Rimstone dams (or gours) are calcite or other mineral barriers that pond streams or shallow pools in caves. They tend to form stairsteps when in a series, and often extend into flowstone deposits above or below (as in the second photo in the group of smaller images). Often flowstone is 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.

There are several rarer subvariations of rimstone, including the lotus variety (3rd photo down) and the horsehoe variety, shown in the first image in the group of four at the bottom.

Choose a thumbnail to zoom in on more images of rimstone

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The Virtual Cave Created: June 19, 1995
Last Updated:March 26, 2019
Author: Djuna Bewley & Dave Bunnell