Turf-cutting has a habit of exposing what peat conceals, and here it has begun to reveal the outlines of a world that pre-dates the bog itself: a buried wedge-tomb on the valley's southern slopes, a hut foundation uncovered about sixty metres to its west, and old field boundaries whose stones poke through the turf like interrupted sentences. On the northern bank of the small river that runs along the valley floor, two low circular enclosures sit among the moorland. The larger of the two measures about 9.5 metres across internally, its bank of earth and stone barely fifteen centimetres high and largely grass-covered, with what may be an entrance on its east-south-east side marked by a single upright stone. The smaller is just five metres across, enclosed by an even lower bank. Neither announces itself with any drama.

The valley's archaeology was surveyed as part of J. Cuppage's 1986 study of the Corca Dhuibhne region, and the picture that emerges is one of accumulated but poorly understood activity across a long span of time. A wedge-tomb, a type of megalithic burial monument associated with the Early Bronze Age, lies largely consumed by the bog on the southern slopes. Close to it, old field systems may represent early agricultural organisation, possibly even predating the onset of peat growth in the valley, though this remains uncertain. On the northern side of the river, a large stony cairn measuring roughly 25 metres by 19.5 metres and about a metre in height sits near what may be a cist grave, a small stone-lined pit defined by five inward-leaning slabs with no capstone surviving, open to the east. A standing stone and associated megalithic structure complete the northern cluster. The presence of the wedge-tomb and standing stone points toward Early Bronze Age use of the valley, and it is plausible that the enclosures, the cairn, and even some of the field walls belong to the same broad period, though surface examination alone cannot confirm this.

The modern field system on the northern bank, documented on the first edition Ordnance Survey map and therefore established by the early nineteenth century at the latest, has been laid directly over an earlier arrangement of boundaries, some of which can still be traced as rows of upright stones or low stony banks in the open moorland. The upper reaches of the valley now serve as rough sheep grazing. What makes An Gleann Mór quietly unusual is precisely this layering: nineteenth-century enclosure over possible Bronze Age organisation over a landscape that the bog has been slowly digesting for millennia, each system of use leaving just enough to suggest the one beneath it.