They are, in fact, the physical remains of charcoal production, an industry that once operated at considerable scale in this corner of County Wicklow.

Charcoal was made by stacking and slowly burning wood under a covering of earth or turf to restrict airflow, a process that required a level, cleared platform, known as a hearth or pitstead, on which to build the mound. These oval earthworks, each measuring roughly nine metres by six metres, are the flattened terraces where that work took place. There are at least seventy-five of them recorded on the northern and southern sides of the Upper Lake and to the west and south-west of Reefert Church, one of the early medieval ecclesiastical ruins for which Glendalough is known. A further forty similar platforms were recorded separately, suggesting the operation was more extensive than any single survey captured. References to the site appear in sources from as early as 1940, with Ua Riain noting the platforms that year, and Healy returning to them in 1972.

The platforms are spread at irregular intervals through the valley, which gives some sense of how the work was organised, opportunistically positioned close to timber rather than laid out to any formal plan. Visiting the area around Reefert Church and the Upper Lake, it is worth looking carefully at the ground beneath the trees. The hearths are subtle features, and knowing what to expect makes them considerably easier to spot.