50 ANS APRES LA DECOUVERTE DE LASCAUX .

JOURNEES INTERNATIONALES D'ETUDE SUR LA CONSERVATION DE L'ART RUPESTRE

DORDOGNE - PERIGORD (FRANCE), 20 - 23 aout 1990

ACTES

--GROUPE ART RUPESTRE DE L'ICOM POUR LA CONSERVATION OFFICE DEPARTEMENTAL DE TOURISME DE LA DORDOGNE Edit. : ATELIER DE RECHERCHES ET D'ETUDES EN PERIGORD A.R.E. 24 - ISBN: 2.9504621-0-3 Photos des pages 1 et 4 de couverture : LASCAUX Salle des Taureaux, paroi gauche (cliches P. Vidal, LRMH)

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SOMMAIRE

AVANT-PROPOS Daniel DEBA YE ....... ............. .. .......... .................................................................... 3

INTRODUCTION J. BRUNET, P. VIDAL, J. VOUVE ........................ ......................................................................................... 5

HOMMAGE A JACQ~ES MARSAL J. BRUNET, P. VIDAL, J. VOUVE ........... .... ........... .............. _ ............... .. .................... _.. ..... ... ...... .... .. ........... 7

LA GROTTE DE LASCAUX : LE POINT SUR LES ETUDES ACTUELLES J. BRUNET, P. VIDAL, J. VOUVE .. ..... ..... ...... .. .. ..... ... ........... ..... . ..... ........ ..... ... ................. ................ ...... .. ............. .. ........ ...... 9

NEW DEVELOPMENTS IN ROCK ART CONSERVATION TRAINING N. STANLEY PRiCE .... .. ..... . .... .. . .. .. .. ........................ . ........... ... ..... ..... .......... ... .................... .. .. ...... .. .. ......... .. ................... 13

THE WEATHERING OF AUSTRALIAN ROCK PAINTINGS A. WATCHMAN ...... .... .... ......... ...... .............. .... . ... . ......... .. . .......... ... ............... ...... ........ . ................................ .. 21

CONSEQUENCES INATTENDUES D'UN PARADOXISME BIO-HISTORIQUE LIE A LA DECOUVERTE DE LASCAUX J. VOUVE ...... ............. ........... ... .. .......... .... .. ....... ... .... ...... ........................ _ ..... ,........................ .. ..... ..... .. .............. .......... 31

CONSERVATION ET RESTAURATION DE L'ART PARIETAL DE LA GALERIE ANBANGBANG I. DANGAS - J. CLARKE.. . ...... ..... . ..... ... . ... ......... ..... .. .. ......... .... ... ... .... ..... .. .. ..... .. .. ... ............ ... . ... ...... . .... ..... ..... . 43

LA METODOLOGIA DE ESTUDIOS CLiMA TICOS EN CUEVAS CON ARTE RUPESTRE APLICADA A TUMBAS EGIPCIAS : TUMBA DE NEFERT ARI E. PORTA ...... ... .. ... .. ..... ......... ... ..... .... ... .... ... .. .. .... ..... ..... , .. ...... .... .. ................................... ... ... : ..... ..... ....... ......... . _ ...... ............. 51

A REVIEW OF STUDIES INTO THE COMPOSITION OF PIGMENTS USED IN AUSTRALIAN ROCK PAINTINGS , A. WATCHMAN .. ... .... .. ... ........... .. .. ................ .......... .... ..... ..... ... .. ............... .......... _ ... _ ..................... ..... .................... , ....... . 57

181

ROCK ART SITES OF BAJA CALIFORNIA: A SYMPOSIUM M.M. RIEDEL .............. , ................... .. .................................................................. , ....................... ....................................... _65

ROCK ART CONSERVATION IN KAKADU NATIONAL PARK NORTHERN AUSTRALIAN Y.P. HASKOVEC ..... ", .. ,., ... ", ... , .. ,., ........ .................................................. , ............................................................................... 75

PROTECTION ET GESTION DU PATRIMOINE RUPESTRE ET DE L'ENVIRONNEMENT ASSOCIE APPLICATION A LA GROTTE DE LASCAUX J. BRUNET, P. VIDAL, J. VOUVE .... ...... ""." .... , .. .. .. .. , ... . , ..... ... . " .. . , .. , ... , .. .. ... " ..... .. ................................... " ................ , ........... 89

