The rock art at Letter sits on a sandstone outcrop at 176 metres above sea level, on a north-east-facing slope that looks out over the Behy River valley. The outcrop itself is modest, barely fifteen centimetres proud of the surrounding ground, but on its western face there is a deliberately decorated surface: a linear row of pickmarks, each roughly four to five millimetres across and two millimetres deep, running just over forty centimetres along a north-west to south-east axis before vanishing beneath the accumulated peat to the north-west. The marks are faint and heavily weathered, and it is entirely possible that the full extent of the carving will never be known.

Rock art of this kind, made by repeatedly striking stone against stone to produce small circular depressions or, as here, a continuous pecked line, is generally associated with the Neolithic and Bronze Age periods in Ireland, though precise dating of individual sites remains difficult. The choice of location is worth pausing over. The outcrop is overlooked on several sides by the surrounding mountains, which gives the site a sheltered, almost enclosed quality despite its open moorland setting. Whether the orientation of the decorated surface, facing slightly south-west while the slope faces north-east, was deliberate is impossible to say with certainty, but it is the kind of detail that prompts the question. The sandstone itself is both smooth and fractured, and the surviving decoration occupies only a small portion of the rock's western side, roughly forty-four centimetres by three centimetres, suggesting a precise and intentional placement rather than opportunistic marking.