There are at least seventy-five of them, each roughly nine metres by six, arranged at irregular intervals along the northern and southern shores of the lake and to the west and south-west of Reefert Church. They are the physical remains of charcoal production, an industry that left its mark on upland woodlands all across Ireland and Britain but rarely attracts the attention lavished on the medieval ruins nearby.

Charcoal-making platforms, sometimes called hearths or pitsteads, were levelled terraces cut into hillsides to create a stable, roughly horizontal surface on which stacks of cordwood could be slowly burned under a covering of turf, earth, or damp vegetation. The controlled, oxygen-limited burn converted the timber into charcoal, which was essential for smelting and smithing before coal became widely available. The oval shape of the Glendalough platforms is characteristic of the type. Their presence in such numbers around the Upper Lake suggests sustained industrial activity at some point, drawing on the surrounding oak and mixed woodland, though the precise period of that activity is not recorded. Ua Riain noted the platforms as early as 1940, and a later survey by Healy in 1972 identified a further forty examples of the same form, indicating that the full extent of the site had not initially been appreciated.