PREMIERS RESULT ATS DE L'ETUDE DES PEINTURES RUPESTRES CONCERNANT LA GROTTE DE LASCAUX S. DEMAILLy ..... " .. .............. .. ..................................................................................... , ......... , ... ..................... ................ ........ 101

L'ART RUPESTRE PREHISTORIQUE DU PARC NATIONAL DE LA SERRA DA CAPIVARA - PIAUI - BRESIL: PROBLEMES DE CONSERVATION A.M. PESSIS ... ... ........ .......... ... .. ....... .. , ... . ,."."." ........... ......... .......... .................... , ..... .. .... ........... , ........................................... 117

INVESTIGACION PARA LA CONSERVACION DEL SITIO ANA-KAY-TANGATA ISLA DE PASCUA - CHILE M. BAHAMONDEZ PRIETO, M.E. VAN DE MAELE SiLVA .... ............ .. .................. , ............... .... ...................... ............... 123

LA CONSERVACION DE LA CUEVA DE AL TAMIRA : PASADO Y FUTURO M. BARRIL VICENTE, C. de las HERAS MARTIN ........... ............. .. .. ... ........................................ : ...................................... 129

MOULAGE D'UNE PAROl GRAVEE DANS LA GROTTE CHABOT (GARD) R. DAViD ..... .. ............ ............. .. ........ .... ...... ... . , ........................... ... ' .. ' .. , .............. .............................. ......................... .. ....... 139

CONSERVATION OF ORNAMENTED TUMULI AND CAVES IN JAPAN AND CHINA S. MiURA ......... .. ........ ... .. ............................................................................................. .... ........ ... ............................. .............. 141

RESTITUTION D'UN REGIME CLiMATIQUE FAVORABLE A LA CONSERVATION DES OEUVRES PREHISTORIQUES DE LASCAUX J. BRUNET, P. VIDAL . .... ................... ... , ......................... .. ..... .. ............................... ' ....................... ......... ... ........ .. ...... ....... 147

PRELIMINARY REPORT ON THE RELATIONSHIP BETWEEN CLIMATE AND CONSERVATION WITHIN THE HAL-SAFLIENI HYPOGEUM IN MALTA J. CASSAR, A. BONNICI, P.J. SCHEMBRI, F. VENTURA .... ... .. ..... .. .. ................................................................................... 155

LES FAC-SIMILES RELIEFS EN MATIERES SYNTHETIQUES AVEC TRANSFERT D'EMULSION COULEUR A. CUNILLERA, S. BRIEZ ..... ," ,., ...... . : ..... .... ,." .. .... ........................... .... .. .......... ..... ................................ .................... , ...... .. ... 163

LA DECOUVERTE DE LA GROTTE DE LASCAUX DANS LA PRESSE Documents: collection A. MERLE , .... , ............................................................ .. ............... ..... ............... ..... .. .... ... ......... ....... 169

REMERCIEMENTS ................................... ............ ................ ...... .................... ................................ .................... .. ......... ... 179

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THE WEATHERING OF AUSTRALIAN ROCK PAINTINGS

Alan WATCHMAN

Research Grantee, Asutralian Institute of Aboriginal and Torres Strait Islander Studies.

Geologist, Centre for Australian Regolith Studies

Geology Department, Australian Natioanl University, CANBERRA, AUSTRALIA

Abstract

Deterioration of Australian Aboriginal rock paintings occurs through natural processes of accumulation of matter on the surface as well as degradation of the ochre and its substrate. Salt crystallisation, mineral deposition, organic accretions and repetitive applications of ochre lead to accumulation on the painted surface. Exfoliation, granular disintegration, and salt fretting cause instability of both paintings and substrate. In this paper these natural processes are briefly described and examples given of the most recent research results.

Introduction

In this paper the issues of rock art deterioration caused by natural weathering forces are examined by focusing on the painted surfaces and the processes which take place at the interface between the arts' substrate and the atmosphere.

Unlike the rock art of France and most of Europe, which is predominantly located in caves, Australian rock art is mainly found on exposed rock surfaces ; a situation which causes considerable instability for paintings (Rosenfeld, 1985). Ochres have been applied to rock surfaces on the back walls and ceilings of deep rock shelters and on vertical rock faces unprotected from the weather. Major painting sites are located in a variety of climatic environments; from the tropical north and the temperate climatic areas to the sub-Antarctic of southwestern Tasmania (the locations of paintings sites mentioned below are identified in Figure 1).

