That quality of concealment is not incidental; it is, in a way, the monument's defining characteristic, and it was enough to defeat at least one serious antiquarian effort to measure it.

The cairn at Carns is a possible passage tomb, a type of Neolithic monument in which a stone-lined corridor leads to a burial chamber beneath a large mound of heaped stone. This particular example is enormous: roughly 53 to 58 metres in diameter and approximately 7.5 metres high, with a flat top some 23 metres across. It sits at the northern end of a ridge, positioned deliberately along the edge of a short but steep western drop, with the hillside falling away on most sides. The Victorian antiquary W. G. Wood-Martin attempted to record it in 1888 and was defeated by the vegetation, noting that its circumference at the base "could not be ascertained, owing both to the uneven nature of the ground and to its being surrounded by trees, and a close undergrowth of briars, furze, &c." It was not until Stefan Bergh's 1995 study that a proper plan was finally published. The flat top carries several hollows and depressions, the traces of earlier diggings into the mound, and there is evidence of minor quarrying in the south-eastern section. A second cairn of very similar size and construction stands on a hilltop roughly 400 metres to the south-west, suggesting the two monuments were conceived as a pair, looking out over the same river valley from slightly different angles.

The site is a National Monument in State ownership. Because it sits within dense deciduous woodland and remains largely unexcavated, there is no interior to enter and no formal interpretation on site. The eastern and southern faces of the cairn are the best preserved, and the ground level on those sides sits about two metres higher than on the north and west, which gives some sense, once you are close enough, of how the mound was engineered into its particular ridge-top position.