They look like natural undulations in the ground, but they are the physical remnants of an industrial process: the making of charcoal.

Charcoal production was carried out on flat or artificially levelled platforms, typically cut into a hillside, where timber was stacked into a carefully managed pile, covered with turf, bracken, or earth to restrict airflow, and then slowly burned over several days. The resulting charcoal was a concentrated fuel used in metalworking and other industries. The platforms at Glendalough number at least 75, each measuring roughly nine metres by six metres, arranged at irregular intervals on the northern and southern sides of the Upper Lake and to the west and south-west of Reefert Church, a tenth-century monastic site associated with the kings of Leinster. A further forty similar platforms were recorded separately, suggesting the scale of activity here was considerable. The site was noted by Ua Riain in 1940 and later by Healy in 1972, though it has received relatively little attention compared to the more celebrated ecclesiastical remains that draw visitors to the valley.

The platforms are most legible on foot, particularly on the quieter paths that skirt the Upper Lake away from the main monastic enclosure. Once you know what to look for, the shallow, flattened ovals become surprisingly easy to pick out in the bankside vegetation, each one a small and functional piece of an industrial landscape that once operated in the shadow of one of Ireland's most significant early medieval sites.