To most visitors walking the valley's well-worn paths, they register as nothing more than gentle irregularities in the ground. In fact, they are the physical remains of an industrial charcoal-making operation, a largely forgotten layer of the valley's past lying quietly among its more celebrated monastic ruins.

Charcoal was produced by stacking wood on a levelled platform, covering the pile with turf or earth to restrict airflow, and allowing it to smoulder slowly, sometimes for days. The levelled oval platforms left behind by this process are known as hearths or pitsteads, and they survive remarkably well in woodland soil where they have not been disturbed by ploughing. At Glendalough, over seventy-five of these platforms have been recorded at irregular intervals along the northern and southern sides of the Upper Lake and to the west and south-west of Reefert Church, one of the valley's early medieval stone churches. A further forty similar platforms have also been noted in the area. The sites were recorded by Ua Riain in 1940 and later by Healy in 1972, though the precise period of activity has not been pinned down in what survives from those accounts.

Reefert Church sits in a part of the valley that most visitors reach only if they continue past the more immediately conspicuous round tower and cathedral. Those who do make the walk will find the church ruins in a quiet clearing, and if they look carefully at the uneven ground in the surrounding woodland, the subtle platforms begin to resolve out of the forest floor, each one a faint but legible trace of the work that once went on here.