Together they measure little more than half a metre across, and yet they represent one of the more quietly ingenious pieces of early medieval engineering on the island. They were not decorative. They were the water supply.

The two rectangular hollows sit close to the western corner of the oratory, the remains of a small stone chapel used by the monks who occupied this extraordinary Atlantic rock from around the sixth century onwards. The north-eastern basin, roughly 50 centimetres by 36 centimetres and just 12 centimetres deep, has a circular depression cut into its base and drains via a small channel into a second, slightly narrower basin to the south-west. Both are fed by a channel running beneath the north-western wall of the oratory and by two grooves cut into the rock face above, directing rainwater down from the cliff. The whole arrangement, modest in scale but precise in execution, formed a cascade: rainfall gathered, was filtered through the system, and was stored in sequence. The existence of the flagged path leading to this terrace was itself unknown until relatively recently. It only came to light after a researcher named Rourke cleared sea campion, a low-growing coastal plant, from around the north wall of the oratory, revealing a route that zigzags down the cliff face along a natural ledge with cut steps. At the foot of the steps descending to the oratory, the basins appeared. Scholars writing in 1990 noted that their shape and location made their purpose unambiguous: they were for collecting water, and without them, the monks occupying this exposed and rainswept terrace would have had no ready source.