing a long-established tradition of environmental

monitoring in other branches of geomorphology,

researchers examined temperature changes experi-

enced by rock surfaces in more detail. Roth (1965)

monitored rock surface and subsurface temperatures

over 24-h periods and moisture content of the same

rock, having split the rock using dynamite; similar

monitoring of diurnal temperature ranges on and

beneath rock surfaces was conducted by Peel

(1974). Rogner (1987) extrapolated temperature

recordings to plot isotherms across tafoni overhangs.

Jenkins and Smith (1990) included data on short-term

fluctuations in rock surface temperature; Hall (1997)

discussed the implications of overlooking surface

temperature changes at intervals as small as 2 min.

Rapid fluxes in temperature because of cloud cover

and shading produce complex thermal regimes

affected by insolation receipt, latitude, altitude, aspect,

and seasonality; averaged climatic data is clearly an

inadequate indication of rock surface and subsurface

temperature cycling.

Recently, the question of a relationship between

insolation and sandstone deterioration has been

revisited. The conventional premise in much of the

twentieth century was that there was no clear-cut

causation between solar radiation-induced temper-

ature fluctuations and weathering (e.g., Fahey and

Dagresse, 1984; Jenkins and Smith, 1990), although

most researchers admitted to some connection (Ollier,

1984). Kerr et al. (1984) and McGreevy (1985)

demonstrated that insolation may work through the

separation of the clast particle from its matrix. The

volumetric change of quartz on thermal expansion and

contraction could also develop microcracks in the

matrix (e.g., Jenkins and Smith, 1990). Numerous

studies found, however, that moisture availability is

inextricably bound up with weathering at the surface,

which is partly influenced by the aspect of the rock

surface. Roberts (1968) attributed the formation of

immense gnammas on the southern slopes of Nor-

way’s Narwick Mountains to more frequent freeze/

thaw cycles that were intensified by the availability of

daytime meltwater. Sancho and Benito (1990) found

sandstone gnamma dimension and frequency

increases toward southwestern aspects, a surrogate

for higher insolation-induced afternoon warming.

Robinson and Williams (1999) observed higher

weathering rates on western than eastern sides of

gravestones and suggested late afternoon sunshine

may be more efficient in drying stone than early

morning insolation. Williams and Robinson (2000),

however, revealed a pattern of east-side weathering of

gravestones in England, indicating the interdepend-

ence between environmental and structural controls

on weathering intensity. Paradise (2002) found that

eastern and western slopes in Jordan displayed larger

weathering features than southern slopes because of

the greater moisture retention of these aspects.

Surface temperatures and thermal gradients created

by insolation depend on the thermal characteristics of

the rock (McGreevy, 1985), and may promote micro-

fracturing of surface grains (Yatsu, 1988), but they are

unlikely to be the sole cause of sudden and dramatic

rock breakdown (Warke and Smith, 1994). Thermal

cycling may, however, have an appreciable effect on

the operation of other weathering mechanisms, such

as salt crystallisation or hydration, oxidation or

solution. The concern of recent research is the control

exerted by temperature and moisture regimes on a

multiplicity of weathering processes.

3.1. The single process model

With advances in transportation during the early

part of the twentieth century, new research followed

into the less accessible arid regions that still focused

on aspects of sandstone deterioration often emphasiz-

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253234

ing tafoni development (Fig. 2). The works of David

and Priestley (1909), Cotton (1922), Bryan (1928),

Blackwelder (1929), Schaffer (1932), Hume (1935),

and Bartrum (1936) established qualitative observa-

tions on sandstone weathering with case study foci on

tafoni development. The work of Hume (1935) on

Egypt’s Nubian Sandstone and pharonic architecture

led to broad speculation on tafoni initiation. Tafoni

and alveolar weathering typically develop in sand-

stone, so it follows that much of this early sandstone

weathering research focussed on these characteristic

recessional features.

During the late 1930s, French and German

researchers believed that tafoni were the product of

aeolian-borne particle abrasion or insolation-induced

expansion in particle-comprised rocks like granite

and sandstone. Popoff and Kvelberg (1938) pro-

moted aeolian corrasion and insolation as agents of

tafoni development. Since this early work, numerous

studies of tafoni formation have collected informa-

tion on morphology, weathering products, and

environmental conditions to infer a causative, dom-

inant process.

Investigations into the occurrence of tafoni have

described their morphology (e.g., Smith, 1978;

Kejonen et al., 1988), micromorphology (e.g., Mus-

toe, 1983; Mellor et al., 1997), salt content (e.g.,

Mottershead and Pye, 1994), cavern microclimate

(e.g., Dragovich, 1981; Turkington et al., 2002), and

rock properties (e.g., Benito et al., 1993). Tafoni and

Fig. 2. Sandstone stonelace (tafoni) in Wupat

alveoli have been commonly attributed to salt weath-

ering (Wellman and Wilson, 1965; Evans, 1970;

Bradley et al., 1978; Mustoe, 1982; Smith and

McAlister, 1986; Young, 1986; Matsukura and Mat-

suoka, 1991), but may be produced by different

processes, or combinations of processes, in which salt

weathering may not be dominant (Martini, 1978).

Even if salt weathering can be proved to be the cause

of rock disintegration, how cavernous forms develop

through this process is still unclear.

Data on salt concentrations and environmental

parameters would suggest that salt crystallization

and hydration may be important disruptive processes,

but Young (1987) and Young and Young (1992)

argued that tafoni are produced by solutional pro-

cesses, which may be enhanced in the presence of

saline solutions. Conca and Rossman (1982) sug-

gested that case hardening of rock surfaces affected

the moisture flux within rock, and thus weathering

rates in tafoni exceed those of the exterior surfaces.

Researchers have also addressed weathering and

tafoni development as a function of bed lamellae

orientation (Martini, 1978; Sancho and Benito, 1990),

the dissolution of matrix cementing agents (Mustoe,

1982; Conca and Rossman, 1982), and the dissolution

of particle boundaries (Young, 1987). Calkin and

Cailleux (1962), Dragovich (1969), Twidale (1983),

Alexandrowicz (1989), and Sancho and Benito (1990)

investigated the hierarchy and association of wetting/

drying cycles, dissolution from hydrolysis, salt

ki National Monument, Arizona, USA.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 235

wedging, bed orientation and sapping, and intersticial

water and chemical mobility.

