Rimstone

Rimstone, also called gours, is a type of speleothem (cave formation) in the form of a miniature stone dam. Rimstone is made up of calcite and other minerals that build up in cave pools. The formation created, which looks like stairs, often extends into flowstone above or below the original rimstone. Often, rimstone is covered with small, micro-gours on horizontal surfaces.

Formation

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.

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.

ources

* [http://www.goodearthgraphics.com/virtcave/rimstone/rimstone.html|The Virtual Cave]