The rock is roughly rectangular, about seven metres long and two and a half metres wide, and its upper surface is covered with a concentration of prehistoric markings that would be easy to walk past and impossible to forget once you have actually stopped to look. There are more than twenty cupmarks, simple shallow depressions pecked into the stone, alongside fifteen or more cup-and-rings, where one or more concentric rings surround a central cup. Then there are the rosettes, eight of them, which are rarer and more intricate: each one consists of a central cupmark roughly four centimetres across, enclosed by a ring of evenly spaced mini-cupmarks each about two centimetres in diameter. Five of these rings are made up of exactly seven mini-cupmarks, two have nine, and one small example has just five. Meandering grooves run across the surface, some intersecting one another, some curling around to enclose individual motifs.

Rock art of this type, sometimes called Atlantic rock art or prehistoric cup-and-ring art, is found across Ireland, Britain, and parts of Atlantic Europe, and is generally associated with the Neolithic and Bronze Age periods, though precise dating remains difficult. The rosette form, with its neat ring of satellite cupmarks, is less commonly recorded than plain cup-and-rings, which makes this outcrop at Letter particularly notable. The site was identified and described by Aoibheann Lambe, whose detailed recording captured not just the count and arrangement of motifs but their spatial relationships, including which marks cluster together, which appear to stand alone, and how the grooves connect or frame the composition. The rock sits about ten metres north of a stream, and from its surface the view opens south-west towards Mullaghnarakill hill and north-east past Laughtshee in the direction of Castlemaine Harbour, a placement that may or may not be coincidental but is difficult to ignore once you are standing there.