THE [KS1] SMOOTHING AND ROUNDING OF THE BOULDERS

IN THE UPPER CHAMBER OF KOONALDA CAVE, SOUTH AUSTRALIA

by

Kevin Sharpe with Helen Fawbert

ABSTRACT. The Upper Chamber of Koonalda Cave contains at least five rockfalls of different ages and degrees of weathering (which, through exsudation, progressively renders the rocks smooth and rounded). The boulders of the oldest collapse show prehistoric human use and line markings, with sparse or no markings on other areas. The most recent rockfalls show no evidence of human use, but underneath them lies the original smooth, rounded, and marked boulder floor. I conclude by discussing the relationship between the geography of the floor, the weathering mechanism, and archaeological evidence.

KEYWORDS. Koonalda Cave, line markings, salt weathering, exsudation, crystallization.

Koonalda Cave is a well-known feature of the Nullarbor Plain of South Australia. Alexander Gallus first recognized the archaeological significance of the cave in 1956, and archaeological work continued in the cave during the following two decades (Flood 1997). The majority of the cave floor comprises two passages ending in lakes. Gallus undertook most of his excavations in the Gallus Site, the first and lower level of the Northwest Passage. The second and higher level of this passage, now dry and called the Upper Chamber, is the section of the cave I focus on. Work proceeded during 1973 in the portion of the Upper Chamber where fine lines incise some of the large limestone boulders on the floor of the passage. Further investigation revealed a large number of marked boulders, with torch stubs and bones sitting on top of or beneath stones in the floor of the cave.

Investigations continued in 1976. I noticed at this time that the whole floor of the Upper Chamber comprised boulders smoothed and rounded to varying degrees. The floor results from a sequence rockfalls and I distinguish between them from their different degrees of weathering. Rockfalls C and F appear the oldest, the most smoothed and rounded and in places under other rockfalls, followed by A, D, and then, most recently, B and E. During the 1973 and 1976 visits to the cave, my investigations concentrated on Rockfall C, whose boulders vary considerably in shape and size.

Lines mark the rocks in Rockfalls C and F and they show evidence of human use. Subsequent rockfalls have obscured much of this rockfall and its human evidence. The origin animal or human of the line markings remains unclear (Bednarik 1995; Flood 1997; Sharpe 2000).

I wish to find a reasonable explanation for the rounding and smoothing process because I want to know if my basis for distinguishing between rockfalls from their different degrees of weathering is appropriate. Further, the process may require conditions, such as the running of water, that affect the underlying line markings. It also influences how we understand the origin of the masses of small twigs on the floor of the cave from natural sources (in water flowing from the surface, for instance), or from human sources (twigs brought in to use as torches).

The floor of the cave comprises boulders mostly fallen from the roof of the cave. The roof had weathered to become smooth and so, on falling from the roof, the majority of rocks would have a jagged face upward and a smooth face downward. How do the boulders then naturally become smooth and rounded on their upper faces?

Any of a number of mechanisms may explain this process. The often-quoted cave weathering mechanisms and those considered especially pertinent include:

1.      By erosion from the claws or guano of bats.

2.      By the action of water running over the rocks, as in a stream.

3.      By the action of water dripping onto the rocks.

4.      By the rocks lying in a pool or a lake and dissolving.

5.      By the action of air rapidly blowing over them.

6.      By a mechanical process, the rocks tending to form stable shapes by forcing off surface particles.

7.      By the weight of the rock forcing off sections from its lower half, or the weight of upper rocks causing the breakdown of lower ones.

8.      By atmospheric humidity changes causing hydration weathering, the molecules in the limestone absorbing and releasing moisture.

9.      By insolation weathering, the rocks expanding and contracting with temperature changes.

10.  By salt weathering, crystals forming in the surfaces pores of the rock and forcing off particles of limestone.

The Upper Chamber contains insufficient bat guano for the first proposed mechanism to apply (Hooper 1958; Jennings 1971: 38; King-Webster and Kenny 1958; Ollier 1969: 47).

