Calcite Dams and Cave Pearls: How Gurs and Rimstone Formations Develop in Caves

Calcite nodules glow under ultraviolet or pulsed light because they contain activating impurities, a property known as luminescence. When irradiated with a pulse lamp, these nodule formations emit yellow, soft green, azure-blue, and blue light.

Some calcite formations give off a dazzling, steady white light that appears to stream from their intricate shapes. The brightest luminescence occurs in colomorphic calcite that contains an admixture of manganese, the impurity most responsible for the strongest glow.

What are colomorphic calcite formations?

Colomorphic calcite formations are rounded, layered cave deposits built up from calcium carbonate precipitating out of water. They include several distinct types found across cave systems:

• Calcite dams, also called gurs or gurus
• Calcite crust
• Calcite films
• Cave pearls, also known as oolites
• Stone milk

Gurs and cave oolites stand somewhat apart from the other colomorphic deposits. Composed mainly of tuff, they differ in structure, porosity, and volumetric weight, which justifies grouping them separately — though this division is largely conditional, since all these forms share the same depositional origin.

What are calcite dams (gurs) and where do they form?

Calcite dams, or gurs, are barriers of deposited calcium carbonate that back up underground lakes and pools. They are quite widespread: in the Soviet Union they have been recorded in 54 caves, occurring mainly in limestone cavities and much less frequently in dolomite ones.

Gurs form in horizontal and inclined passages when calcium carbonate precipitates out of solution. This precipitation is driven by the release of carbon dioxide as the temperature of the water flow changes during its movement through the underground gallery. The outline of each dam — usually a regular or curved arc — is determined mainly by the original shape of the protrusions on the cave floor.

How large can calcite dams grow?

Calcite dams range widely in size, with heights from 0.05 to 7 metres and lengths reaching 15 metres. The largest gur in the world was found in the abyss of Petit Saint-Cassien in the Var department of France, at a depth of 229 metres; its height reaches 13 metres.

By their morphological features, gurs are subdivided into two types:

• Area gurs — broad dams spread across the floor of wider chambers.
• Linear gurs — developed mainly in narrow passages with underground streams, dividing the flow into separate water bodies of up to 1000 m² and more.

How are calcite dams destroyed?

The same water flow that builds calcite dams also destroys them. When flow rates and groundwater salinity change, erosion and corrosion cut holes, breaches, and notches into the gurs.

This destruction produces dry gurs that can no longer hold water. With further dissolution and erosion, the calcite weirs may vanish almost entirely, leaving only highly corroded protrusions on the floor and walls of the cavity to mark where the dams once stood.

How old are the calcite dams in the Red Cave?

The calcite dams in the Red Cave are about 9 to 10 thousand years old. V. N. Dublyansky determined this age by measuring the thickness of the seasonal half-layer, which deposits at roughly 0.1 mm, and counting the accumulated layers.

Which caves have the most remarkable calcite weirs?

The most striking calcite weirs are found in the Red, Shakuran, and Kutuk IV caves, each notable for the scale or unusual form of its dams:

• Red Cave. In the far part of the cave, a 340-metre stretch holds 36 calcite cascades between 2 and 7 metres high and up to 13 metres long, with widths sometimes reaching 6 metres.
• Kutuk IV Cave. In the Gallery of the Big Gurus on the upper floor, a 102-metre passage has the bed of its underground stream blocked by 34 dams of milky-white calcite, reaching 2 metres high and 15 metres long. So-called sealed gurs, or calcite chambers, were found here, where the submerged pools are completely covered with calcite film.
• Shakuran Cave (Caucasus). One passage about 400 metres long is divided by calcite dams into 18 lakes between 0.5 and 2 metres deep.