The Beginnings of the Broch

The Beginnings of the Broch

In the summer of 2017, Historic Assynt and AOC Archaeology Group undertook a major programme of structural conservation and archaeological excavations at Clachtoll broch, Assynt. In a series of posts over the coming weeks, we’ll share with you the preliminary results of the excavations.

aerial shot

Aerial view of the broch before our excavations began © John Wood

Conservation works were concentrated on the south wall, an area previously identified as vulnerable to collapse, the broken wall ends on the seaward side and consolidation of newly exposed lintels and wall faces. Excavations were carried out on a series of archaeological levels relating to destruction and occupation layers within the broch, including evidence for a large – probably final – fire event and three phases of hearth construction. Deep floor deposits and midden material were found that related to each of the phases of occupation. A sizeable artefact assemblage was recovered including a large quantity of ceramics, stone, bone and metal items including tools, implements and items of personal ornament. We’ll share more about these finds in a later post. The broch was stabilised on completion of the project through the installation of a concrete supporting plinth in the south wall void and supports for broken lintels over several of the gallery and entrance openings. The wall was further consolidated through the addition of pinnings by the project stone masons.

The builders of the broch selected a prominent knoll of outcropping bedrock located on a spur of Torridonian sandstone, flanked by sharp crags to the seaward side, and a steep slope toward the land. The bedrock footings for the broch were evidently quarried – perhaps for the purposes of partially levelling the interior as well as a source of building stone – though this quarrying did not result in a smooth, flat surface on which to build. The bedrock within the broch is steeply undulating, requiring the builders to accommodate a change in elevation of over 1.7m from one side of the broch to the other.

Where not quarried, the bedrock was covered by a fine layer of sandy till. It is likely that a thin skeletal soil had developed over the bedrock knoll in places prior to construction on the site. In several places, clefts and crevasses in the bedrock had been levelled up at an early stage in the construction using rounded cobbles and small stones. The broch is almost exclusively built from Torridonian sandstone, immediately local to the site, though blocks of Lewisian gneiss and conglomerate were occasionally observed within the rubble debris.

Calculations based on wall thicknesses, architectural features and quantities of rubble have suggested that the original building could have stood over 13m tall. The surviving roundhouse is 16.8 m in external diameter, with massive walls of around 4m in thickness. The entrance passage is located in the north-east, and gives access to two guard cells. Two further intra-mural galleries survive: a tall corbelled chamber; and a gallery containing the stair and stair foot return cell. The stairs ascend clockwise to the west, leading to the fragmentary remains of a first floor landing on the seaward side of the broch. A further cell, the vestigial remains of which were first encountered during this project, was located on the west side of the broch, in the area now lost to coastal erosion. A ledge type scarcement, to support the first floor, runs around the interior of the broch at a height of 1.8 m above the bedrock.

The entrance passage displays many archetypal broch architectural features such as orthostatic door jambs and bar hole, and contains the distinctive triangular lintel in the outer wall face. Conservation works undertaken in 2011 on the entrance passage established that the lintelled passage incorporated structural features designed to relieve load stress on the spanning lintels. This evidence, coupled with the vestigial remains of a relieving stacked void above the entrance passage, supported the interpretation that the broch had been designed to be a tall building: when planning the lower courses, the builders gave careful consideration to the engineering challenges posed by building skywards.

laser scan ortho image

3D laser scan data of the broch at the close of our excavations

In several areas of the broch it was observed, both prior to and during the excavations reported here, that there was evidence for significant reconstruction and remodelling of the original building in antiquity. It is likely that the failure of lintels and other load bearing elements precipitated partial collapses in some areas of the building that necessitated rebuilding and buttressing within the Iron Age use of the monument. Full analysis of evidence for these processes will be undertaken as part of the post-excavation programme for the project, and can be expected to fill out our understanding of the history of construction and modification of the broch.

Next week’s post will focus on the earliest deposits relating to the occupation of the broch. Check back here to find out more, or subscribe to receive these posts straight to your inbox, using the button in the sidebar!