A technique for dating rock surfaces from measurements of the diameter of lichens growing on them. It is especially useful in environments where trees cannot survive. The first step is to find lichens growing on rock surfaces of a known age (e.g. gravestones or made ground) and to determine the mean diameter of the five largest lichens growing on them. That provides a lichenometric dating curve against which other exposed surfaces can be dated. An alternative method, useful where there are few surfaces of known age, involves measuring the diameters of selected lichens at intervals over several years to determine their rates of growth. Using very large samples of lichens makes it possible to define confidence limits for the calculated ages.