rimstone

thumb|300px|Rimstone – Endless Caverns, Virginia thumb|Rimstone with odd color, From Bristol Caverns Rimstone, also called gours, is a type of speleothem (cave formation) in the form of a 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. Rimstone basins may form terraces that extend over hundreds of feet, with single basins known up to 200 feet long from Tham Xe Biang Fai in Laos.

==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.