A conceptual requirement for field-based weath-

ering studies using morphometric techniques that aim

to elucidate process is a bone-to-oneb relationship

between process and form (Thorn, 1988). This is

rarely the case; a weathering form may be produced

on a sandstone surface by a range of processes.

Spatial co-variation of aspects of weathering forms

(e.g., material loss) with process variables (e.g., salt

content) does not necessarily reveal formative

mechanisms (Lane and Richards, 1997). This is the

conundrum of equifinality, which denotes the possi-

bility of similar landforms being derived from

different initial conditions in different ways by

possibly different processes (Haines-Young and

Petch, 1986). Studies which have endeavored to find

causal explanations for weathering forms such as

tafoni and alveoli have demonstrated that it may not

be possible to distinguish between several theories

for the formation of particular forms. Within sand-

stone weathering research, the problem of equifin-

ality remains, because of the complexity of

interactions between a suite of weathering and decay

processes; particular forms are not merely dependent

on the nature of causative processes, but also their

sequence and the specific bconfigurationalb condi-

tions (Lane and Richards, 1997) that affect their

action. The concept of equifinality expresses, in

short-hand form, the impossibility of distinguishing

between many possible histories of different initial

conditions and different possible process mecha-

nisms on the basis of available evidence (Beven,

1996).

Examination of microscale features of rocks dis-

playing cavernous weathering forms often reveals

weathering forms of both a physical and chemical

nature (Turkington, 1998). For examples, disaggrega-

tion of grains and pore-infilling by salts may coincide

with etching of quartz grains and feldspars (e.g.,

McGreevy and Smith, 1984; Butler and Mount,

1986). Salt weathering is not invariably the cause of

tafoni formation, but salts are often present in tafoni

backwalls and flakes and cause disintegration both

through physical and chemical processes. Problems in

interpretation of single causative weathering processes

have arisen due to significant gaps in the under-

standing of processes that may produce microscale

features, or suites of features, and of the relationships

between features on micro- and mesoscales. These are

compounded by the complexity of natural weathering

systems, where feedbacks may exist between process,

form, rock properties, and environment; the intricacy

of interactions between these variables cannot be

easily elucidated through field-based observations.

The spatial distribution of sandstone weathering forms

has received less attention from weathering research-

ers than the rate at which stone weathering proceeds.

This may, in part, be due to the perception that

weathering processes are time-dependent not distance-

dependent processes (De Boer, 1992). An under-

standing of the reasons for spatial variability is,

however, pivotal to weathering studies. On a relatively

homogeneous surface, such as a subaerial sandstone

cliff face, the occurrence of specific weathering forms

provides an inconstant pattern. The distribution of

tafoni on sandstone outcrops was demonstrated to

represent a dynamically unstable weathering system,

which suggests fragmentation of any sandstone sur-

face by tafoni (given appropriate boundary condi-

tions) regardless of the dominant causative process

(Turkington and Phillips, 2004), as illustrated sche-

matically in Fig. 3.

3.2. Reductionism and weathering studies

Recently, reductionist studies have gained prom-

inence in sandstone weathering research, where the

time- and space-scales of inquiry have been collapsed

in the endeavor to fully understand weathering

processes and controls on stone response. Since the

1960s, geomorphology became dominantly process-

oriented, having progressed from an historical

approach; weathering research was no exception.

Empirical measurements of weathering forms were

supplemented by experimental investigations, which

focused primarily on physical processes, specifically

salt and frost attack. The preponderance of simulation

studies within rock weathering studies is partially due

to the difficulties of sampling rock in inhospitable

environments or from stone in the architectural

environment. Apart from the logistic advantages of

laboratory tests, they have often taken precedence

over field studies because the level of environmental

monitoring required complicates interpretation of

outdoor studies, so to separate the effects of various

Fig. 3. Sandstone weathering model diagram: morphology and chronology.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253236

mechanisms of deterioration, controlled conditions

must be available.

Evans’ (1970) work addressed the many aspects,

causes, and forms due to salt weathering in various

rocks including sandstone, and this body of work set

the stage for a series of experiments designed to

elucidate mechanisms of salt attack and controls on

stone response. The most cited cause of salt weath-

ering is salt crystal growth (Goudie and Viles, 1997).

Experimental studies have suggested that, of the three

mechanical mechanisms of salt weathering, crystal

growth is the most significant in causing rock break-

down (Goudie, 1974; Cooke, 1979). Crystal growth is

promoted by a decrease in solubility as temperature

falls and/or evaporation occurs, or by mixing of

certain salts in solution, termed the dcommon ion

effectT (Goudie, 1989). To cause rock disruption,

crystals must exert pressure on pore walls. In other

words, salt crystallization must produce disruptive

forces that exceed the cohesive forces of the rock

(Dibb et al., 1983). Lewin (1990) suggested that

spontaneous precipitation of crystals from a super-

saturated solution supplies free energy, which allows

crystals to grow against confining pressure suffi-

ciently to cause mechanical failure. Correns (1949)

originally suggested that continued growth of a crystal

against confining pressure can occur when there is a

film of solution at the salt/rock interface. Mortensen

(1933) first recognized the significance of salt

hydration as a weathering mechanism; hydration

cycling can cause internal cracking of stone samples

(Cooke and Gibbs, 1995). While simulation studies

have not demonstrated differential thermal expansion

of salts to be an effective weathering mechanism, one

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 237

empirical study of coastal cliff weathering concluded

that thermal expansion of halite accounted for a rate of

weathering on sunny south-facing cliffs 10 times that

of weathering of shaded north-facing cliffs (Johan-

nessen et al., 1982).

