Rimstone, also called gours, is a type of

speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending ...

(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 Flowstones are sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave. They are typically found in "solution caves", in limestone, where they are the most common speleoth ...

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 In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...

, and hence flow, over the edge of a pool. Crystallization begins to occur at the

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...

interface. The turbulence caused by flow over the edge of the building dam may contribute to the outgassing or loss of

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

from

, 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.

Concrete derived micro-gours

Rimstone or gours can be formed by the secondary deposits derived from concrete, lime or mortar. These secondary deposits consisting primarily of calcium carbonate, are called

calthemite Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4 ...

s and mimics the shapes and forms of cave speleothems to create stalactites, stalagmites, flowstone and gours.Smith, G K. (2016). "Calcite straw stalactites growing from concrete structures". Cave and Karst Science 43(1), pp4-10. Gours form beneath

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

structures on a floor with a gradual sloping surface or on the side of rounded stalagmites derived from concrete. Most of the calcium carbonate carried by the leachate is deposited as stalactites (when the drop rate is slower than one drop per minute), leaving little in solution to be carried to the ground to create stalagmites, flowstone and gours. The leachate which does reach the ground usually evaporates quickly due to air movement beneath the concrete structure, hence micro-gours are more common. The secondary deposits derived from concrete are the result of concrete degradation, where

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

are leached out of the concrete in solution and redeposited on the underside of a concrete structure. Calcium carbonate deposition as micro-gours occur when the solution drops to the ground under the concrete structure. Carbon dioxide is absorbed into the alkaline leachate solution, which facilitates the chemical reactions to precipitate any calcium carbonate remaining in solution as a stalagmite and micro-gours. This chemical reaction creating

s, is different from that which creates speleothems in limestone caves. Secondary deposits, which create stalagmites, stalactites, flowstone, rimstone etc., outside the natural

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

environment, are referred to as “

s”. These concrete derived secondary deposits cannot be referred to as “

s” due to the definition of the word.Hill, C A and Forti, P, (1997). Cave Minerals of the World, Second Edition. untsville, Alabama: National Speleological Society Inc.Moore, G W. (1952). "Speleothems – a new cave term". National Speleological Society News, Vol.10(6), p.2.

References

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Sources

The Virtual Cave's page on Rimstone
Speleothems