Back in 1961 or 1962 my mother bought a beautiful first edition of Professor W Boyd Dawkins' Cave Hunting (1874) for my Christmas present. She paid rather more than the three pound ten shillings we later found pencilled on the rear fly leaf... she also had her purse stolen in the same bookshop. Not a good start to Christmas! The book had been recommended to me by Joe Jennings. My mother, who died last month, Joe and our next door neighbour in Canberra, Francis Ratcliffe (of Flying Fox and Drifting Sand fame) were the people who created my appreciation of nature. In fairness, I must say that it was my father who first took me into Jersey Cave at Yarrangobilly in about 1954 thus starting the rot that has me writing for you today. However, my mother also reinforced the trend by giving me Norbert Casteret's Ten Years Under the Earth. Has anybody not read this?
What has all this to do with cave coral? On page 67 of Dawkins' book is a cross section of one type of cave coralloid. The sketch has been used in a number of other texts (often without acknowledgment) and looks like a couple of mouse or bat ears! Dawkins' accompanying text provides a more than adequate mechanism for the "popcorn' variety of cave coralloid:
Several microscopic sections showed that each was formed originally on a slight elevation of the general surface, which would cause a greater evaporation of water than the surrounding portions, and therefore be covered with a greater deposit of calcite. The evaporation is greater at the point further removed from the general surface, and therefore the apex is larger than the base (Dawkins, 1874: p67-68).
Those of you who have listened to my diatribes in the past or even, perhaps, read ANDYSEZs will know that something more than evaporation is involved. Hopefully ...
Why do I say cave coralloid rather than cave coral? I am following the usage of Hill and Forti (1986) who point out that there are a number of types of cave coral. They state that:
"Coralloid" is a catch-all term describing a variety of nodular, globular, botryoidal, or coral-like speleothem. After stalactites, stalagmites and flowstone, coralloids are probably the most common speleothem type ... [a series of fairly ghastly American neologisms or colloquialisms follow]... Coralloids range in size from tiny beads to gargoyle-like masses over 1m in diameter (page 31).
Some developed under water, others in the cave atmosphere/rock interface in both wet and dry environments. For these latter subaerial forms Hill and Forti suggest the following mechanisms for development:
- by water seeping through the cave bedrock and through the crystal structure of the coralloid itself
- by thin films of water flowing over wall irregularities
- by splash from dripping water
- by water moving upward from pools onto walls by capillary (action) and
- by condensation water.
As is usual, and as I have said so many times before, more than one mechanism can be, operating at any one site and thus we can get additional complexities of form.
In their Figure 23 Hill and Forti show a portion of Pig Pen Cave in Idaho which appears to possess coralloids ranging from the "popcorn" variety to the coral-like "mitten popcorn" which resembles hands, cut off at the wrists, reaching up from the rock surface. In such a site it would seem likely that more than one mechanism is operating and that the dominance of mechanisms changes over the scale of a few metres so that we have be spectrum of forms.
Hill and Forti state that seepage is probably the most universal of the five mechanisms. However, they go on to discuss the importance of evaporation in the development of many of the subaerial forms citing Dawkins' original explanation.
It would appear that similar morphologies of coralloids can be produced by different mechanisms. In the North Glory Cave at Yarrangobilly there is a patch of spectacularly well developed "mitten popcorn" which cannot arise by any of the first four mechanisms as they are situated on isolated fallen blocks nor probably by evaporation given the site environment. In the Queenslander Cave at Chillagoe there are essentially identical forms which could certainly arise from capillary seepage or, more likely, from seepage and evaporation. Interestingly, Dawkins also mentions coralloids on isolated blocks.
We will return to cave coralloids, and then to helictites, in the next few ANDYSEZs. I believe that Dawkins' book, which deals mainly with palaeontological and archaeological aspects of caves, has been re-issued as a facsimile edition. It is certainly worth reading. Try and find it - and the works of Norbert Casteret. Thanks, Mum.