Early simulation experiments imposed artificial

conditions with little facility for controlling relative

humidity; more recently, the use of environmental

cabinets has facilitated close control of temperature

and humidity. Simulation experiments give consid-

eration to the relationship between environmental and

laboratory conditions, but cannot replicate exposure

conditions (McGreevy and Smith, 1982). Rock sur-

face temperatures have been measured in various

locations (e.g., Kerr et al., 1984; Jenkins and Smith,

1990), but detailed information on microclimatic

conditions at the rock–atmosphere interface, or within

rock substrate, is sparse. More recent experimental

designs have made some progress toward simulating

salt-rich environments with a greater degree of fidel-

ity, through the application of salt solutions of low

concentrations (e.g., Goudie, 1993; Warke and Smith,

1994), or through the application of salt mixtures

(e.g., Jerwood et al., 1990a,b). Combinations of salts

have not been widely used (Robinson and Williams,

2000; Williams and Robinson, 2001); further inves-

tigation is required to elucidate the combined effects

of salts in causing damage. Steiger and Zeunert (1996)

suggested that it is virtually impossible to predict the

behavior of a salt mixture from the properties of the

pure components; however, chemical models of

behavior of salt mixtures over ranges of temperatures

and humidities may extend our understanding of the

properties of salt systems in sandstone (e.g., Price,

2000). The patterns of breakdown achieved in

simulations are disparate from those observed in field

studies; normally granular disintegration is the only

mode of breakdown, with assessment of weathering

based predominantly on weight loss. Field surveys in

salt-rich environments have illustrated the importance

of flaking, scaling, and crust development (e.g.,

McGreevy et al., 1983; Fitzner and Heinrichs, 1991;

Samson-Gombert, 1991).

Geomorphological experimental investigations

have been conducted in parallel with testing of stone

to predict durability in urban environments (Turking-

ton, 1996). Durability tests do not closely simulate the

natural conditions and processes to which stone may

be exposed, exemplified by test methods to predict

frost damage (e.g., Ross, 1984; Van Gemert and Ulrix,

1987). In the U.K., artificial frost testing was

discontinued because of the bdifficulty of developing

an inexpensive test that would accurately reproduce

the observed behavior of stones under natural

conditionsb (Price, 1977; Building Research Estab-

lishment, 1989). This summarizes the quandary faced

by all researchers working on artificial weathering

tests or simulation experiments; all tests aim to

compromise between the representation of actual

conditions of exposure in a laboratory, rapid produc-

tion of information on the weathering resistance of

stone samples, and easily reproducible conditions.

Given the importance placed on frost testing by

many building researchers and the evidence of frost

damage to sandstone outcrops, buildings, and monu-

ments, there has been little investigation of the

combined action of frost and salt weathering pro-

cesses. Laboratory studies (Goudie, 1974; Williams

and Robinson, 1981) have indicated that rocks

disintegrate more rapidly when soaking in salt

solution precedes freezing. McGreevy (1982) found,

however, that salts inhibited frost action, particularly

if the salt supply is limited. Several studies have

indicated that greatest deterioration is accomplished

by solutions which contain 2–6% salt by weight

(Trenhaile, 1987). Jerwood et al. (1990a) found that

sodium sulphate enhanced breakdown of stone speci-

mens; sodium chloride increased breakdown under

intense freezing conditions; magnesium sulphate

inhibited the breakdown of cubes subjected to intense

freezing.

Given the state of knowledge regarding weathering

mechanisms, and the structural and environmental

factors that control their action, investigating empiri-

cally the role of environmental regimes and rock

properties on sandstone breakdown would seem

appropriate. Without a more complete understanding

of process, fully explaining the effect of these

parameters is not possible. As more specific combi-

nations of conditions have been examined, more

detailed information on the weathering behavior of

sandstone has become available. These studies have

many limitations regarding their applicability to real

exposure conditions; artificial weathering experiments

have, nonetheless, proved instructive. Further inves-

tigation of individual factors controlling rock break-

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253238

down by salt attack has recently been suggested to be

imperative (Rodriguez-Navarro and Doehne, 1998):

the environmental conditions under which crystalli-

zation takes place, the characteristics of porous stones,

the crystallization characteristics and growth patterns

of salts, and the distribution of salts within porous

stones, a function of solution flow and evaporation.

Weathering research seems set to continue to be

increasingly reductionist in the sense of integrating

process at more detailed scales, particularly making

use of microscope technology to examine process at

micro-, or nano-, scales (Viles and Moses, 1998) and

most recently, spectroscope technology (Edwards et

al., 2002; Friolo et al., 2003).

3.3. Urban stone decay

Rock weathering research has become integrated

with applied research into urban stone decay, an

interdisciplinary body of research concerned with the

performance of stone as a building material and

strategies for its conservation. The cultural and

financial implications of decay of stone buildings

and monuments, with an increasing awareness of the

hazards of atmospheric pollution and stone decay, has

engendered a political imperative for effective con-

servation of stone structures (Cooke and Gibbs,

1995), to which rock weathering research can offer

a significant contribution (Pope et al., 2002). From the

geomorphologist’s perspective, architectural stone

provides an accessible analogue for rock breakdown

in dry salt-rich environments (Winkler and Wilhelm,

1970), but offers a suite of decay processes particular

to historically high concentrations of air pollutants

during exposure in urban areas.

Studies of stone response to pollution attack under

laboratory conditions (e.g., Sabbioni et al., 1993) and

exposure conditions (e.g., Jaynes and Cooke, 1987)

have focused attention on carbonate stones; fewer

researchers have investigated sandstone deterioration

in simulated atmospheres, notably Vale and Martin

(1986) and Kirkitsos and Sikiotis (1995). This

contrasts with the popularity of sandstone samples

in simulations of salt weathering, and the case studies

reporting on decay of sandstone buildings and monu-

ments. Studies of sandstone response to pollution

include examples of black crusts on buildings (e.g.,

Nord and Tronner, 1995; Thomachot and Jeanette,

2002), deterioration of exposed sandstone tablets

(e.g., Halsey et al., 1995; Turkington et al., 2003),

and salt-induced material loss from building facades

(e.g., Turkington and Smith, 2000; Warke and Smith,

2000; McKinley et al., 2001). Smith et al. (1994)

stated that the mechanical breakdown of quartz

sandstones in polluted urban areas is overwhelmingly

associated with salt concentration, primarily gypsum.

