Rimstone dams (or gours) are depositional calcite (or other mineral) barriers that pond streams or shallow pools in caves. They can range from massive, as in photo #1, or tiny, as in photo #7. They tend to form stairsteps when in a series, and often extend into flowstone deposits above or below. Flowstone itself is often festooned with tiny micro-gours on horizontal surfaces.

When dams form under running water, they tend to be higher when the passage is steeper. Shallow-gradient dams tend to be lower and more sinuous in nature. Rimstone is one of the most common cave formations, after flowstone, stalactites, and stalagmites.

Rimstone dams form where there is some gradient, and hence flow, over the edge of a pool. Crystallization begins to occur at the air/water/rock interface. The turbulence caused by flow over the edge of the building dam may contribute to the outgassing or loss of carbon dioxide from water, and result in precipitation of mineral on this edge. Rimstone also often occurs on the top of broad stalagmites, such as in photo #4.

There are several rarer variations of rimstone, including the lotus variety (photo #9) and the horseshoe variety, photo #8.

Several records for rimstone dams, such as longest continuous series, tallest gour, and largest single rimstone pool, are noted and pictured on the page on the legacy Virtual Cave site devoted to the world’s largest speleothems.

AUTHOR: Djuna Bewley & Dave Bunnell