Abstract

Blocks of Forest of Dean Stone, an arkosic, clay-rich sandstone of Lower Carboniferous age, which were used in the construction of a sea wall at Weston-super-Mare in the 1880s, now display extensive honeycomb weathering features. Spatial variations in the degree of weathering, together with geochemical evidence, suggest that marine salt spray has played a key role in the development of the honeycombs. Petrographic microscope and SEM studies indicate that the principal weathering mechanisms involved are granular disintegration and micro de-lamination induced by chemical alteration along inter-grain boundaries and differential swelling and contraction of clays within the rock.

Salt crystallization and hydration pressures within the rock pores appear to have played a minor role in this case. The formation of alveoli has been favoured by initial irregularities on the surfaces of the stone blocks which allowed differential ingress and retention of saline solutions. Progressive enlargement of the alveoli continues until the boundary walls between adjacent cavities are breached and the honeycomb layer disintegrates. Over the past century the maximum rate of surface lowering of the honeycombed surfaces at Weston-super-Mare has exceeded 1 mm a-1.

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