The chemistry of crust formation appears to be

fairly well understood, but the physical phenomena

relating to crust decay is not (Lewry, 1988). Smith and

Magee (1990) suggested that an essential precursor to

contour scaling caused by pollution-derived gypsum

is near-surface microfracturing. Porous building

stones are inherently permeable, permitting moisture

penetration; pore spaces, capillaries, and microcracks

may be extended and exploited by salt crystallization

at or near the surface. Detachment of contour scales

has been attributed to the concentration of salts at a

particular depth (McGreevy, 1982), and the efficacy of

this process has been demonstrated in experimental

work (Smith and McGreevy, 1988). Nord and Tronner

(1995) found that gypsum had formed all over the

Gotland sandstone facades of the Royal Palace,

Stockholm, and that flaking and contour scaling were

the dominant decay forms. Processes of crust detach-

ment commonly involve the loss of a surface layer,

which has a weakened or altered layer of stone

immediately below, as some sort of stability threshold

is breached. Bluck and Porter (1991a,b) postulated a

similar process of detachment of iron-rich patinas on a

sandstone building; Puhringer et al. (1992) described

the formation of a crust due to surface cementation of

sandstone by amorphous silica, followed by formation

of contour scales. Thus, an outer crust will eventually

spall because of a zone of decementation and/or

mineral expansion behind it. A model proposed by

Smith (1996) suggested that when thresholds of

weathering resistance are breached and sandstone

surfaces experience rapid decay, two outcomes are

possible: the surface may re-stabilize or continue to

lose material, or decay patterns may be repeated many

times before thresholds of rapid decay are breached.

Spatial variability of gypsum crust occurrence on

sandstone structures provides an indication of the

importance of micro-environmental differences, due

to building morphology, and the nature of building

materials, in producing dissimilar decay forms.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 239

3.4. Physical myths and chemical realities

Weathering research displayed trends in the post-

World War II period similar to much of physical

geography (Gregory, 2000) when quantification and

reductionist approaches became predominant. This

period also ushered in new studies that investigated

chemical causes or influences in weathering. The

earlier emphasis on mechanical effects persisted in

tandem with a new focus on chemically induced

alteration. White (1944) investigated the influence of

case hardening on weathering rates in an arid climate,

while gnamma development in sandstone was studied

by Frye and Swineford (1947) whose findings

corroborated Kirk Bryan’s earlier works emphasizing

the importance of matrix dissolution in sandstone

disaggregation. Tallman (1949) then assembled the

descriptive statistics from different locales for the

sandstone cementing agents, particle types, shapes,

and sizes and helped standardize descriptions used in

arenaceous geology since these earlier days.

Schumm and Chorley (1966) addressed the prev-

alence of springtime sandstone spalling and rockfalls

in the American Southwest, attributing this occurrence

to the springtime’s greater diurnal temperature ranges

and increased moisture availability accelerating par-

ticle disintegration. Many studies followed that high-

lighted the significance of surface moisture retention

in accelerating weathering from matrix dissolution

(Alexandrowicz, 1989; Sancho and Benito, 1990) or

case hardening and core softening (Conca and Ross-

man, 1982; Robinson and Williams, 1989). Young

(1987) analyzed the ability of chlorides to disintegrate

quartz constituents in rocks by dissolving the particle,

and not the matrix as previously observed, in addition

to elaborating on the role of moisture in its stimulation

of salt crystallization and the resulting wedging and

particle disruption (Young and Young, 1992).

In spite of research on chemical attack on sand-

stones, a widespread belief that siliceous, or quartz-

rich, sandstone is chemically inert and all but carbona-

ceous sandstone is immune from solutional weathering

persists. However, karst landscapes are formed on

many highly quartzose rocks, with several accounts of

sandstone karst (Young and Young, 1992; Wray,

1997). Research into weathering of building sand-

stones has acknowledged the importance, even prev-

alence, of solutional attack on stone (e.g., Smith et al.,

2002; Mottershead et al., 2003), but few researchers

have tested this idea.

Research strategies prevalent in process studies

have been more gradually adopted in investigations

into ancient landscapes, in which rocks have existed

through much of geological time. Many Australian

sandstone landscapes have persisted since the Meso-

zoic era; weathering forms cannot be simply related to

contemporary weathering processes as marked cli-

matic changes throughout the Tertiary and Quaternary

periods would presumably have initiated variable

processes and process rates. Landform response to

current processes must also be controlled by inherited

weathering effects. Description and photographic

evidence of sandstone landforms in Arnhem Land

were presented by Jennings (1979), in central Aus-

tralia by Twidale (1980), in western Australia by

Young (1986), and in the Sydney Basin by Young and

Young (1988). Despite the high quartz content of

many Australian sandstones, landform assemblages

are suggested to have been sculpted by solutional

processes.

Young (1986) assessed morphological character-

istics of sandstone in the Bungle Bungles, from

mapping the arrangement of towers to examining

quartz overgrowth surface textures using scanning

electron microscopy; thin sectioning, and X-ray

diffraction complemented these techniques in inves-

tigating the role of solutional processes in weathering

of chemically resistant rocks. Young (1987) recog-

nized convergence of form, where landforms devel-

oped on siliceous rocks may, through prolonged

weathering, become akin to those on chemically

weaker rocks. Young (1988) presented quartz grain

surface textures indicative of slow, widespread solu-

tional processes, which may be critical in producing

friable sandstone, subsequently eroded to form tower

karst.

3.5. Sandstone landscapes

Difficulties in linking weathering processes and

forms have been highlighted by several studies of

sandstone terrains. Robinson and Williams (1987)

suggested that surface crusts are widespread on

sandstones; many weathering features, such as honey-

combing and polygonal cracking, are intimately

bound up with crust formation and destruction. The

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253240

two-stage model of crust formation and loss is a

recurring theme in research into sandstone weath-

ering, and despite its simplicity may form the basis of

a conceptual model of surficial sandstone weathering.

This model is by no means comprehensive, however.

Polygonal cracking, for example, has been suggested

to be the result of diagenesis, unloading, insolation

weathering, clay shrinkage, frost weathering, salt

weathering, lichen weathering, and surface crusting

(Williams and Robinson, 1989). Polygonal cracking

may have more than one origin, although surface

crusting, and cracking due to deposition of secondary

minerals and structural discontinuities between the

crust and the substrate, seems to provide the most

widely applicable explanation (Robinson and Wil-

liams, 1989). Other processes may be influential, or

dominant, depending on site-specific environmental

conditions or rock properties. A catalogue of sand-

stone weathering features in the High Atlas of

Morocco indicated a role for surface crusting in the

formation of many features (Robinson and Williams,

1992), but links between crust development and

features such as rillenkarren are tenuous at best. Many

sandstone terrains exhibit a suite of weathering

features that combine to produce a distinct and

characteristic morphology (Young and Young, 1992;

Robinson and Williams, 1994); however, the range of

origins postulated for individual features on various

rocks in various environments and the interplay of

physical, chemical, and biological and surface and

subsurface processes has precluded establishment of a

comprehensive model of sandstone weathering.

