Easy to mistake for natural features or the remnants of forgotten field systems, they are in fact the physical traces of charcoal production, an industry that would once have transformed the woodland here into fuel for iron-working or other heat-intensive trades.

Charcoal was made by stacking wood into a mound, covering it with turf or earth to restrict airflow, and burning it slowly over several days. The process required a flat, stable working surface, and so charcoal-makers, known as colliers, would cut into sloping ground and create level platforms, called hearths or pitsteads, on which to build their stacks. At Glendalough, sometimes referred to historically as Sevenchurches after the number of ecclesiastical buildings in the valley, the evidence survives in remarkable quantity. Seventy-five oval platforms, each roughly nine metres by six metres, have been identified at irregular intervals on the northern and southern shores of the Upper Lake and to the west and south-west of Reefert Church, a small Romanesque church dating to the early medieval period. A further forty platforms of similar dimensions have also been recorded nearby. The platforms were first noted in print by Ua Riain in 1940, and later by Healy in 1972.

The sheer number of platforms suggests organised, sustained production rather than occasional or opportunistic burning. Set against the better-known history of Glendalough as a place of early Christian monasticism, the charcoal-making site introduces a more industrial dimension to the valley, one that tends to go unnoticed by visitors whose attention is drawn to the round tower and the cathedral further down the glen.