This article is about the thick deposits of dust often seen just inside cave entrances and discusses deliquescent mineral deposits in the entrances of Eastern Australian caves.

Contents
Introduction
Sources of Dust
Tourist Do's and Don'ts
Discussion
References

Introduction

The earliest accounts of cave exploration in NSW record a large quantity of dust in dry cave entrances. In the 1800's people have written saying that they sank "up to their knees" in dust (see References below). With the passage of people, this dust usually becomes compacted and loses its soft qualities however in some tourist caves such as at Jenolan and Wombeyan (NSW) this dust can still be seen off the sides of the track and in little-visited areas. I have never encountered knee-deep dust, though.

Sources of dust

The origin of dust in dry cave entrances is usually ascribed to the wind (ie aeolian source) however there are other sources, depending on what the dust is made of.

• Aeolian source: In this case, the dust is made of topsoil blown into the cave during dust storms. Expected materials include silica, organic matter such as grass and leaves, pollen and clays.
• Bird droppings: Most notably owls, swallows and swiftlets in Eastern Australian caves. Sooty owls (Tito tenebricosa) have favourite sites which are used by successive generations however the "dust" in this case is specific to only one place in the cave and does not have the soft qualities of other sources of dust. The droppings of owls are the voided remains of small animals and are a good indicator of the type of prey available to generations of owls over a long climatic period.
• Bat guano: This is often a constituent of cave entrance dust. Some caves have large deposits of bat guano which have been mined in the past. Guano heaps, however, do not usually produce the fine dry dust that one sinks into in cave entrances however some of the constituents of bat guano may play a role in its formation.
• Wallaby droppings, Possum's nests, Rat's nests: The droppings and accumulations of Rats are discussed in Moore and Sullivan as a good source of nitrogenous material in caves.
  In Tasmania, I have encountered a Possum's nest in a cave, being an accumulation of plant material and (from the smell) Possum droppings and urine. In NSW there is a fairly rare wallaby, the Brush-Tailed Rock Wallaby, (Petrogale penicillata) which used to be common around the tourist caves (Jenolan and Wombeyan). I have noted scats from these animals fairly deep in Wineglass Cave (Wombeyan) and presumed that they sheltered in the cave during cold or snowy weather.
  For more information on the wallaby, or other animals, search online or look in the species lists on NSW National Parks https://www.nationalparks.nsw.gov.au/plants-and-animals
  There are deep deposits of dry wallaby scats in the Grand Arch at Jenolan Caves. Associated with these deposits is nitre, presumably from the wallaby urine. This tends to be precipitated underneath the boulders on which the droppings accumulate. The animals usually pick a dry spot to rest, so there is little chance that the droppings will be washed away. Anecdotally, dry scats have been implicated in occasional fires in the 1990's in the Grand Arch at Jenolan, supposedly caused by a cigarette tossed onto a pile of wallaby scats and dry leaves.
• Crystal wedging of ceiling deposits: This can occur in very dry conditions if salt (NaCl) or gypsum is present. Generally caves in Eastern Australia are too moist for salt to precipitate however gypsum wedging is a factor in some caves at Bungonia (NSW). This type of crystal wedging is not restricted to cave entrances though.
• Crystal heaving of mud substrate: On a several occasions I have visited the Fig Tree Cave at Wombeyan when the floor of the entrance chamber was festooned with needles, blobs, ropes and balls of a white fluffy material. The material has not been analysed however its deliquescence would infer that it is a nitrate (eg nitre). It usually occurs during the colder months when the air is drier. Often there are little particles of soil and dust adhering to the surface of the material, inferring that is is extruded from the substrate. The white fluffy material disappears in humid weather, leaving the dust behind very soft like a dark grey talcum powder. A tiny bit in the rockpile of the entrance chamber of Fig Tree Cave was tasted (!) and its intensely cold and bitter salty taste was reported (SUSS Bull 48(3) page 9). If this is nitre, then presumably it is from the scats of wallabies. The cave has been gated for over 100 years so it's unlikely to be a recent deposit. Nitre has been reported from Jenolan (see references below) Hill and Forti, referencing Mingaye) and Colchester, D., Pogson, R., & Osborne, R. A. L. (2001). Crystal wedging and crystallization of nitre and syvite could be a significant source of soft dust in cave entrances in Eastern Australia.

