This cave formation is known as a “rimstone dam”. It is a calcite dyke that terraces a sloping surface, holding back mirror-smooth pools of water. I am reminded of rice paddies of molten wax. Sometimes a skin of calcite grows on the surface like a sheet of floating paper. In absolute darkness they expand until their weight might sink them or some unexpected ripple drags them down.
In Honduras’s Talgua Cave the dams reach a staggering three feet high. “Natures Hoover Dams on the underworld”. It is an aberrration linked to the extreme age of the deposits. Most startling of all are the so called “glowing skulls” within. The caves had been used as an ossuary and the bottom of the pools have numerous bones and skulls cemented to the floor by calcite deposition. The skulls themselves are covered in tiny sparkling calcite crystals, it is this that imparts their luminous qualities when lit by the explorer’s torch.
As a general rule, rimstone dams form where cave water, saturated with calcium carbonate, flows down an inclined surface. Tiny irregularities cause turbulence and the release of carbon dioxide. This change in the water chemistry causes the liquid to become “supersaturated and the mineral that was held in suspention immediatly drops out. The depositions form part of the dam wall. Where the gradient is shallow the dams tend to be low and sinuous. Where the slope is steep the dam walls are higher and more regular.
I had taken this picture in the cave that I had explored this past weekend. The deposition from side rifts appeared to ooze across the surface slipping down toward the floor in tiny steps. Special care must be taken not to bump or touch the dams as they are very delicate.