Excavation of a Souterrain in the Outer Hebrides

Knockaird souterrain viewed from entranceIn early 2018, a team of AOC's archaeologists was called out to the Western Isles after the surprise discovery of a souterrain (right) during construction of a new house at Knockaird, Ness, on the Isle of Lewis. The site was identified by the Comhairle Archaeology Service at the start of construction, below the collapse of lintel capstones under the weight of a mechanical excavator. These underground, or sous terrain, structures are associated with Iron Age Scottish sites and, while their functions is still up for debate, the most common interpretation is that they served as a secure, cool structure for food storage.

The souterrain comprised a curvilinear, stone-built structure with a stepped entrance and lintelled doorway that led into the main chamber, measuring overall 6.6m long NW-SE by 1.2m wide and up to 1.4m high. The souterrain was accessed by a shallow entrance pit (in the foreground in the image to the right) that was roughly revetted with stone to access three slab-built steps. The passage walls had been well-constructed and lined with corbelled, drystone walling of small sandstone slabs capped with lintels. This short passage led to a doorway built of upright slabs supporting lintels and several courses of drystone walling to connect it to the main chamber of similar drystone construction. While the main chamber was substantially waterlogged, it contained a rich organic silt base layer.

This excavation revealed not only the construction and detail of the souterrain, but, also significantly, revealed that the entrance had been closed off during a single depositional event. Substantial sherds of decorated Iron Age pottery, potentially representing a single vessel, and several coarse stone tools were recovered from this layer. Also of interest was the discovery of a c.1.2m long hearth pit built into the surface of the east side of the structure, containing charcoal and peat ash overlying a sandstone slab.

Knockaird souterrain and pottery

Above, L-R: view from the entrace with the chamber at the back; view from the entrance through the doorway into the main chamber; large sherd of decorated Iron Age pottery recovered from the backfill of the souterrain.

The Knockaird souterrain is the latest of several souterrains known in Ness parish, some of which are associated with domestic Iron Age structures known as wheelhouses. The purposeful backfilling of Knockaird souterrain entrance is similar to a recently excavated souterrain in Broadford on the nearby Isle of Skye. Similar in size and construction, the Broadford souterrain also revealed a single closure deposit containing fragments of a large Iron Age decorated pot and a hearth setting next to the surface of the structure.

Funded by the Comhairle nan Eilean Siar, the rescue excavation and full recording of Knockaird souterrain ensured that important archaeological information was not lost forever during the development.