Sandstone landscapes evolve under three broad

sets of controls: lithologic; structural; and erosional.

The effects of these are manifest at varying scales.

Spectacular sandstone features, such as arches, col-

umns, mesas and pedestals have been attributed to

exploitation of localized intense fracturing by weath-

ering and erosion (e.g., Nicholas and Dixon, 1986;

Cruickshank and Aydin, 1994). It is evident that many

exposed sandstone slopes are controlled by joint

patterns on a large scale, but assumptions that rock

strength, porosity or cementation (i.e., lithology) do

not influence these features may be erroneous.

Lithology plays an important role in creating, for

example, bpulpit rocksb or pedestal rocks (Froede andAkridge, 2003) where differential weathering and

erosion of sandstone strata creates these relatively

stable features, often combined with case hardening of

the uppermost exposed sandstone layer. On exposed

sandstone slopes, material loss is often by granular

disintegration, solution and peeling of thin weathering

rinds. Young and Young (1992) described the removal

of weakly cemented individual grains from outcrops

in the Bungle Bungle Range; Howard et al. (1988)

observed weathering pits and runnels on exposed

sandstone slip rock surfaces. Thus, exposed sandstone

features are weathering-limited slopes. Combining

large scale structural or tectonic effects with small

scale grain-by-grain loss is problematic, yet progress

has been made in integrating rates of weathering and

erosion over varying time scales using, for example,

cosmogenic nuclide dating to complement field

observations (Matmon et al., 2004).

3.6. Rates of weathering

Determination of the rate at which weathering

proceeds is difficult, and attempts to quantify weath-

ering rates often have recourse to simulation experi-

ments and exposure trials; disintegration or surface

recession of stone on structures of known age (e.g.,

Takahashi et al., 1994) or of gravestones (e.g.,

Robinson and Williams, 1999; Williams and Robin-

son, 2000) is also indicative of average weathering

rates. Weathering research is constrained by the

normally slow rates of stone decay and rock weath-

ering; linear extrapolation of short-term or intermittent

information on decay rates is often inappropriate. In

fact, relatively few field observations have been made

of the rate at which weathering proceeds on natural

rock outcrops. The main limitation on quantification

of weathering rates is the method of assessment. The

weight of material lost from rock surfaces (e.g.,

Rogner, 1988) and the rate of surface recession

(e.g., Gill et al., 1981) are most commonly used;

weathering rates of up to 0.7 mm/year were recorded

by Rogner (1988), which are comparable to the rate of

honeycombing of 1 mm/year recorded by Grisez

(1960). These do not, however, account for changes to

material properties that occur prior to material loss;

nor do they elucidate the episodicity of rock break-

down (Colman, 1980).

In his field studies, Gilbert (1877, 1890) differ-

entiated between the concepts of weathering and

erosion. He illustrated the concept that weathering

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 241

was the in situ alteration of stone, while erosion was

that influence that occurred through entrainment and

movement. These early propositions by Gilbert

remain germane and important, as weathering is

commonly manifest as material loss and minimum

weathering rates are most usually determined by

measurement of erosion of material over time.

Recent studies have investigated the rates at which

sandstone deteriorates (Table 1). Schmidt (1989)

discussed the various agents affecting cliff retreat as

important in determining sandstone weathering rates

in the arid American Southwest. He estimated rates

for cliff retreat (500–6700 mm/1000 years); however,

his research could not isolate true weathering rates

because of his inclusion of catastrophic rockfalls in

cliff retreat. Later, Paradise (1995, 2002), working in

southern Jordan (with Nabataean architecture), deter-

mined sandstone recession rates ranging from 13 to 66

mm/1000 years on horizontal surfaces to 7–18 mm/

1000 years on vertical surfaces.

Yaalon (1996) raised the point that over such a

long period the weathering rate is likely to follow an

exponential curve from a rapid initial rate to a slower

steady-state rate. Mottershead (1989) used a micro-

erosion meter to conduct a detailed 5-year study of

rates and patterns of rock weathering, which

demonstrated strong seasonality in recession rates

and emphasized the spatial variability, because of

extremely local factors, of surface lowering at the

microscale. Pentecost (1991) investigated patterns of

microscale weathering by comparison of sequential

small-scale photographs of exposed outcrops, iden-

tification of lost sand grains, and calculation of grain

weights; this facilitated estimation of weathering

rates over short time periods, but these cannot be

Table 1

Examples of measurements of rates of sandstone erosion

Reference Method of assessment

Rogner, 1988 Weight of material lost

Mottershead, 1989 Surface lowering using micro-erosion

Schmidt, 1989 Retreat of cliff surface

Mottershead, 1994 Surface lowering using false datum

Takahashi et al., 1994 Surface retreat using false datum

Paradise, 1995 Surface lowering using false datum

Paradise, 1995 Surface retreat using false datum

Petuskey et al., 1995 Photographic survey and surface retre

Sancho et al., 2003 Surface topography using contour-plo

extrapolated over millennia because of the control of

changes in climate and vegetation cover on weath-

ering processes and rates. Petuskey et al. (1995)

examined weathering of sandstone ruins exposed for

around 750 years; however, compared with weath-

ering of test walls and exposure trials samples, they

found that present weathering rates were consistent

with inferred long-term rates of surface recession,

despite modifications to local air chemistry and

microclimatic conditions.

Pope et al. (1995) suggested that spatial and

temporal variability in weathering reflects variations

in weathering on a microscale. While microscale

weathering undoubtedly affects macroscale, long-term

landform evolution, the relationship between the two

remains unclear.

4. Weathering in the twenty-first century

Research into sandstone weathering remains a

somewhat peripheral part of rock weathering

research, and many of the shortcomings of existing

approaches may be viewed as common to weath-

ering geomorphology. First, biological impacts on

sandstone deterioration have only recently been

given much consideration, with many studies con-

ducted on historic, or cultural, stone. Second, scale

issues clearly need to be examined in weathering

research, with only a handful of papers addressing

this need (e.g., Smith, 1996; Goudie and Viles,

1999; Viles, 2001). Related to questions of linking

scales of enquiry with scales of information, there is

a need to revisit a holistic approach to weathering

research.

