Tufa is a porous variety of limestone. It is formed by the precipitation of carbonate minerals from water at normal temperatures.
Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits known as travertine.
Do not confuse with the rock made of volcanic ash, which is called tuff.
Images for kids
Barrage Tufa at Cwm Nash, South Wales
Tufa at Trona Pinnacles, California, USA
Timothy O'Sullivan, The Pyramid and Domes, Pyramid Lake, Nevada, USA (1867)