The process of water action (2) is sometimes cited as the smoothing and rounding mechanism for the Upper Chamber boulders (Jennings 1963: 54). It might at first seem the most reasonable and predominant form of erosion. After all, the appearance of the rocks resembles those found in a river; a smooth and hard outer crust covers them (Jennings 1971: 40-41); and smoothed tubes more than ten centimeters in diameter run through them.

Several factors counter this suggestion. The undersides of the boulders are not necessarily smooth and rounded, and are often heavily crystalline. Water flow would have worn all surfaces. The major difficulty with the process of water action arises from the direction of water flow. In the Upper Chamber, water must flow uphill. Jennings (1963: 54) notes that the flow marks in the squeeze area of the cave suggest a southward flow. The water would have flowed through the squeeze and then over the boulders. However, the rounded boulders which a rapid and turbulent flow appears to have rounded lie somewhat higher than the squeeze. Jennings explains this by positing an uphill flow under hydrostatic pressure. He later considers (Jennings 1978), though, that some of the larger rounded boulders are so large as to make the currents required to round them too great to be likely in the cave.

A water-worn proponent might suggest the existence of a similar cave just above the present Upper Chamber. A rapid flow of water in this cave smoothed the boulders, which then collapsed into the present chamber. The present smooth and rounded boulders thus derive from the floor of the higher chamber. The shape of the Upper Chambers ceiling and the even greater flow problem this hypothesis implies, however, negate it as a suitable explanation. Additionally, the lack of evidence for the remains of an opening into such a chamber and the need to explain why the smooth surfaces of the boulders sit uppermost when we might expect a random distribution further abrogate this suggestion.

Beneath Rockfall A sit incised smooth and rounded boulders, and any water-dissolving process acting on the rocks above would remove the markings. The process cannot explain the smoothing and rounding of unincised boulders, other than for the oldest rockfalls.

What about the two features of the rocks that suggest the water-warn hypothesis? The smoothed tubes more than ten centimeters in diameter and that run through the rocks may have formed when the boulders were part of the roof during the phreatic preparation phase in the evolution of the cave (Jennings 1978). Second, the smooth and hard outer crust that covers the rocks may have formed through a process such as oxidization or the formation of surface salt crystals.

To cite water flow as one of the mechanisms for the smoothing and rounding of the boulders is, therefore, not as simple as might first appear.

We can dismiss the third possible eroding force water dripping on to the boulders as irrelevant because of the absence of carbonate speleothems in the Upper Chamber (Frank 1971b: 32; Jennings 1967: 26; Lawler 1953: 339-40, 345; Lowry and Jennings 1974: 72-73). Moreover, a drip would round a boulder only by deposition; solutional erosion by a drip would only sculpt channels (Jennings 1978).

We can also discount the process of dissolving (Jennings 1967: 24; Lowry and Jennings 1974: 69). Quite angular and rough boulders sit in Nullarbor Cave lakes and show no signs of rounding because the water is evidently too saturated. (Limestone does dissolve, however, in exceptional unsaturated portions of some lakes in Nullarbor caves.) Dissolving as the boulder weathering process also encounters two problems I raised above: those of water flow and the rounding of boulders over line markings.

Wind action as the weathering process for the cave boulders also proves unsatisfactory. Anderson (1964: 129) and Wigley, Wood, and Smith (1966) suggest air movement as the reason for tafoni and other erosional forms in the Mullamullang Cave. Jennings (1967: 25) thinks this does not offer an adequate explanation. The erosion lacks, he writes, the streamlined forms which surface wind erosion produces, and the wind velocities are not high enough over adequate duration to be responsible for cave features. Any streamlined features, he further suggests (1978), would be aligned in a common direction unless disturbed since fashioned by the wind. No aerofoil-shaped rocks appear in the Upper Chamber of Koonalda Cave and no air currents of any great magnitude pass through it. (However, some of the caves and blowholes of the Nullarbor do have large air movements through them (Anderson 1964: 128, 130-31; Lawler 1953: 342-43; Wigley 1967; Wigley and Brown 1976: 333-34; Wigley and Wood 1967: 32-33; Wigley, Wood, and Smith 1966). The breathing phenomena (Flood 1997) reported for these chambers suggests an air flow reversal within them, making any wind-eroded rocks more round in shape than aerofoil in one direction.)

