↑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.
File:Kalktuff-Block Schloss-Tuebingen 2.jpgTravertine 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.