Though not on the modern 1:25000 map, the 1891 map shows what might read "well" just south of the top of Pen-Y-Fan (which is the higher of the two major peaks of the Brecon Beacons). Alternatively, it may read "wall", but this seems unlikely given the absence of a wall on the modern maps, and the survey line(?) extending from it.

You can find a description of a visit to "the spring" (probably the "well" on the map above) before 1893 here (note 56), which suggests Gerald (See Note) mixed up the spring and a pool on the north-west of the mountain, though it is hard to see why the latter wouldn't account for the whole description.