Neither do the temperature changes recorded in the Nullarbor caves seem adequate to cause insolation weathering of the boulders (Lowry 1968: plate 22[KS2] ).

This leaves process (10), that of salt weathering (or exsudation, crystal wedging, salzsprengung, salt frittering, flaking, or granulation). Many authors refer to it as the cause of small-scale weathering in Nullarbor caves and blowholes (Frank 1971a: 101; 1971b: 41; Hunt 1970: 17-18; Jennings 1967: 25; 1968: 50; 1971: 38; Lowry 1964: 16; 1968: 42; 1970: 31; Lowry and Jennings 1974: 59, 71; Wigley and Hill 1966) and some consider it an important geomorphological process in many environments (Goudie 1986; Mustoe 1982). A well-recognized spatial association exists between weathering and the presence of soluble salts (Young 1987: 962).

The salt weathering classification contains three potential mechanisms (Cooke and Smalley 1968):

1.      The heating of salts within confined spaces in rock surfaces could exert pressure on the rock and cause flaking and granulation. Even with a relatively limited temperature range, salt weathering produces rapid splitting and granular disintegration of rock (Goudie 1986).

2.      The expansion of anhydrated salts passing to a hydrated state within confined spaces in a rock surface could cause sufficient stress to force off particles. Alternatively, salts passing from one hydrated state to a higher one under temperature and humidity changes could cause a similar effect (Goudie and Wilkinson 1977: 20). Nocita (1987) suggests that this process mostly operates in hot, arid environments.

3.      Salt solutions within a few millimeters of the surface of rocks could, on evaporation, precipitate salts that wedge off the surface grains of limestone (Lowry 1964: 16). This process can wedge off surface grains, flake and scale off various sized rock fragments, as well as split rocks (Goudie 1986: 284; Jutson 1934: 255; Mustoe 1982: 111; Thornbury 1954: 38-39).

Experiments by Goudie in 1974 show that the salt heating process (1) is relatively ineffective. Further experiments published in 1986 show that temperature plays a significant role, however, but the temperatures used simulated those on desert surfaces rather than cave interiors. Koonalda Cave does not have the necessary temperature fluctuations to promote this granulation process (Cooke and Warren 1973: 66-67; Young 1987: 963).

Evans (1969-70: 154-55) reviewed exsudation as a cause of weathering (see also Jutson 1918 and 1934: 254-56 for an early Australian use of this explanation; and Twidale 1968: 140-42 in relation to Jutsons use). In fact, it is often cited as the cause of decay in building stone (reviewed again in Evans 1969-70: 156-57; see also Goudie 1986; Reed 1947; Schaffer 1967; Winkler and Wilhelm 1970). In-depth interest and experimentation have looked at the mechanisms particularly promoting salt weathering (Buckley 1951: 468-79; Cooke and Warren 1973: 66-71; Evans1969-70; Goudie 1974; Goudie 1986; Goudie, Cooke, and Evans 1970; Mabbutt 1977: 27-29). They show that salt weathering mechanisms (2) and (3) would both work effectively in the limestones of Koonalda Cave (Goudie 1974; Goudie, Cooke, and Evans 1970: 45). Further, Koonalda contains more than ample deposits of limestone dust covering the boulders and the floor; this may be the rock meal or rock flour, a diagnostic indicator of salt weathering (Goudie 1986; Higgins 1990: 296; Pohl and White 1965: 1464; Wellman and Wilson 1965: 1098). In Goudies experiments (1986) using Lower Carboniferous siliceous sandstone, the process also produced substantial quantities of fine sediment. Another important sign, crystallization, appears in the Upper Chamber both under the rocks and in the scaling and slicing off sections of the wall (Frank 1971a: 96, 101 and 1971b: 32, 41; Lowry 1967; Maynard and Edwards 1971: 64; Wigley and Hill 1966: 38). Jennings (1967: 25 and 1971: 38) accepts salt weathering as an explanation of some cave formations (see also White 1976: 308-9). He (1978) agrees that it probably caused the frittering of the roof in Mullamullang Cave to produce the Dune, a ten-meter-high pile of dust. Salt weathering, in fact, seems the most likely cause of the smoothing and rounding of the rockfalls in the Upper Chamber of Koonalda Cave.