Tourist Do's and Don'ts

• Do keep a record of where you see deep dust, when you see it, what time of the year and whether there are any fluffy deposits associated with it.
• Do look in the dust for footprints of small animals such as beetles, lizards, birds, mice and spiders. This gives an idea as to what kinds of animals live in the vicinity.
• Don't go round poking the fluffy material or stomping on the dust. Poking the fluffy material spoils its appearance for others and there may be histoplasma spores in the dust which can cause illness if inhaled.
• Note to tourist cave owners / operators: Be careful during cave cleaning operations. It is probably not a good idea to hose down areas of deep dust as it may wash out the minerals responsible for its formation.

Discussion

The origins of deep dust in dry cave entrances is an interesting problem. It seems to be more prevalent in caves with multiple entrances, although it is also present in large caves with single entrances.
Lint is certainly a modern constituent of dust (N. Michie, pers. comm.) in tourist caves.
Regarding the depth of dust as reported by the earlier accounts: I do not question the depth, rather one wonders why we do not find dust to that sort of depth these days. If the dust was due to the extrusive effect of nitre on cave sediments, then perhaps there is no longer the quantity of nitre present in the cave entrances. One possibility is that the nitre was being formed from wallaby scats. These days, wallabies no longer inhabit the caves so possibly the source of dust is not being replenished. Another possibility is the entrance dust has been compacted by repeat visitation.
I have looked at some very small holes in limestone, too small to call a cave, in which there was no dust, just turned-over soil as might be expected from (say) earthworms or insects.
So in order for cave entrances to have dust, there needs to be:

• An animal to inhabit the cave
• A source of nitrogen compounds eg animal droppings
• Dry mud or soil to act as a substrate (source of dust)
• Bacterial decomposition of nitrogen compounds to form nitre
• A dry location to avoid washing away the materials
• A period of dryness to allow nitre to precipitate
• A period of higher humidity to allow nitre to deliquesce

These last two activities would need to be repeated in order for the dust to form, as distinct from soil.

Photo shows deliquescent material on rockpile, Fig Tree Cave entrance. Photo: Jill Rowling with Mike Lake (3.4 MB).

In the case of Fig Tree Cave at Wombeyan (NSW), to test the hypothesis a number of measurements would need to be made:

• What is the fluffy material made of? Is it nitre?
• Is there any correlation between humidity and appearance of the fluffy deposit?
• Is it the same as Jenolan's fluffy deposits?
• Does the same fluffy material occur elsewhere?
• Does the soft dry dust occur elsewhere?
• What is the dust made of? Is it organic, silica, gypsum or clay?
• What is the temperature and humidity where the dust occurs?
• What is the air flow rate and direction?
• How long does it take for an impression in the dust to be filled in?

References

Personal comments with Armstrong Osborne in 2000 and with Neville Michie in 1999.
Rowling, J. (2008). Possible solutions to puzzles in Fig Tree Cave, SUSS Bull 48(3) October - December 2008. Sydney University Speleological Society, p.9.
The Australian Museum has analysed Niter and Sylvite from the dust in the Devils Coach House and the Grand Arch, Jenolan Caves. This was presented at the International Union of Speleology congress in Brazil in 2001 and is in the Proceedings: Colchester, D., Pogson, R., & Osborne, R. A. L. (2001). Niter and sylvite from Jenolan Caves, New South Wales Australia. In I. U. of Speleology (Ed.), Proceedings of the 13th International Congress of Speleology, Brasilia d. f., Brazil (133–S1, 4p). International Union of Speleology. See https://uis-speleo.org/index.php/proceedings-of-the-international-congress-of-speleology-ics/
Hill & Forti
Speleology by George Moore and Nicholas Sullivan.