A petrological examination of the provenance of stone masonry from three medieval churches in Shetland

BGS has been commissioned by Allen Fraser and Jenny Murray, on behalf of Shetland Museum & Archives, to conduct a petrographic examination of sandstone masonry from three churches in Shetland and sandstone bedrock from two historic quarries in Orkney, and use the results to test a hypothesis that the stone forming the Shetland masonry was sourced from Orkney quarries. The client has provided seven samples of sandstone from masonry blocks, and details of the sampled localities. The blocks are believed to originate from three medieval churches in Shetland, but are now spolia (stone fragments removed from one built structure and incorporated in the masonry of another, later structure). All three of the medieval churches were constructed in the C12th and demolished in either the C18th or C19th. Ashlar blocks and rubble from St Magnus’s Church (Tingwall, Mainland) and St Laurence’s Church (Papil, West Burra) were incorporated within the masonry of new churches that were built on the same sites. St Mary’s Church (Ireland, near Bigton, Mainland) was not replaced after demolition, but stone blocks believed to be from the church have been found in the walls of nearby buildings. The client has also provided one sample of sandstone from Head of Holland quarry (Mainland, Orkney). Four samples of sandstone bedrock from the BGS Rock Collection – two from Head of Holland quarry and two from Fersness quarry (Eday, Orkney) – have been included in the assessment, so that the full petrographic character of these two historic quarries can be assessed. Both quarries are sited within bedrock assigned to the Eday Group, a geological formation that crops out exclusively in parts of Orkney. A thin section (a slice of stone cut thin enough to be transparent, so it can be examined by microscope) was prepared from each sample. The petrographic description for each sample includes information from the hand sample and the associated thin section. Brief details of each sample, with a hand sample photograph, are provided in section 2 of this report. A full petrographic description for each sample, with thin section photographs, is provided in Appendix 1. The key petrographic characteristics are described in section 3, and these are discussed with reference to the provenance of the masonry samples in section 4. Key conclusions are summarised briefly in section 5.