Going Underground: a Souterrain within a Broch?

Posted by on Feb 16, 2018 in Artefacts, Excavation, Public archaeology | No Comments
Going Underground: a Souterrain within a Broch?

In our last post we looked at the earliest hearth, found in the centre of the broch. Today we turn our attention to an unusual design feature: integral to the broch’s internal layout was a drystone-lined lower passage. This feature was formed by the bedrock shelf on one side (see earlier post for more on the quarried bedrock) and the broch wall on the other. This lower area was formed by the accommodation of the bedrock outcrop by the broch wall, forming a step down to a curving, passage-like space in the eastern side of the broch. This space was lined by three distinct sections of walling, none of which was particularly well constructed and were apparently prone to collapse. It may be that these three sections of walling represent rebuilds or repairs of the original lining. However, it does seem probable that the lower area was divided into at least two compartments, the northern area being paved with sandstone flags. This overlay a dark grey/black silty organic clay which in turn directly overlay the bedrock. The upper stone of a rotary quern was found lying flat on this flagstone floor.


A feature we are tentatively interpreting as a souterrain or sub-floor storage chamber

It is not certain how this structure integrated with the remainder of the broch floor. However, two features lend weight to the interpretation that it was roofed or floored over, providing a continuation of the broch floor and creating a subterranean passage. Firstly, the entrance to a cell is located c. 1m above the foot of the inner broch wall, at roughly the same level as the top of the revetment wall, so that a timber floor spanning the passage might have rested on the revetment wall and met the broch wall at the threshold to the cell. A projecting ledge of rough stonework south of the entrance to the cell may have provided additional support for this roofing. If this feature was roofed, it seems likely that the sub-floor space would have been accessed from the south, where the bedrock steps down to the flagstone floor. The floor itself would most likely have been lightweight, perhaps wicker, since there was no evidence for designed integration with the broch wall.

It is not possible to be certain as to the function of this structure but the sub-floor space shares characteristics with ‘souterrain’ and other sub-floor voids commonly found in brochs of the north mainland and Orkney. Such features are often described as wells, though their practical function as a source of fresh water is dubious, and this cannot have been the purpose of the Clachtoll structure. It is more probable that the structure was formed by necessity in accommodating the bedrock knoll, and that by continuing the floor across to the entrance to the cell, a convenient sub-floor space was created.

In removing the infill of rubble and soft deposits from this feature, thick deposits of clay/peat ash were excavated, overlying a mixed deposit containing large quantities of animal bone and charcoal flecks and beneath this, another mixed, rubbly deposit containing numerous finds including spindle whorls, stone lamps, ceramics and worked bone. This occupation debris might be interpreted as infill or deliberate dumping of refuse, but it was considered at the time of excavation that this might instead represent the collapse of the flooring over the lower feature, resulting in the accumulation of large quantities of artefacts and other debris as they fell down into the sub-floor space.

For now, our interpretation is tentative, and we await further analysis! In the meantime, look out for our next post, which will look at the second of three successive central hearths. To make sure you don’t miss a thing, why not sign up to receive these blog posts direct to your inbox? Just pop your email address into the box in the right hand toolbar!