Geograph - photograph every grid square

A brass or copper stud fixed into the stonework of one of the viaduct piers, possibly used as part of a scheme to monitor water and flood levels. Its origins aren't entirely clear - although the size and shape of this one closely matches those installed by the Environment Agency, such items generally have the letters EABM embossed upon them. And this one's blank, so I'm recording it as 'unknown origin' for the moment.