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Travertine terraces at Mammoth Hot Springs, Yellowstone National ParkCalcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation.
↑Dictionary of geological terms, 1962. A Dolphin Reference Book
↑A glossary of karst terminology, 1970. Geol. Surv. Water-Supply Paper 1899-K, U. S. Gov. Print. Off., Washington.
Travertine in a 400 year old wall of the castle Hohen Tübingen. The material is extremely hard and weatherproof. The cavities and impurities - remnents of small debris, scrub and biotic materials - make the rock porous and lightweight.