Several significant points need answering, however:

1.      Several experts tentatively discussed the conditions necessary for exsudation (Higgins 1990: 296; Lowry 1964: 16; Lowry and Jennings 1974: 59, 71). Others then readily accepted exsudation as the explanation of breakdown in Nullarbor caves (Hunt 1970: 17-18). We must adopt a more cautious approach. The conditions necessary for it to take place probably occur in these caves, which Lowry (1968: 42-43) cites for exsudation to cause the dome development in the shallow Nullarbor caves as at least three: the rock must be sufficiently porous; the interstitial fluid saline; and . . . the air is not saturated with water. Whether these constitute the necessary conditions for exsudation to occur requires further research. Whether they are satisfied for the boulders in Koonalda remains unknown.

2.      Ollier (1969: 12-14) notes a common criticism of the salt precipitating process: it is rather hard to explain why crystal growth should continue against the pressure of the confining rock. Evans (1969-70: 167-73) cites experimental evidence for the process under certain conditions: high super-saturation and rapid evaporation (Rice 1977: 120). Thus, many authors suggest it as the mechanism explaining weathering in arid areas such as deserts (Nocita 1987) and Antarctica, and where wetting and drying is common as in coastal regions (Cooke and Warren 1973: 68; Evans 1969-70: 159-64; Mustoe 1982; Ollier 1974: 20; Young 1987). Writes Mabbutt (1977: 27-28), Super-saturation is favored under desert conditions where surface heating and drying winds cause excessive evaporation, and crystallization forces are therefore likely to reinforce thermal stresses. The forces may be cumulative with repeated solution and re-crystallization under high super-saturation. In Koonalda, even if a salt or salts highly super-saturate the moisture in the boulders, is the cave atmosphere conducive to rapid evaporation?

3.      The conditions necessary for the hydration of the salts offer another challenge (Cooke and Warren 1973: 67-68; Higgins 1990; Mabbutt 1977: 28-29). Crystallization in the anhydrous form usually occurs under high temperatures and low humidity, and moisture is later absorbed, particularly after a wet period. The process repeats itself. Drops in humidity cause dehydration and anhydrous salts fill the resultant spaces. Re-hydration follows this, and the process continues. This alternative wetting and drying creates the most effective disruption to the surface of the rock. Do these conditions occur in Koonalda?

4.      The origin of moisture presents another problem when looking at exsudation as the smoothing and rounding mechanism. Salt solutions supposedly evaporate close to the surface of the boulders and deposit crystals whose growth or hydration exert pressure and force off surface grains of limestone. But, where does the salt solution come from? The situation differs from the disintegration of building stones by exsudation. Here, rain falls onto the stone wall and percolates down through the stones to accumulate in the lower portion of the wall. Most of the damage occurs here. The situation also differs from exsudation on the walls and roof of the cave. Here, water percolates down from the surface of the plain and, when it reaches the cave walls, it evaporates.

Two potential sources may provide the moisture in the boulders: the atmosphere and the possibly moist floor. The Wick Effect that Goudie (1986) propounds applies here. Numerous field observations record the upward migration of saline solutions into rock and the subsequent damage of the rock. Buildings with foundations in the capillary fringe or saturated zone (such as the ruins of Mohenjo-Daro on the alluvial plain of the Indus) suffer from this process. Boulders transported onto moist playa surfaces from alluvial fans decay rapidly as salt migrates into them. Capillary action can draw up salts from deep zones of saturation (Baker 1990: 237). However, if the atmosphere in Koonalda Cave usually remains constant humidity-wise and temperature-wise, as it seems to, the rock may not absorb moisture that then evaporates back into the atmosphere. Seasonal changes may change the inside climate if the temperature in the cave varies from one season to another. It makes more sense, though, for the moisture to rise in the boulders by capillary action from the floor of the cave, especially after rain (Jutson 1934: 347; Winkler and Wilhelm 1970: 568). Rain outside the cave may cause large changes in humidity inside.