Paintings are found on a variety of rock types: namely sandstone, quartzite, limestone, arkose and granite. Sandstone provides the substrate for most of the paintings throughout Australia. Understanding the present day processes which are taking place at the substrate/painting! atmosphere interfaces is critical to the study of the deterioration and conservation of rock pair.ltings. Rocks on which art has been painted are regarded as continually changing under dynamic environmental, structural, biological and geomorphological forces.

Both accumulation and degradation must be considered in the weathering of paintings; both processes reflect the competing natural forces. Accumulation of matter on the surface of paintings can only take place where the substrate is relatively stable. Degradation of paintings takes place as a result of processes which remove either the substrate, the pigment or both substrate and pigment; consequently in these circumstances the interfaces are unstable.

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Processes and products of accumulation

Salt crystallisation

Accumulation of salts on rock surfaces obscures the paintings from being viewed. The salts occur as individual small pimple-like growths, coalescing granular lumps and interlocking rosettes, composite sheets and globular nodules. Hydrated mineral salts found at rock art sites come either from evaporation of rain water or ground-water. Rain water has variable chemistry depending upon the season and proximity to the sea (Hingston and Gailitis, 1976 ; Probert, 1976). The chemistry of ground-water also changes with time depending on its volume, the season and rock type through or over which it has flowed.

Rain water which falls in the early wet season storms in Kakadu National Park in the northern extremity of the Northern Territory is highly acidic, with an excess of sulphate (Noller et al i 1985). Monsoonal rain which falls later in the wet season is less acidic and contains more ionic components derived from a maritime rather than a terrestrial source (Wetselaar and Hutton, 1962 ; Noller et al ; 1987).

The assemblage of evaporite-type minerals found on rock faces includes nitre, gypsum, halite, polyhalite, darapskite, anhydrite, morhite, hydroxyapatite (Watchman, 1987), jarosite, syngenite, various iron, calcium and potassium phosphates, sveite and sylvite (Clarke, 1989).

In shelters, ground-water often flows freely out of bedding planes. Such outlets for ground-water arise because of mineralogical and grain size differences in the original sediments. The intersec­tion of joints and bedding planes also controls the hydrological regime of a rock mass.

Crystallization of salts and deposition of suspended particles from ground-water' may close discharge points in a rock face. Once the escape route of ground-water has been blocked off hydrostatic pressure and dissolution of soluble matrix and cement minerals eventually forces another opening through which the ground-water can be discharged. In this way a series of openings along bedding planes and within the more impermeable beds open and close through time and control the discontinuous flow of grqund-water over art.

Mineral deposition

Airborne matter ,

Local and regional sources of fine dust particles arise from disturbances of the adjacent archaeological deposit, unsealead access roads and tracks, and wind-transported dust from deserts, dry plains and bush-fire ravaged areas. The mineralogy of the dust particles comprises quartz, clay, ferruginous minerals and carbonised plant material. The grain size of the wind-blown material depends on the proximity of the source ; coarse particles being derived from nearby sources and finer ones from more distant locations.

In the Laura area of Queensland, airborne particulate matter has been deposited at many painting sites. Samples of these surface crusts, taken from painting sites on sandstone and conglomerate in that region are all thin (1 mm), and comprise lower, middle and upper grey layers or bands separated by gypsum layers. These layers contain mixtures of fine to very fine particles of quartz, clay, and carbonsied organic fragments.

Although local conditions are the major contributing factor in the deposition of salts, dust and . other matter on painted rock surfaces, regional trends can also be identified. For example, the effect of regional climatic conditions is evident in crusts over limestone at painting sites near Chillagoe in Queensland. Cross-sectional analyses of cortexes from four sites reveal almost identical micro-stratigraphy, despite the observations that the thicknesses of the crusts range from 0.5 mm to 4 mm. Cortexes above the limestone substrate from large open rock shelters

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consist of a basal layer containing calcium carbonate fragments from the underlying rock, calcium phosphate and small amounts of potassium, calcium and magnesium sulphate and chloride salts. Immediately above this layer is a thin band of amorphous silica, minute quartz grains and tiny particles of black carbonised plant remains.

Above this band is a t.hicker layer made up of many fine wey lamination~, the bulk of whic~ is composed of whitlocklte (Ca3 (PO,J2)' trace amounts of calcium and potassium sulphate, chloride salts and quartz particles. Gypsum laminations drape across the eroded lower layers ; these, together with trace amounts of potassium "and calcium phosphate, quartz and clay form the surface.

