There are at least 75 of them, each roughly nine metres by six, arranged at irregular intervals along the northern and southern shores of the Upper Lake and to the west and south-west of Reefert Church. They are charcoal-burning platforms, the physical signature of an industrial process that once took place quietly in the woods above one of Ireland's most visited monastic sites.

Charcoal was produced by stacking wood into a carefully managed pile, or hearth, covering it with earth or turf to restrict airflow, and allowing it to smoulder slowly over several days. The process required flat, level ground, which in a steep-sided glacial valley like Glendalough meant cutting platforms into the slope. The resulting terraces, oval in shape to suit the circular hearths built upon them, are sometimes called pitstead platforms. They leave no dramatic ruin, just a subtle levelling of the ground that reads as natural unless you know what to look for. Ua Riain noted the Glendalough examples as early as 1940, and a later survey by Healy in 1972 recorded a further 40 similar platforms in the same general area, suggesting the operation was substantial rather than occasional. Charcoal production on this scale would likely have served iron-working or other fuel-intensive industries, though the valley's monastic community would also have had use for a reliable supply.