This symposium will examine the impact of lichens and other microflora on natural and human-manipulated rock substrates. Decomposition of rock is a natural phenomenon, with biological and non-biological factors interacting in a complex fashion, the effects of one accentuating the effects of the other, relentlessly breaking down rocks, large and small, to form the mineral component of soil. The process is inherently slow but constant. Biologically enhanced degradation of prehistoric and historic rock structures is receiving increasing attention from biologists, geologists, archaeologists, and cultural conservators. Papers in this session will address several aspects of this phenomenon from a variety of perspectives including the role of lichens in the biodeterioration process, the effects of other rock degrading microflora, changes in rock structure due to biodeterioration, issues related to the control of microflora on culturally significant substrates, and archaeological concerns relative to biodeterioration.

Key words: Biodeterioration, Lichens