Geograph - photograph every grid square

A bench mark is an Ordnance Survey arrowhead sign found on walls, bridges, churches and specially erected triangulation pillars 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.
With rivets and pivots there is no datum (horizontal line) cut, the pointed-to horizontal surface defining the datum height.
For a view of the church NY7801 : St Mary's Church, Outhgill and Link