They predate the peat that now surrounds them, meaning they were already old when the bog began to form and simply kept going, preserved under the accumulation of centuries. These are pre-bog walls, a category of field boundary that tends to escape casual notice precisely because the landscape has grown up around them and, in many cases, over them. Here, the walls emerge in both straight and curving stretches, the longer runs following the natural fall of the slope, with few intersections between them. The longest individual stretch reaches approximately 50 metres; most are roughly half a metre high and slightly less than a metre wide, built from slabs and boulders without mortar.

The walls are only part of what survives at Treangarriv. Scarped into a slight ledge on the slope are the remains of a circular drystone hut with a corbelled roof, a construction technique in which stones are laid in overlapping courses that gradually close inward to form a self-supporting vault without the need for timber or mortar. The southern portal of its entrance, just 75 centimetres wide, is still in place, and two large stones lying nearby to the east may be the displaced second portal and lintel. The interior measures 2.4 metres across, and the walls, now largely collapsed inward, survive to a maximum height of 40 centimetres. A short distance to the east stands a roughly built subcircular enclosure of boulders, with an internal diameter of just under 3 metres. Whether the hut and enclosure were used for sheltering animals, as a seasonal habitation, or for some other purpose is not recorded, but the combination of managed field boundaries, a small dwelling, and a stock enclosure suggests a coherent, if modest, agricultural presence on this mountain slope at some point before the bog overtook it all.