Soluble rocks undergo erosion by solution when rain falls on an unprotected rock surface. Lichens may act to protect the surface from such erosion, and the protected area may subsequently emerge from the surrounding lowering surface to form an area of higher relief. Specimens of Aspicilia calcarea and Diploschistes diacapsis act in this way on a gypsum surface, creating conical or rounded mounds up to 15 mm in height. In the case of Aspicilia, the lichen centre decays, re-exposing to surface solution the central area, which then corrodes to create a cratered gypsum cone. The gradient of the cone side-slopes is a function of the relative rates of lichen radial growth and rock surface lowering.