The deep canyons of limestone formed by carbonation. Initially the limestone shows the wide, deep tissues known as bogaz; these widen and deepen into long gorges known as karst streets with other, cross-cutting, lines of erosion. The remnant of this carbonation is tower karst. See Zhu and Chen (2006) Speleogen. & Evolution Karst Aquifers 4, 1 on cone karst, tower karst, and tiankengs (giant dolines), and Harmon et al. (2006) GSA Spec. Paper 404 for perspectives on karst geomorphology, hydrology, and geochemistry. Karstic features do occur in non-carbonate rocks; see Uagodo et al. (2011) Zeitschr. 55, 1, 1.