It measures sixteen metres in length, thirteen metres in width, and barely half a metre in height, and its opening faces west. To most eyes it might read as an unremarkable rise in a wet field, but what it is made of gives it away: the mound consists almost entirely of burnt and fire-cracked stone, the characteristic waste material of a fulacht fia.

A fulacht fia is a Bronze Age cooking site, typically found near a water source or in low-lying, marshy ground. The working principle was straightforward: stones were heated in a fire and then dropped into a water-filled trough, bringing the water to a boil quickly and efficiently. Over repeated use, the cracked and spent stones were raked out and piled to the side, gradually building up the distinctive horseshoe-shaped mound that survives today. The opening in the horseshoe is where the trough would have sat, and at Killavallig that gap faces west. There is a slight indentation on the south-east side of the mound, thought to be the result of more recent disturbance rather than anything from the original period of use. Roughly fifteen metres to the south-west, a second fulacht fia has also been recorded, which suggests the area saw repeated or sustained activity during prehistory. Such pairing is not unusual; these sites often cluster near one another, following the same logic of wet ground and available water.