Siddall, Ruth; (2023) Ferricrete: London's forgotten Medieval building stone. Presented at: III International Workshop-cum-Seminar on Natural Stones and the Conservation of Heritage Stones, Jaipur, India.

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Abstract

The London Basin lacks any high quality building stones, and this has always posed a problem for local builders. From the Roman period, Kentish Ragstone (a Cretaceous sandstone) was imported from the south-east of England for the construction of major monuments and buildings, including the amphitheatre and city walls. This stone went on to be used in the Medieval period for the construction of churches and prominent secular monuments such as the Guildhall. In outer London and particularly west and north of the city of London, such prestigious building stone was less readily available. Wood, wattle, plaster and brick would have been the main vernacular building materials, and this is in part responsible for the poor preservation of buildings from this period. However a number of churches and (few surviving) secular buildings such as Harmondsworth Barn were constructed using ferricretes, iron-cemented duricrusts primarily obtained from Pleistocene and Holocene fluvial deposits associated with the ancestral River Thames and its tributaries. Somewhat unprepossessing as a building stone, these materials are iron-cemented sandstones, and flint-rich conglomerates and breccias which would have been relatively soft and unpromising as a construction material on extraction but would have hardened considerably during exposure to the elements before being used for building. Ferricretes are also encountered as building stones in East Anglia, where, in similarity to the London Basin, the geology is dominated by Tertiary and Quaternary deposits. The formation and occurrence of ferricretes in the Ypresian Bagshot Sands and Pleistocene Thames Gravels and Kesgrave Sands and Gravels have been investigated and their potential for providing stone for north and west London’s medieval churches will be presented. Such structures include those at High Barnet, Hornsey, Harrow, Pinner and at the important monastic complex of Waltham Abbey as well as at Harmondsworth Barn.

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