Rate of material loss (mm/year)

0.7

meter 0.625

0.5–6.7

1.05

0.022–0.052

0.013–0.066

0.007–0.018

at using false datum 4�10�6–5.5�10�4

tting frame 0.039–0.084

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253242

4.1. Biodeterioration

Recently, interest in the nature of biodeterioration

of stone, and in the possible roles of biological

material in eroding stone surfaces, has increased

(Papida et al., 2000; Naylor and Viles, 2002; Stretch

and Viles, 2002). Since the early works of Fry (1922,

1924), the effect of biotic contact on sandstone

substrates has gained great interest (Paradise, 1997).

Several papers have provided comprehensive reviews

of the processes involved in biodeterioration (e.g.,

Kumar and Kumar, 1999; Warscheid and Braams,

2000), and it is difficult to conceive of circumstances

within which stone surfaces are not colonized by

microorganisms. Conventionally, biological material

has been viewed as deleterious on sandstone surfaces,

with attention focused on lower-order plants (e.g.,

Wessels and Schoeman, 1988; Young and Urquhart,

1999). Lee and Parsons (1999) and Silva et al. (1999)

have suggested a protective rather than destructive

role for epilithic lichens on silicate rocks, a finding

supported by recent work on desert sandstones (Kurtz

and Netoff, 2001). Other researchers contend that

microbial contamination contributes significantly to

the acceleration of weathering processes (Warscheid,

1996; Warscheid and Braams, 2000). The presence of

biofilms has been shown to exacerbate the damage

caused by salts (May et al., 2000; Papida et al., 2000);

the presence or activity of bacterial cells was not

related to the decay state of sandstone (Tayler and

May, 2000). Biogeochemical effects of organisms

might promote salt and frost attack by increasing pore

volume and moisture content, precipitation of sul-

phates and oxalates, and mineral alteration or may

inhibit the effectiveness of salt weathering by reduc-

ing effective porosity–permeability, preventing pollu-

tant accumulation, and altering thermal characteristics

and wetting times of stone (Bjelland and Thorseth,

2002).

4.2. A systems approach

Research into sandstone weathering is concerned

with morphological elements (F), processes operating

(P), environmental conditions (E) controlling the

processes, and the materials (M) upon which the

processes operate over periods of time t. Based on a

bphysical geography equationb expressed by Gregory

(2000), the sandstone weathering system may be most

simply expressed as:

F ¼ f E;M ;Pð Þdt ð1Þ

The recognition of this system and its characteristics

and interactions is essential for the understanding of

sandstone breakdown, but this has rarely been

explicitly stated. While the methodological applica-

tion of systems analysis to the problem of stone

weathering promotes a holistic perspective, focusing

on the adjustment between form and process and

emphasizing the multivariate character of the problem

(Sack, 1992), it does not require acceptance of general

systems theory per se. A detailed model of rock

weathering at the microscopic scale was presented by

Pope et al. (1995), who offered a multivariable, open-

system weathering boundary layer model. This model

incorporates numerous variables pertaining to envi-

ronmental (E) and lithological/structural (M) controls

on rock weathering at a microscale, but emphasizes

that weathering proceeds slowly, rendering time a key

factor, and that each of the variables will also change

over time. This type of conceptual model offers

considerable potential as a predictive tool, and as a

vehicle to link disparate spatial scales, as it is bin the

microscopic details of the boundary layer that the

processes of weathering are most clearly revealedb(Pope et al., 1995, p. 57).

Examination of the suite of processes that may

cause weathering of sandstone perhaps should focus

on more rigorous investigation of the interactions

between processes, including biological processes

(destructive or constructive). The interactions between

weathering and processes can be conceived as having

three possible configurations. First, they may be

synergistic; second, they may be mutually exclusive;

third, they may be independent. Controls on weath-

ering processes, such as moisture availability, may be

perceived as enhancing rates of all types of processes;

whereas other types of control, such as atmospheric

pollutant concentrations, may increase the operation

of some processes (e.g., salt attack) and decrease the

rate of others (e.g., surface disruption by organisms).

These interactions and feedbacks within the weath-

ering system deserve further attention as their

elucidation would improve explanation of weathering

forms.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 243

In a survey of sandstone weathering features in a

range of environments, Turkington (1999) demon-

strated interactions between processes at both the

granular scale and outcrop/building scale. Extrapola-

tion between scales is not possible, however; no

correction factors exist that allow extrapolation of

laboratory-generated weathering processes and rates

to those documented on sandstone landscapes,

although these have recently been proposed for

silicate mineral weathering rates (e.g., Malmstrom et

al., 2000). Instead, the survey of sandstone weathering

tends to validate a conceptual model of sandstone

surface alteration and material loss based on crust

formation and detachment. A multivariate analysis of

the processes creating tafoni, solution basins, and

block retreat on buildings demonstrated the tendency

of sandstone surfaces to display selective material loss

(Turkington, 1999); Fig. 4 portrays a simplistic model

for each weathering form.

Efforts to establish the nature of process interac-

tions, or co-associations, might benefit from recourse

to some long-established geomorphic theoretical

constructs. Magnitude and frequency, for example, is

an apposite concept for weathering research (Wolman

and Miller, 1960), as weathering processes, such as

dissolution or salt crystallization, may occur with high

frequency without causing severe or rapid material

loss, while catastrophic loss may be triggered by a low

frequency event, such as a severe frost or, in the built

environment, stone cleaning. Alternately, the oper-

ation of high frequency, low magnitude processes

over time may create cumulative stresses that induce

catastrophic loss without contribution from low

frequency, high magnitude events, often due to a

breach of some intrinsic stability threshold. There is a

fundamental distinction between events that initiate a

new landform and those that perpetuate or enlarge an

existing form, although initial processes do not

necessarily cease once exploitation of a form begins.

Cavernous weathering forms provide an example:

those processes or events that initiate caverns may be

of differing, or lower, magnitude to those that enlarge

caverns or may be entirely different processes. The

relaxation time of the system to such disturbances, or

variations in processes, controls how the system may

be perceived as being in some equilibrium and how

far the magnitude–frequency of system responses can

be related to the magnitude–frequency characteristics

of processes, particularly in terms of dominant or

formative processes (Wolman and Gerson, 1978;

Brunsden and Jones, 1980; Brunsden, 1993).

