Geograph - photograph every grid square

The lichen covered bench mark is to be found on the wall of St Martins Church. SU0125 : St Martins Church, Fifield Bavant and Link

A bench mark is an Ordnance Survey arrowhead sign found on walls, bridges, churches and specially erected concrete posts where the altitude above sea-level has been accurately measured by surveyors. The arrowhead points to a horizontal line above it which marks the exact altitude. At places below sea-level the arrowhead points down.

The walls of the church are covered in large patches of 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, south-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.