Materials borne by ground-water

Thin, matt-buff, and pale brown deposits are often found associated with areas of regular water wash zones. An example of such an occurrence is found at Split Rock, near Laura in northern

d. Analysis of the surface deposits and the substrate indicate that the kaolinite ....... Ar\""ited on the painted surface is derived from the matrix of the medium-grained sandstone

per. comm. ; Watchman, 1990a). Kaolinite, in fine suspended particles, is re-mobilised the rock by ground-water and re-deposited on the painted surface.

obvious affect of water at art sites is to create dark-coloured eroded zones, bounded by -coloured regions where minerals have been deposited from the running water. Ate Ubirr, in

"o,'\a""Y National Park, tor example, although there is an unsightly water-wash stain covering a the art itself is in nearly pristine condition because of a protective film of silica. The stain

on the edges of the main water wash zone is caused by crystallization of sparingly soluble growth of algal colonies and their consequent vermiculation structures (interconnecting

..-uJnrrYhc:haped ridge and hollow networks), and deposrtion of suspended mineral matter in the of slower water flow.

skins in Australia were first recognised by Dolanski (1978) as being important natural devices protecting rock surfaces from deterioration. Lambert (1979), also described the protective role thin transparent silica films in protecting rock paintings. Essentially, silica skins form by

Ut~:IV,ution of amorphous silica, and crystalline quartz to a lesser extent, in the quartzite and one substrates, then transportation in ground-water and deposition on the surface. Factors ling dissolution, transportation and depOSition are still unclear, and are currently being

Ochre coverred by thin films of silica is protected from surface abrasion, erosion and salt damage. &foliation of thick white silica deposits on a painted rock surface can cause damage to art SUrfaces. as at Gnatalia Creek in New South Wales, but otherwise silica skins are highly beneficial for rock art preservation (Lambert, 1989 ; Watchman, 1989).

~ skins ~ary in thickness, ranging from a few microns up to 1mm. Dolanski (1978) attributed ~~f.ormat,on ~o mobility of silica in surface water and ground-water, reflecting the volume and utCm,stry.?f rain water. However, rain water itself contains negligible quantities (less than 0.2

of Silicon.

preliminary investigation into the formation of silica skins at rock art sites in Kakadu National . ~a~chman (1985) concluded that these heterogeneous siliceous films were formed from

eclPlt3tlon of silica from solutions which flowed across the rock face .

.. rese.arch (Watchman, 1989) has revealed at least four compositional and textural of s,hca:sk,n ; these four types can be differentiated in terms of their complex formational

"lpr·OC .• ~S.4 ~ MI~!o-organisms, as well as inorganic chemical processes, are involved in the tlon of Silica skins. There is strong evidence that both ground water and surface water are

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critical to their development. Such findings have great implications for conservation of rock art, especially regarding the installation of artificial driplines to control surface water flow.

Oxalate deposits

Dusty surfaces on the gently sloping back walls of many rock shelters contain the oxalate-bearing mineral whewellite (calcium oxalate monohydrate). Th is mineral is thought to form mainly from the products of a lgal growth but the role of organic acids in aerosols from rain water should not be completely discounted (Watchman, 1990 b), especially where organic pollutants are present in the atmosphere. Stable, rough and relatively inert substrates are favourab le environments for micro-organic habitation. This is. especially apparent when t.he microbes deriv~ nutrients directly from the weathered products In the substrate, from particulate matter which settles on the surface, and from ground-water and rain water which occasionally wet the surface.

In Kakadu National Park, oxalate-rich crusts appear to form after polyhalite has crystallised directly on the substrate. Then there is developed an alternating succession of whewellite, gypsum, dust and other salt layers depending upon fluctuating past environmental conditions.

Whewellite is presently forming on painted rock surfaces in Kakadu National Park and it has been actively forming, albeit intermittently, for at least the last 9000 years (Watchman, 1987)

Organic accretions

Irregularities on the walls of rock shelters, such as depressions, fractures and hollows, lead not only to the accumulation of particulate matter and salts, but also to a build up of insect and microflora debris. This is because these locations provide favourable habitats for insects and other small animals. W asp-nests, termite-mounds and trackways, and mud ne-sts ' of birds cover parts of artistic expression on rock faces (Watson and Flood, 1987, Hughes and Watchman, 1983, Naumann, 1983).

Vermiculated patterns of micro-organic debris and particulate matter can also obscure paintings (Clarke, 1989). These develop on rock faces which receive water from slow, surfac;e seepage. Ridge and hollow surface crenu lations, mainly created by accumulation of algal fi laments, trap moisture and enable deposition of finely dispersed days and other suspended matter.

