Australian Archaeology

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Australian Archaeology

Archived at Flinders University: dspace.flinders.edu.au

Full Citation Details:

Mortlock, A.J. Price, D. & Cardiner, G. 1979. The discovery and preliminary thermoluminescence dating of two Aboriginal cave shelters in the Selwyn Ranges, Queensland. 'Australian

Archaeology', no.9, 82-86.

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T H E D I S C O V E R Y AND P R E L I M I N A R Y T H E R M O L U M I N E S C E N C E D A T I N G O F TWO A B O R I G I N A L CAVE S H E L T E R S I N T H E

SELWYN R A N G E S , Q U E E N S L A N D

A . J . MortZock,

D.

Price and G. Cardiner

Two apparently undisturbed cave shelters near Selwyn in the Selwyn Ranges in Queensland were discovered by one of us (G.G.) during 1977. The first of these, referred to as Site 1 is located at Lat. 21°23'; Long. 140°32'. The second referred to as Site 2, is located approximately lOkm SE of the first. Rock paintings were present in both shelters but were not recorded in detail.

At Site 1 the paintings consisted of finger

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paintings in faded red ochre of a figure 25cm high, two boats 20cm long with figures in them, and smaller symbols.

Near this site t h e r e

was

a

flaked edge stone tool and numerous flakes.

At Site 2 there were numerous paintings in red ochre similar to some at Site 1 (not boats) plus many others in yellow, white and red of (among other things) rainbow snake; kangaroo; dingo

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birds

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and hand silhouettes. The yellow and white paintings cover some of the red and are more complex, suggesting they are younger.

Small fire hearth samples were carefully taken from each site, wrapped in aluminium foil and forwarded to Canberra for trial thermoluminescence dating. This was carried out using the now well-known quartz grain inclusion method. In the present case after washing in HCL acid the separated grains were finally etched for 20 minutes in concentrated HF acid to remove the near-surface alpha radiation dose. The identity of these grains was established

by XRD.

Glow curves associated with these samples are shown in Figures 1 and

2.

The derived plateaux, which are required to be flat if the thermoluminescence is induced by radioactivity, are displayed in Figure 3. It will

be

observed that the general characteristics of these are good in the region of the read-out temperature, namely 375'~.

Measurements of the U, Th and K for the two samples were made at the ANU

by

XRF. The concentrations are listed below:

Site 1

U 17 PPm

Th 51 P P

K20

2.16%

Site 2 17 PPm 37 PPm 1.35%

The calculated annual dose rates using these data and

assuming burial in surrounding soil of the same composition were:

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SELWYN

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1

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SlTE I

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Figure 3

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B e c a u s e o f t h e n o m i n a l n a t u r e o f t h e c a l c u l a t i o n s a t

t h i s s t a g e and l a c k o f r e l e v a n t e x p e r i m e n t a l d a t a , no c o r r e c t i o n h a s b e e n made f o r t h e p o s s i b l e e f f e c t s of t h e w a t e r c o n t e n t o f t h e s o i l a n d , a l s o , any r a d o n g a s l o s s t h a t m i g h t have t a k e n p l a c e . The d i r e c t i o n o f t h e s e c o r r e c t i o n s , i f o p e r a t i v e , would b e t o d e c r e a s e t h e c a l c u l a t e d a n n u a l d o s e r a t e .

A p p a r e n t t h e n n o l u m i n e s c e n c e a g e s were c a l c u l a t e d on t h i s b a s i s t o b e a p p r o x i m a t e l y :

S i t e 1: 1 9 0 0 y BP S i t e 2 : 1 1 0 0 y BP

Two a d d i t i o n a l f a c t o r s p o s s i b l y p r o d u c e f u r t h e r s y s t e m a t i c e r r o r s i n t h e s e a g e f i g u r e s . F i r s t l y , t h e sample m a t e r i a l was n o t baked h a r d a n d , on a r r i v a l i n C a n b e r r a , was r e l a t i v e l y l o o s e . T h i s c o u l d mean t h a t l i g h t had r e a c h e d t h e i n n e r q u a r t z g r a i n s a n d d r a i n e d some t h e r m o l u m i n e s c e n c e . I f s o , t h e a p p a r e n t a g e s would b e l o w e r l i m i t s . T h i s s t a t e m e n t r e m a i n s t r u e when t h e two c o r r e c t i o n s a l r e a d y mentioned f o r w a t e r c o n t e n t and r a d o n l o s s a r e i n c l u d e d .

U n f o r t u n a t e l y t h e l a s t and f i n a l f a c t o r which c a n n o t b e a c c o u n t e d f o r n u m e r i c a l l y may go e i t h e r way i n i t s e f f e c t on t h e c a l c u l a t e d a g e , p a r t i c u l a r l y f o r S i t e 1. The t e s t sample f o r t h i s was t a k e n f r o m t h e b o t t o m of t h e o v e r l y i n g s o i l l a y e r s , t h e s u b s t r a t e b e i n g r o c k . T h i s r o c k may b e o f h i g h e r o r l o w e r r a d i o a c t i v e c o n t e n t t h a n t h e n e a r b y s o i l . I f l o w e r , t h e n t h e c a l c u l a t e d a n n u a l d o s e r a t e used i s t o o h i g h and t h e age f i g u r e f o r t h i s s i t e r e m a i n s a l o w e r l i m i t . However, t h e r a d i o a c t i v e c o n t e n t c o u l d b e h i g h e r . I n t h a t c a s e t h i s meaning c a n no l o n g e r b e a t t a c h e d t o t h i s p a r t i c u l a r a g e f i g u r e . The f a c t t h a t t h e c a l c u l a t e d a g e f o r S i t e 2 i s l o w e r t h a n t h a t f o r S i t e 1 i s c o n s i s t e n t ( a l l t h i n g s e l s e b e i n g e q u a l ) w i t h t h e r a d i o a c t i v e c o n t e n t o f t h e s u b s t r a t e r o c k b e i n g h i g h e r t h a n t h a t o f t h e n e a r b y s o i l . The t e s t s a m p l e f o r S i t e 2 was t a k e n lOcm from t h e r o c k s u b s t r a t e , w h i c h i s s t i l l w i t h i n t h e r a n g e o f gamma r a y s f r o m t h e r o c k b u t s h o u l d b e l e s s a f f e c t e d t h a n t h e t e s t s a m p l e f r o m S i t e 1.

From w h a t h a s b e e n s a i d t h e w e i g h t o f t h e e v i d e n c e i s t h a t t h e a g e s o f b o t h f i r e h e a r t h samples a r e n o t l e s s t h a n 1000 y e a r s . The a g e s o f t h e w a l l p a i n t i n g s may v e r y w e l l be d i f f e r e n t f r o m t h i s , a n d c o v e r a r a n g e of y e a r s . C l e a r l y a f u r t h e r v i s i t t o t h e s i t e s made w i t h c a m e r a s and o t h e r a p p r o p r i a t e s u r v e y e q u i p m e n t s u c h a s i n - f i e l d r a d i a t i o n d o s e m e t e r s would b e w o r t h - w h i l e . A d d i t i o n a l e x p e r i m e n t a l i n f o r m a t i o n c o u l d t h e n b e o b t a i n e d t o remove t h e l i m i t a t i o n s on t h e p r e s e n t r e s u l t s .

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Acknowledgements

I t i s a pleasure t o acknowledge the help o f Mr J . Pennington and Mr R . S . Freeman of the Department o f Ceology, School o f General Studies, ANU, f o r carrying out the XRD and XRF a n a l y s e s r e s p e c t i v e l y

Australian Archaeology