THE SMOOTHING
AND ROUNDING OF THE BOULDERS
IN THE UPPER CHAMBER OF KOONALDA
CAVE, SOUTH AUSTRALIA
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 Chamber’s 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).
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 Jutson’s 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 Goudie’s 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.
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.
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by Kevin Sharpe. All rights reserved. Submitted for publication.