The prevalence of nonequilibrium in landform

systems is now widely recognized (e.g., Harvey,

1992), as is the notion that many geomorphic systems

exhibit multiple equilibria, many of which are unstable.

These posits hold some promise for weathering studies.

Equilibrium concepts are developed or derived from

hydrological, fluvial, or coastal systems where land-

forms represent a compromise between erosion, trans-

portation, and deposition processes. Weathering

systems, however, are affected by many irreversible

processes. Once the structure of stone is altered, or

breakdown is achieved, the stone cannot be reconsti-

tuted. Equilibrium within weathering systems is likely

to exist as weathering forms that retain their state within

a range of imposed stresses; beyond this range (thresh-

old), the form becomes unstable and a new config-

uration of the stone is produced. This must be defined

as dynamic or metastable equilibrium; although stone

surfaces may appear stable and weathering forms may

be sustained, deterioration of the stone will continue,

particularly in subsurface or near-surface layers. Stone

breakdown may be manifested by intermittent changes

in forms, but the trend is continuous throughout the

blifetimeb of the stone or its system. Phillips (1992a)

suggested that equilibrium is more likely to emerge as

spatial and temporal scales expand; in the context of

sandstone weathering, equilibrium is more likely to be

identified as time scales are shortened.

Phillips (1992b) and Trofimov and Phillips (1992)

postulated that the traditional equilibrium approach,

which offers the hypothesis that there is a single

dynamically stable equilibrium state to which a

geomorphic system will evolve, should be rejected

or expanded, as multiple stable and unstable equilibria

exist that correspond to system attractors or concords.

This is evidently a pertinent standpoint for weathering

research; stone response to weathering processes must

not only be considered to exhibit a tendency to pursue

attractors, states which the system approaches, but

more appropriately a tendency to pursue concords,

balanced forms adjusted to climatic regime, or

lithological controls (Trofimov and Phillips, 1992).

The presence and prevalence of thresholds in stone

response to weathering processes suggests that the

weathering system is, in fact, inherently nonlinear,

Fig. 4. Sandstone weathering processes producing (A) cavernous weathering, (B) solution basins and (C) building stone retreat.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253244

which can lead to complex, chaotic behavior and

patterns (Nahon, 1991; Mayer, 1992; Phillips, 1992b,

1999, 2000, 2001; Renwick, 1992; Ortoleva, 1994).

This is evidenced by the high degree of spatial and

temporal variability of weathering and decay on

buildings, monuments, and outcrops.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 245

A major problem in understanding sandstone

weathering in the landscape lies in the inheritance

of stress by exposed stone, which is extremely

difficult to estimate, measure, or recreate artificially.

Weathering features are a result of the cumulative

effect of weathering processes over time and may be

as much relict as produced by contemporary pro-

cesses. Thus, the operation of weathering processes

must be set within an historical context of changing

conditions associated with environmental change or

alterations to material properties. On a larger scale,

weathering forms may be polygenetic. Given the

irreversible nature of weathering and decay process

outcomes, stone decay is arguably perpetually

affected by previous processes. Mostafavi and Leath-

erbarrow (1993) postulated that the past is always

present, as the context from which the present has

emerged; thus, there is theoretical continuity between

material changes in the past, present, and future.

However, Trofimov and Phillips (1992) proposed that

threshold-dominated development allows bifurcation

of system responses, so the system does not abso-

lutely remember past states. In other words, an

increase in stress derived from external variables

may induce a weathering route, such as surface

exfoliation, that has not originated in the effects of

previous weathering or decay, such as etching of grain

surfaces. Nevertheless, existing material properties

exert a profound influence on development of weath-

ering and decay forms, and changes to these structural

and compositional properties on a microscale are

likely to play a significant role in stone response to

processes of deterioration across a range of spatial

scales.

4.3. Scale issues

Finally, as sandstone research attempts to identify

and interpret influences, features, and their often

complex relationships, interscale investigations have

continued to confound research as case study

approaches based on inspection of outcrops and

analysis of samples consistently encounter these

scale problems. Viles (2001) provided a ground-

breaking review of scale issues pertinent to weath-

ering research in general and limestone weathering

research in particular. In weathering studies, theories

and understanding of process developed in small-

scale, laboratory-based experiments are often

expected to hold true in the larger scale of land-

forms. Conversely, data from landform observations

may be used in smaller scale explanations and

predictions. This involves extrapolation of informa-

tion and theoretical constructs across scale bounda-

ries, and the associated problems are scale issues

(Bloschl and Sivapalan, 1995). With regard to

sandstone weathering, processes may operate simul-

taneously at a range of scales. Extrapolation may

involve upscaling, downscaling, or regionalization;

the major complication is the heterogeneity of

materials and variability of conditions. Clearly, this

is of great significance in weathering research as

rock properties and environmental conditions may

exhibit an intractable degree of heterogeneity and

variability over space and time, and subtle variations

may have a significant impact on deterioration

processes.

Investigations of sandstone weathering under salt

weathering regimes, for example, require some

inductive reasoning to provide theories on the

operation of processes and to form a bridge

between the scales at which processes can be

examined and the generally larger scales at which

explanations are sought. Climatic cabinets have

been used to investigate different rates of rock

breakdown under artificially controlled conditions

of salt input, temperature, and humidity cycling;

experimental results allow ranking of the suscept-

ibility of different rocks to mechanical breakdown

by salt crystallization and/or hydration, and ranking

of the efficacy of different salts. However, obser-

vations of the nature and rate of breakdown do not

easily identify the precise weathering mechanisms

or the precise rock properties that control weath-

ering susceptibility. Generalizations based on such

empirical relationships risk being the product of

sampling and experimental design rather than

intrinsic to nature (Lane and Richards, 1997).

These mechanisms and controlling parameters can

be identified, however, by theoretical considerations

of crystallization pressures and crystal distribution

within pores of varying sizes. In extrapolating

results from such studies to the broader scale of

sandstone landforms, it is not the empirical evi-

dence of form or product that is extrapolated but

the theoretical understanding of processes that

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253246

allows explanation of the behavior of weathering

systems of which the experiment or case study is

representative.

