When 'carved in stone' is the work of lichens

By Jeff Mitton

The phrase "carved in stone" refers to a decision or policy that will not change -- it is permanent. While stone may seem permanent, the fact of the matter is that many forces erode stone.

I recommend a visit to the Colorado Plateau for an awe-inspiring reminder of all of the elements that erode rock. Extensive areas of the western portion of the Colorado Plateau contain layers of red, white, pink and caramel sandstone that were heaved and twisted ages ago, and now the colorful formations are being eroded by water and buffed by wind.

But another process, etching by lichens, adds a unique pattern that is so prevalent it is now a distinctive feature of the landscape.

Lichen etchings form elaborate mosaics that extend for miles on the San Rafael Swell, in the Grand Staircase of the Escalante National Monument and in the Paria Canyon-Vermilion Cliffs National Monument. These pervasive etchings are probably caused by tile lichen, Lecidea tessellata.

Lichen etching is prominent and widespread on white sandstone, but not common on red, pink or caramel sandstones. Sandstones are predominantly quartz crystals cemented together by carbonate or iron oxide or silica cement or clay. Sandstones with contrasting colors have different mixtures of constituents, modifying their suitability as substrate for lichen.

A lichen is a symbiotic association of a fungus and an alga. When you look at a lichen, you see the thallus, which is the fungus. Algal cells form an inner layer that is slowly consumed by the fungus. However, the algal cells reproduce faster than they are consumed, so the partnership is stable and mutualistic.

Crustose lichens on rock surfaces send specialized hairs, actually fungal hyphae, into the rock to anchor the lichen. Hyphae might initially take advantage of surface texture and minute cracks, but they penetrate solid stone, by dissolving the matrix that cements quartz crystals into stone. On limestone, lichen hyphae have been detected a half-inch beneath the surface.

Lichen hyphae dissolve rock by producing weak acids, depsides and depsidones, and by releasing carbonic acid, a byproduct of normal metabolism and a stronger acid. Carbonic acid dissolves calcium carbonate, a major constituent of limestone and the predominant cementing matrix of some types of sandstone. As hyphae dissolve the matrix and grow through sandstone, they make it friable. After the lichen dies, the riddled rock crumbles, leaving a shallow depression.

The depressions are conspicuous, for the stone exposed by etching usually has a richer color than the surrounding surface. Some etchings have delicate ridges of stone arching over the depressions from the peripheries. These are the last remnants of dwindling lichens. The delicate arches are dotted with crozzled apothecia, or reproductive structures.

In some places, etched ellipses covered sandstone benches for miles, suggesting that a rich carpet of lichens covered most of the land in the past. I fear that the lichen etching is the signature of a disappearing community and indisputable evidence of a changing climate.

Lichen etchings are commonly enclosed within polygons formed by weathering. When sun bakes sandstone, it causes the surface layer to expand, and then cool evening temperatures cause it to contract. Cycles of expansion and contraction interact with cycles of wetting and drying to create narrow, shallow cracks forming irregular pentagons and hexagons two to four feet across on flat surfaces and smaller squares on rounded or steep surfaces. Tile lichens on a tiled surface.

The combination of lichen etching, weathering and erosion by wind and water have sculpted magnificent sandstone structures and textures that appeal to the artist in most of us. Natural art is one of the most rewarding aspects of natural history.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

March 2012