5.      If so, exsudation would affect the lower portions of the boulders more (and thus perhaps render them more cavernous) than the upper portions. The upper portions are, on the other hand, exposed to a more evaporation-inducing atmosphere. This would result in more smoothing and rounding on the upper surfaces. Large crystals grow on the undersides of many of the otherwise smooth and rounded boulders. The weathering of a rockfall of several boulders depth would, in a similar way, attack the outer boulders rather than those deeper down but the lower boulders would be more subject to migrating salts than those on top. The situation could be complex and warrants a thorough investigation. Jennings (1978) also points out that weathering of the rocks in a pile will lead to some rotation with lower surfaces becoming upper surfaces and therefore subject to weathering. So, he suggests, a stratigraphy of rounding should exist.

6.      Ollier (1969: 186), in his discussion of the flaking by salt growth in granite, says, individual blocks that weather by flaking become rounded because the process attacks corners and edges more than faces. When a boulder is quite rounded it shrinks, and the radius of curvature becomes smaller. He notes also that flaking does not extend below ground surface and that, when it does occur on concave surfaces, it tends to exaggerate the curvature. Evans (1969-70: 152, see also 157-58) writes: rounding by granular disintegration is the commonest effect of salt crystallization.

I assume that exsudation weathers jagged surfaces to smooth and rounded surfaces; that is, jagged parts wear down to the same level as the valleys between them. This makes sense (but needs further research) because a jagged portion has two faces to force off particles from, whereas a hollow has less of a tendency to become deeper (see Mowat 1962). The process should, therefore, develop in stages. Stage I represents a jagged, relatively recently fallen rock. It becomes smooth and humped (stage II) in the process of exsudation and the curvatures of both concave and convex surfaces become exaggerated (Ollier and Tuddenham 1961: 264). The humps more or less disappear in the third stage as the weathering removes convexities at twice the rate of concavities, and the boulder becomes smooth and rounded. If this scenario is correct, the boulders of Rockfalls B, D, and E collapsed the most recently and are in stage I of weathering. The rocks of Rockfall A follow in stage II, and those of Rockfalls C and F in the final stage III and constitute the oldest collapse.

7.      Boulders become less and less susceptible to exsudation as they progress through it because their surface area decreases and becomes a crust. This crust becomes crystalline, harder, and impervious to the flow of moisture (Lowry 1970: 31; Mabbutt 1977: 30; Ollier 1969: 80). This process needs further investigation. Evans (1969-70: 157) describes skins of crust peeling away from limestone; portions of skins with line markings are peeling off boulders in the Upper Chamber. The formation of the crust means that lines incised on rocks in stage III of weathering will last longer than lines on rocks at a lower stage. This may explain why few line markings appear in Rockfall A and only faint ones in Rockfall D.

The ceilings and walls of Nullarbor caves show the results of exsudation, including hollowings and tafoni (the convex and rounded projections left between the hollowings) (Howard and Kochel 1988: 28; Jutson 1934: 255; Lowry 1970: 31; Lowry and Jennings 1974: 71). The suggestion that exsudation causes smoothing and rounding of boulders, then, departs from what scholars and other observers have seen as its work in Nullarbor caves. As the way for the rounding and smoothing of the Upper Chamber boulders, exsudation offers a promising hypothesis; many details need clarification and verification[KS3] .

Summary and Conclusions

The archaeological remains and wall markings in the Upper Chamber of Koonalda Cave on the South Australian portion of the Nullarbor Plain have been known for many years. The floor of the Upper Chamber comprises at least five rockfalls of different ages and degrees of weathering (which renders the rocks smooth and rounded). Incised lines appear on rocks of the oldest collapse, Rockfalls C and F, with few or no markings appearing on rocks in the other collapses. The rocks in Rockfall A are fairly smooth and rounded. Those in Rockfall D are more rough and jagged but have been used significantly by humans. No evidence of human use exists in the most recent rockfalls, B and E, but underneath them and the others lies the original smooth, rounded, and incised boulder floor.

What mechanism smoothes and rounds the rockfalls? This process, whatever it is, operates on rocks on top of already incised smooth and rounded boulders. This counts against water flow as the process, despite what many writers suppose. After examining this and a number of other mechanisms, I favor exsudation or salt weathering, in which crystals form in the surface pores of the limestone and force off particles. Several details need confirming with this as the weathering mechanism before I could suggest it with certainty.