As scales of investigation in studies of rock

weathering have increased, the pertinent parameters

have tended to become more complex, with prolif-

erating interactions between them. The effect of

processes at finer scales must modify process rates at

coarser scales. For example, exploitation of pore

spaces, capillaries, and microcracks by salt crystal-

lization may influence long-term rates of surface

recession of stone. However, salt crystallization may

also affect the effective pore size distribution within

stone and thus alter the solution ingress/egress

regime, consequently affecting salt precipitation

patterns and rates of loss of surface material.

Furthermore, the weathering history of the stone is

likely to control the evolution of microcracks and

secondary porosity–permeability, a feedback loop

that cannot be ignored. Theories of the nature of

rock weathering at each scale often make general-

izations that are not necessarily compatible with

those on another scale, therefore further effort must

be directed toward reconciling theories across scale

differences (Viles, 2001).

One sticking point in the adoption of theoretical

constructs to link scales in weathering research is

likely to be the scarcity of information on weathering

rates. Phillips (1995a) outlined four theoretical argu-

ments that may facilitate linking processes that

operate over fundamentally different time scales, each

of which is based on a comparison of rates or

durations of geomorphic changes. As Viles (2001)

argued, even basic description of characteristic tem-

Fig. 5. Scale-dependent we

poral distributions of weathering processes and forms

is difficult to make because of dlack of data on life

spansT of these features. Although many weathering

features observed on sandstone surfaces have been

defined on the basis, at least in part, of their size (e.g.,

alveoli and tafoni, flaking and scaling), characteristic

spatial scales of weathering forms are also difficult to

limit.

Spontaneous pattern formation, sometimes termed

self-organization (Phillips, 1995b), is typical of many

sandstone weathering features, exemplified by the

constant width/depth ratios of gnammas (Wray,

1995) or tafoni (Turkington, 1999) or geometric

regularity of polygonal cracking (Young and Young,

1992). However, morphometric similarity across

scales does not imply that the same process–form

relationships hold across scales, although this may be

possible. Fig. 5 illustrates the range of spatial scales

across which regular pits may be observed on

sandstone surfaces; arguably, a hierarchy of forms

exists. Viles and Moses (1998) suggested that in the

case of limestone dissolution microscale features

may be diagnostic of specific processes and thus be

used to predict the development of larger features.

Links between micro- and mesoscale features on

sandstone are tenuous, due largely to the polygenetic

origin of most larger features; the assumption that

microscale features remain constant is perhaps

unfounded. Quantification of microscale features,

and more accurate definition and classification,

may contribute to establishment of direct links

between features on a range of scales.

Sandstone weathering is controlled by environ-

mental conditions, genetically controlled heteroge-

athering morphology.

Table 2

Factors contributing to scale issues in durability studies

Variability of external

conditions

Long-term pollution levels and climatic regimes do not remain constant; regional differences in

climate and pollution levels; geometry of buildings and outcrops produce marked spatial variability

in environmental conditions.

Heterogeneity of internal

conditions

Rock properties may vary at a range of scales and through time, in porosity and permeability,

pore size distribution, microcracking, composition, jointing, bedding, etc.

Inheritance effect Changes to stone, or potential for particular changes, may result from previous processes or stresses

in operation since initial exposure.

Inconsistent response Stone response to decay processes may vary due to different sequences of processes and change,

due to subtle variations in rock characteristics, due to nature of inheritance effects.

Episodicity of processes

and response

Most decay processes are episodic, and may be cyclic or effectively random; response may be

delayed, or absent until some external or intrinsic threshold of stability is breached.

Singularity Each weathering system displays particular combinations of conditions at instances in space and time;

specific, or contingent, properties may be regarded as unique, or unpredictable.

Inherent complexity Variables and interactions within weathering systems may be introduced at larger scales that are not

derivable from smaller scales.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253 247

neities in rock properties, and inherited changes due

to past weathering behavior or anthropogenic

effects. Thus, weathering in situ can never be

absolutely defined (Niesel, 1981). Difficulties in

extrapolating across scales arise due to the inherent

complexity of weathering systems, variable response

within systems, episodicity of processes and their

manifestation and inheritance effects (Table 2).

These issues are central to geomorphological

theory; consideration of the geomorphological con-

text may provide a viable framework for linking

varying scales of inquiry in weathering studies now

and in the future. Interscale research may represent

the new trend in sandstone deterioration studies

and, from it, new insights will emerge, shedding

new light on these often unique features and their

often complex ancestry of influences.

5. Conclusion

As a new global focus on cultural heritage

protection and management emerges, it becomes

evident that sandstone architecture is the thrust of

much of this focus. Although relatively neglected in

weathering research, both as a landscape component

and natural building material, sandstone is increas-

ingly recognized as a heterogeneous material, which

displays diverse assemblages of weathering features

created by a multiplicity of causal processes.

Research on sandstone weathering continues to

explore previously underdeveloped fields, such as

subsoil weathering (Certini et al., 2003) and the

impact of fire (Dorn, 2003). Many recent studies

have examined the nature of sandstone decay in

urban environments, an interdisciplinary field of

study that is making both significant empirical and

theoretical contributions. Sandstone weathering

studies in this last century have proven to be a

strong avenue of innovation in field studies and

laboratory-based research.

The development and evolution of most empiri-

cal fields follow similar paths, and sandstone

weathering research has largely followed a typical

sequence (Mason, 1977; Sagan, 1977). In many

sciences, observations are often first addressed,

followed by feature classification and categoriza-

tion, influence/cause determination and rate assess-

ment, conceptual development, and finally network

(scale integration) and system analyses. This review

has highlighted the progress made, and the

progress to be made, in the development in this

subfield of weathering geomorphology. As this

interdisciplinary field attempts to link complex

systems at diverse scales, more relationships will

be divulged, more questions answered, and new

questions asked. Answers to questions associated

with how sandstone landscapes evolve or how

decaying architecture may be conserved will hope-

fully emerge as the critical field of sandstone

weathering research continues to evolve, stretch,

and mature.

A.V. Turkington, T.R. Paradise / Geomorphology 67 (2005) 229–253248

Acknowledgements

The authors wish to thank Jonathan Phillips and

David Robinson for their insightful comments on this

manuscript.

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