Repetitious ochre applications (repainting)

Throughout Australia it has been traditional Aboriginal practice to repaint the art (Edwards, 1979 ; Merlan, 1989; Mowaljarlai and Watchman, 1989) and there is evidence from one art site in the Ki mberley region of Western Australia of 44 visible layers of ochre in a surface 5 mm thick (Clarcke, 1976). Evidence for widespread repai nting throughout Australia is revealed by studying many cross-sections through painted rock faces.

Processes and products of degradation

Exfoliation

The application of a single layer of hematitic ochre to a rock surface usually ensure long-term stabil ity for the art. Finely dispersed ochre particles are able to penetrate easily into the rock th rough capi llary ·cracks. Deposition of these particles in the relatively stable micro-environment of the rock face does not allow loss by water-wash erosion.

~- '"

There is a major problem with the present surface paintings at sites where over-painting has been repetitively carried out using platy pigments; this is the failure of the latest ochre layer because

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of differential expansion, with changes in moisture levels, of the surface pigment from the underlying one (Clarke, 1976). However, loss of cohesion in a painting is not as significant a problem as the failure of adhesive forces to hold the platy ochre minerals on the substrate.

The adhesion between successive layers of pigments is affected by the chemistry, mineralogy and particle size of the ochre layers. The strength of the adhesive bonds is diminished where a dust or clay layer partly covers the underlying ochre layer, leading to failure of the painted surface.

Exfoliation of the rock surface, and consequential loss of pigment on that surface, takes place by meGhanical leverage through formation of subflorescent salts and thermal expansion. Gypsum, which crystallises a few millimetres or centimetres below a rock surface, will cause inter-granular stress which eventually leads to microspalling of the rock face. The time scale over which this process happens is not certain but photographic records from Split Rock, near Laura, suggests hundred of years are required for sufficient accumulation of stress.Cross section examination of the many different substrates reveals capillary cracks and micro-fissures within two centimetres of the surface. Micro-fractures which are developed sub-parallel to the surface are of concern to conservators because their presence and extent are generally not known until a section of the rock face begins to exfoliate.

Granular disintegration

A major problem at painting sites which are exposed to wetting and drying cyles because of their orientation, aspect and location is deterioration of the matrix and cement which bind the rock together. Some clay minerals, particularly the smectite clays, noticeably expand in the presence of water; this affect generates large inter-granular forces. In sandstone, these stresses eventually weaken and loosen the cement around individual quartz grains, and leads to granular disintegration. Paintings on the surface of sandstones containing high proportions of smectite clay are therefore most susceptible to erosion where moisture permeates the rock.

The composition of the clay matrix is highly significant in the weathering of sandstones. For example, the kaolinite-dominated matrix of sandstone in Carnavon Gorge, Queens/and, is relatively stable during hydration and dehydration. The rock is, however, vulnerable to abrasion because the grains are not strongly bonded. In comparison, the sandstones from coastal New South Wales which contain up to 30 % illite are unstable; during both wetting and drying cycles; under moist conditions exchange of small cations for larger cations lead to increases in the structural dimensions of the clay. Salt fretting and granular disintegration of these sandstones causes rapid weathering of the painted substrate (Hughes, 1978 ; Lambert 1989).

Salt fretting

Mechanisms by which salts cause deterioration to paintings include chemical reaction, .physical disruption, expansion and contraction stresses, solubility and ionic exchange.

Examples of chemical reaction are provided by the crystallisation of sylvite (KC1) and halite (NA C1) ; these salts are found in coastal painting sites where the concentration of chloride ions in rain water is high. Because of their deliquescent nature, sylvite and halite cause exfoliation of small flakes from the surface. Fragments of pigment and charcoal from the art may be attached to or incorporated in silica films and so will be lost from the painted surface by this process.

Clarke (1989) identified sveite (KA 17(N03)4C12(OH),6.8H10) as the major salt in active weathering of sandstone io several locations in the Kakadu National Park (sveite absorbs water up to ten times its initial volume before it dissolves). The degree of hydration of this mineral at different times of the year reflects local humidity conditions and such changes in hydration are Significant in fragmenting porous rocks.