Acknowledgements

This paper derives from work I carried out in 1976 under the auspices of the National Geographic Society and the South Australian Museum. I wish to thank the two institutions, as well as Sandor Gallus and the Gurney family of Koonalda Station.

References

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Baker, V. R. 1990. Spring sapping and valley network development. In C. G. Higgins and D. R. Coates (eds.), Groundwater Geomorphology: The Role of Subsurface Water in Earth Surface Processes and Landforms. Special Paper 252. Boulder, Colorado: The Geological Society of America, pp. 235-66.

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Buckley, H. E. 1951. Crystal Growth. New York: John Wiley and Sons.

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Cooke, R. U. and A. Warren 1973. Geomorphology in Deserts. London: B. T. Batsford.

Evans, I. S. 1969-70. Salt crystallization and rock weathering: a review. Revue Geomorphologie Dynamique 19 (4): 153-77.

Frank, R. 1971a. Cave sediments as palaeoenvironmental indicators, and the sedimentary sequence in Koonalda Cave. In D. J. Mulvaney and J. Golson (eds.), Aboriginal Man and Environment in Australia. Canberra: Australian National University Press, pp. 94-104.

Frank, R. 1971b. The sediments. In R. V. S. Wright (ed.), The Archaeology of the Gallus Site, Koonalda Cave. Canberra: Institute of Australian Studies, pp. 31-44.

Goudie, A. 1974. Further experimental investigation of rock weathering by salt and other mechanical processes. Zeitschrift fr Geomorphologie Supplement 21 (3): 1-12.

Goudie, A. 1986. Laboratory simulation of The Wick Effect in salt weathering of rock. Earth Surface Processes and Landforms 11: 275-85.

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Copyright 2000 by Kevin Sharpe. All rights reserved. Submitted for publication.


 [KS1]AR01 words 18 September 2000

Writing Method in Outline

        Answer questions.
OVERALL

        Check name introductions.

        Complete bibliography (Chicago A).

        Print. Check for dumbing up. Revise.

        Print. Check chapter for variety in paragraph lengths. Check for consistency of person. Check tenses.

        Read aloud. Revise. Spell check.

        Print. Mary read. Revise.

        Answer questions.

        Spell check. Word count.

        Print. Send out.

Theme:

 

Tension:

 

Summary:

 

Lead:

 

Hook:

 

Ending:

 

Outstanding Points and Questions:

        Suggestions for future work here:

      Experimental work including XRD or SEM to identify the nature of the surfaces or the deposits on them.

      Need morphometric data from proper field studies, e.g., comparing degree of rounding among different rockfalls, plus petrology of each deposit. Microscopic study of surfaces and sections.

        Boulder smoothing and rounding see Robert's recent article (1992a) on macro rounding for refs.

        Porosity is a good indicator of age.

        So is rounding Robert has an instrument to measure this.

        Compare with Bednarik's paper. Rounding process. Dating how useful?

        What is the archaeological significance of the rockfalls? Floors and flake in some suggest human use of them. Could date the falls over them.

        This sets the stage for the smoothing/rounding process and whether Robert's techniques of use.

        Start with a description of the different rock falls.

        Could combine this with the results of the 1976 expedition.

        Comments on Bednarik's "rounding" paper:

      The probability curve and the assumption about linearity of wear seem a problem.

      The example if wearing from exsudation, the more the build up of crystals near the surface, the less frittering.

      Question of my work: where does the water come from? But otherwise try to explain the build up of surface crystal crust and its pealing off.

      The technique does not seem to apply to limestone it does not seem to be one of the rocks he lists. Is it too soft?

      If not exsudation, what causes the surface to come off?

      Are there research questions here for koonalda? Yes.

      Relate the rounding process to smoothing.

      Do I relate the smoothing/rounding to the REAL human markings in Koonalda?

      Still I'm plagued by why these large animals got into the Upper Chamber another entrance?

       

 [KS2]???what about the other possibilities???

 

 [KS3]???discuss the relationship between the geography of the floor, the weathering mechanism, and archaeological evidence.???

 [KS4]???Sharpe animal or human?? Description of UC