English: Woodland gravestones in St Marys Churchyard Many of the gravestones are covered with lichen.

Lichens do not have roots, but absorb water and gases through their upper surface, and are therefore sensitive to atmospheric pollution. For this reason they are rarely found around cities and grow best on the wetter west side of the British Isles. Those on trees thrive best on the sunny, sough-west aspects of trunks and branches. Disappearance of lichen species can be used to detect rising levels of air pollution.

There are three main types. The encrusting forms, including the bright orange/dark yellow 'Xanthoria' grow on roofs, walls, gravestones, signs and tree trunks. Leaf-like species develop flat lobes spreading over bark or stones, and shrubby forms which grow vertically from the ground or hang from trees. Few have common names.