Rainbow Arcs Over Hueco Tanks

Hueco Tank’s coarse and abrasive rock, which gives so many great boulder problems, is called syenite porphyry, a type of granite. The rock formed about 34 million years ago when a mass of molten rock pushed upward into overlying layers of limestone. Before reaching the surface, the molten rock slowly cooled and solidified. Erosion then went to work, eroding the softer limestone layers and slowly weathering the granite until all that was left was today’s humped rock mountains. The eroded rock surface gives the immediate impression that this is rock that was made for climbing. The surface of the boulders and cliff faces are pocked with spherical depressions, called huecos in Spanish, which make ideal handholds. The rock surface also has a hard surface coating, locally called “ironrock,” which again forms sharp flake handholds.