Approaching the mind of the builder: analysis of the physical, structural and social constraints on the construction of the broch towers of Iron Age Scotland

Abstract

Following a review of the paradigmatic context of broch towers in 2012, a revised standard model (the RSM) was defined. The then prevailing paradigm supports a view of broch remains as single monuments of highly variable form that continued in use over perhaps a millennium or more, without significant modification of their original tectonics i.e. their people/constructed-space relationships.

This thesis challenges the pre-2012 paradigm by testing the hypothesis that brochs were built to the standard canonical form of the RSM and that their apparent diversity results from anthropic and, or natural modification, not design variability. The fieldwork tests could but did not find refutation of these hypotheses in the observable evidence and offered more profound interpretations of several surviving feature-types.

The loading on the stone lintels of the entrance passage through the massively built outer wall and the structurally overladen inner wall created a major structural challenge, evoking a complex engineering solution. Its elements were individually noted pre-2012 but the significance of the engineering response to compression management had not been identified. This structural response was necessary for a tall structure with massive loads, and meaningless without one and its elements are therefore, jointly and severally, clear diagnostics of a broch tower. The entrance engineering was probably the inspiration of one individual or of a small group of master mason-types, not vernacular responses, contra the 2012 paradigm. Isolated stacked voids high in the inner wall are relict features indicative of significant modification of the inner wall. Other anomalous features are shown to be relict stacked void fragments.

The East/West differences in brochs across Scotland have long been identified and these are generally attributed to their lithologies. Accepting that, this thesis argues that the principal differences are attributable to the social processes that gave rise to centralisation of settlement around, in and over brochs in the east and north, possibly during the first century BC, and the absence of centralisation in the west; perhaps also explaining the differences in the scale and composition of the artefact assemblages between the two zones.

The canonical form facilitates calculation of the relative social costs of broch building for hard-rock and sedimentary stone types. This indicates that the costs of building, increase between 16-, and 32- fold over the buildable range of brochs. Constraints of design down-scalability, design weakness in ground loading, and design cost were major constraints on the mind of the broch builders. Canonicity and the limitations of drystone building technologies predicated specific forms of decomposition on the canonical broch, further complicating their autobiographies and their conservation: the main challenge now being that of finding ways to conserve the evidence for a sequence of processes while conserving the products of those processes.