In the last issue of the Journal I introduced the subject of cave coralloids.  "Coralloid" is a catch-all term describing a variety of ...... speleothems. We discussed the forms that occur above water that is, subaerial forms.  Now we will turn to subaqueous forms - those found in pools or in areas previously or intermittently inundated.

Cave pools become supersaturated with respect to calcium carbonate as carbon dioxide outgases from the water surface and as evaporation removes water.  Calcite is deposited on the walls and floor of the pool. Sometimes, especially in quiet, barely supersaturated pools various forms of perfect calcite crystals develop.  These forms include nailhead spar, dogtooth spar and various rhombohedral forms.  These are usually clusters of single crystals like those found in Jersey Cave, Yarrangobilly or in Jewel Cave, Western Australia.  These are not cave coralloids. However, if conditions are changeable, various microcrystalline forms may be found ranging from things looking like broccoli to vertical towers reminiscent of trees.  The rounded forms underwater are difficult to explain.

Those of you that have been lucky enough to visit the Wollondilly Cave at Wombeyan will recall the forests of speleothems that look like fir or pine trees in old rimstone pools.  These are but one example of subaqueous coralloid and are called coral towers.  They are, of course, found in many other caves but these are spectacularly good examples.  This area of the cave has been trenched through to create a tourist pathway.  The trenching has exacerbated the draining of the pools but they may well have been seasonally dry before the developers (= vandals?) arrived.

Coral towers are found where we have still pools (i.e. no drips, for example) with evaporation and outgassing a vertical gradient of saturation will result more saturated near the top - and thus growth will be upwards as calcite.  Sometimes the tower corals will be 10 or 20 centimetres tall but more usually they are less than around four centimetres.

As water levels go up and down and as the quality of water changes as a result of seasonal fluctuations and so on we will get the development of a variety of forms - thus producing complexity.  Shelves, reflecting differing water levels, and spar crystals may develop and become superimposed on each other and on any coralloids.  This is one of the many mechanisms producing the complexities which we see every time we visit a cave but which do not appear in the textbooks.

The next ANDYSEZ is going to turn to the horrible subject of helictites which seem to be something that people have a morbid interest in.  Please bone up on earlier ANDYSEZs that deal with crystals and so on.