GeoGraph - photograph every grid square

This image is taken from the opposite side of Blaen Nanmor, but shows this amazing dry stone wall climbing almost vertically up the very steep mountainside on the opposite side of the valley. On flat land two men can build about 2 metres of dry stone wall a day, I wonder how long this took. There are ancient settlements at the top of the hill, you can just make out some of the stone circles on the plateau just right of centre on the ridge in the picture.