Ninety per cent of the decorated surface is now covered in lichen, which makes the carvings extraordinarily difficult to read, and the rock itself sits at 277 metres above sea level on an ENE-facing slope, overlooked on three sides by mountains and within earshot of a watercourse draining from Coomnacronia Lake.

The boulder is a smooth, unfractured piece of sandstone measuring roughly 3.10 metres east to west and 1.55 metres north to south, with a maximum height of about 60 centimetres at its eastern end, where a natural step sits some 27 centimetres below the decorated surface. What has been recorded across that surface is a composition of considerable variety for its size. Cupmarks, the simplest form of prehistoric rock art, are shallow circular depressions pecked into the stone, here ranging from 3 to 12 centimetres in diameter. Around several of these, the carver added concentric rings, producing what are called cup-and-ring motifs, the most characteristic type of Atlantic rock art found across Ireland, Britain, and parts of western Europe. At Letter, there are eleven plain cupmarks distributed across the central portion of the rock, seven cup-and-one ring motifs concentrated towards the south, and three cup-and-two ring motifs slightly off-centre to the south. Several of these more complex motifs have a radial groove running outward from the central cupmark through the rings to the exterior, a detail that appears consistently in prehistoric rock art but whose precise significance remains unresolved. One cupmark has been placed directly on the outer ring of a cup-and-two ring motif at the western end of the surface, a small compositional decision that feels deliberate, even if its meaning is entirely lost to us.

The site sits near a scatter of glacial boulders flanking the watercourse to the south, with a view across the valley towards Coomaglaslaw Lake to the southeast. The combination of faint, lichen-obscured carvings and the remote, boggy terrain makes this a site that rewards patience and low-angle light rather than a casual glance.