The boulder is barely the size of a large chopping board, low and roughly square, but what distinguishes it is the deliberate geometry pressed into its stone. These are cupmarks, shallow circular depressions ground or pecked into rock by prehistoric hands, and here they are arranged with unusual precision into two concentric rings.

The decorated surface is subcircular and generally flat, with a slight southward tilt. Fifteen cupmarks in total occupy it, averaging around four centimetres in diameter and three to five millimetres deep. Seven form an inner ring enclosing a roughly circular central area, and eight form an outer ring, some of them pressed close to the very edge of the carved surface. The effect is concentrated and considered, two loops of small hollows radiating outward from a shared centre on a piece of smooth, unfractured sandstone. A second piece of rock art lies only about twenty centimetres to the southeast, which suggests this particular stretch of hillside was a place people returned to, or at least marked more than once. The Seefin and Droum Mountains form the backdrop to the northeast, and the Behy valley opens out below. Whether the location was chosen for its views, its geology, or reasons entirely lost to us is impossible to say.

Cupmark rock art of this kind is found widely across Ireland and Britain, and dates broadly to the Neolithic and Bronze Age periods, though precise dating of individual sites is rarely possible. What makes the Letter example quietly arresting is the small scale and the care with which the rings are laid out on such a modest stone. The outer ring follows the edge of the decorated surface almost exactly, as though the carver was working to a boundary already understood before the first mark was made.