Lichen: the challenge for rock art conservation

Abstract

This study investigates the effects that lichens have on rock surfaces in which ancient rock art (petroglyphs and pictographs) may be found. The study area includes four sites in the United States: one quartzite site in southwest Minnesota, two sandstone sites in Wyoming, and one volcanic site in Central New Mexico. One additional granitic site studied is located northeast Queensland, Australia. The questions driving the pursuit of this dissertation research are: 1. How does the chemistry of the rock change with weathering and how deep is the profile? 2. Do lichens cause differential chemical changes? 3. How does the chemistry of the unaltered rock influence these changes? 4. Do lichens strip the patina or “desert varnish” from the rock surfaces? The results of this research confirm from elemental chemical analyses that geochemical changes do take place in the presence of lichens. The combined mechanical and chemical processes contribute to the degradation and greater erodability of all the rock surfaces studied. Chemically, we have demonstrated that cements that hold grains of rocks together can be dissoluble by lichen byproducts in the presence of an aqueous environment whether the rocks are sandstone or granite. This information regarding the mechanical and geochemical processes at work in natural environments has significant practical benefit for the management, conservation, and preservation of rock art sites everywhere.