1.5

~ : • I . '

Gypsum is found widely throughout rock art sites in Australia (Walston and Dolanski 1976 ; Clarke, 1978; Watchman 1987 ; Lambert 1989). Its limited solubility in water, together with the nucleatlng ability of smaller crystals to promote the growth of larger crystals, make this mineral another structurally debilitating mineral. Crystallization of gypsum in the porous matrix immedia­tely beneath the surface of many sandstone rock faces leads to spalling of the rock.

Clarke (1989) has found evidence that gypsum forms from natural sulphuric acid attack on carbonate pigments in paintings in Kakadu National Park; the wetting and drying cycles promote the gradual growth of gypsum rosettes.

Other salts are also found at painting sites throughout Australia. The most common of these minor salts include jarosite (KFe3(S0.J2(OH)6 ; this is derived from dissolution of iron minerals in the substrate, and from ferruginous ochres attacked by sulphurous ground waters.

Hydrated sulphate salts such as syngenite (K2Ca(S04)2' 2H20) (Clarke 1989), polyhalite (K2Ca2Mg(S04)4' 2H20), mohrite «NH.Jle(SO.J2.6HzO), potassium darapskite (K)NO)S04H20) (Hug­hes and Watchman, 1983), tal'l!arugite (NaAI (SO.J2.6H20) (Hall et ai, 1989) and gorgeyite (K2Cas(SO.J6.HzO), occur at the substrate/painting interface but their roles in damaging the art have not been clearly established. They may only be surface deposits which are the products of crystallisation from saline ground-waters, and they may not be harmful to the art.

Various calcium, aluminium, iron and potassium phosphate-rich minerals, for example tinsleyite (KAI2(P0.J2(OH).2HzO) and whitlockite (Ca3(PO.J2), are also found on painted rock faces but, except for covering small areas of the painted surface, do not appear to be damaging the. art (Hughes and Watchman 1983, Clarke 1989).

Conclusion

The stability of the representative images on painted rock in Australia is dependent on the outcome of conflicting natural forces balanced by skillful conservation actions where the latter are applied. In many cases images have been in place for hundreds of years and show signs only of dust and salt accumulation. Other examples reflect stable environments for perhaps thousands of years and current research is attempting to date some of these surfaces in order to understand the factors enhancing their longevity. The most critical need for research and conservation action is on sites where active granular disintegration and exfoliation are rapidly eroding the art. Broad scale research carried out in the last ten years has established basic knowledge about the weathering of rock art but, if galleries of paintings are to be protected from further degradation and loss, much more site-specific research is essential.

Acknowledgments

The Australian Institute of Aboriginal and Torres Strait Islander Studies and the Centre for Australian Regolith Studies provided financial assistance, and laboratory and writing facilites respectively, for the conduct of this research and preparation of this paper. I also appreciate the constructive comments made by Dr Graeme Ward on an early manuscript.

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References

Figure 1 - Locations of Australian painting sites mentioned in the text.

WESTERN AUSTRALIA

: I

: HORTt£RH TERRITQAY :

I : i I I I OUEENSlN«) I I I I I I

L. __ .... __ . _____ .i_ ..... I '

: : I I

I SOUTH AUsmAL1A r------ ... -----.. -...... ~, .. -

500 Km '-----'

J i

r~~\ : VlCI~i.C- - u,. I ~._

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Merlan,. F., 1989. The interpretive framework of Wardaman rock art : a preliminary report. Journal of the Australian Institute of Aboriginal Studies, 14-24

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Noller, B·.N., N. Currey, G.' Ayers, R. Gillett, A Harrison and R.A. Rasmussen, 1985. Chemistry of rain water. In Alligator Rivers Region Research Institute Annual Research Summary for 1984-85. Australian Government Publishing Service, Canberra.

Noller, B.N., N. Currey, G. Ayers, R. Gillett, A Harrison and R.A RAsmussen 1987. Chemiwy of rain water. In Alligator Rivers Region Research Institute Annual Research Summary for 1985-86. Australian Government Publishing Service, Canberra.

Probert, M.E., 1976. The composition of rain water at two sites near Townsville, Queensland. Australian Journal of Soil Research, 14, 397-402

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Watchman, AL., 1985. Mineralogical analyses of silica skins covering rock art. In R. Jones (ed') Archaeological Research in Kakadu National Park, Australian National Parks and Wildlife Service, Canberra (Special Publication 13), 281-289.

Watchman, AL., 1987. Preliminary determinations of the age and composition of mineral salts on rock art surfaces in Kakadu National Park. In W.R. Ambrose and J.M.J. Mummery (eds) Archaeometry : Further Australasian Studies, Australian National University, Canberra, 36-42.

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