My first thoughts were limestone fern a species that's something of an enigma to me. Apparently recorded in their thousands on exposed east facing slopes around Blaenavon I've yet to find them myself. That's why when examining a disused agricultural building complete with post-medieval enclosures north of the Keeper's Pond known as Pen-Rhiw-Ifor, it was a fair assumption that the extensive population of ferns growing from the mortar of the brickwork were in fact the infamous limestone fern. Alas after closer examination they weren't, but there were a interesting diversity of ferns on view nonetheless.
|Brittle Bladder Fern|
|Maidenhair Spleenwort & Harts Tongue Fern|
|Lemon Scented Fern|