Sandstone Heritage in a Climate Change(d) Future: Weathering of Torridonian and Devonian building stone and its implications for conservation

Abstract

Sandstones are important to Scotland’s heritage as a traditional building stone from the Neolithic to the present day and are particularly susceptible to weathering. This thesis focuses on two Scottish sandstones; Torridonian Sandstone and Stromness Flagstone, found in Clachtoll and Borwick Brochs, respectively. Culturally significant sandstones of these types were obtained from archaeological spoil associated with each site, and are weathered stone initially exposed above ground during the Scottish Middle Iron Age (Clachtoll C14 date; ca. 2025 ± 30 BP). Samples of each sandstone were also collected from geological outcrops as unweathered control rock. The baseline condition of control rock and broch stone has been determined with focus placed on how the stone had weathered since their emplacement above surface. Petrographic thin section analysis, X-Ray diffraction (XRD) analysis, and scanning electron microscope with dispersive X-Ray analyser (SEM-EDX) analysis results have identified weathering features brought about by exposure above surface, particularly in the Stromness Flagstone. Conversely, the Torridonian Sandstone broch stone appears robust. These findings formed a baseline of each sandstone type and how they have weathered to date. Through exposure to warmer, wetter winters and warmer, drier summers in a climate changed future in Scotland, the weathering of these sandstones may accelerate. To understand this, innovative controlled environment facility (CEF) experiments have been conducted, whereby sub-samples of control rock and broch stone were subjected to systematic accelerations of climate change parameters, informed by UKCP18 data. In both the ‘Climate Changed Year’ and ‘30-Year Seasonal Climate Change’ experiments, broch stone, particularly from Borwick, is seen to weather at an increased rate under climate changed conditions, compared to the present day. This is evidenced by surface failures, salt formation and dolomite dissolution exacerbated by sulphide weathering in Stromness Flagstone samples, as well as increased granular loss and vertical cracking in Torridonian Sandstone.