Subtle enough that a casual walker might take them for natural undulations in the terrain, they are in fact the remnants of an industrial process that once shaped much of upland Ireland: the making of charcoal.

Charcoal production required flat, level ground on which to stack and slowly burn timber under a covering of earth and turf, a technique that left behind distinctive platforms known as hearths or pitsteads. At Glendalough, researchers identified as many as 75 such platforms on the northern and southern sides of the Upper Lake, each measuring roughly nine metres by six metres and arranged at irregular intervals through the landscape. A further 40 platforms of similar dimensions were recorded separately. The sites were noted by Ua Riain in 1940 and again by Healy in 1972, suggesting the features were substantial enough to attract repeated scholarly attention across several decades. The proximity to Reefert Church, one of the early medieval monastic buildings within the Glendalough complex, raises quiet questions about the relationship between the monastery and any organised woodland industry in the valley, though the precise period of the charcoal-making activity is not firmly established from the available records.

Visitors walking the woodland paths along the Upper Lake are, in a sense, already moving through this site. The platforms are dispersed across a relatively wide area, and without knowing what to look for they read as little more than gentle shelves in the slope. The oval shape and the consistent dimensions are the telling features, distinguishing these levelled hearths from the ordinary contours of a glacially carved valley floor.