I was intrigued by these boulders of a very distinct rock type found on the shore between Chapmans Pool and Egmont Bight in Dorset, England. The rocks have many inclusions – either continuous rows or bands, or a series of nodules, of a harder substance than the surrounding limestone matrix. It seems that these boulders are derived from the Portland Cherty Series which is part of the Portland Limestone Formation belonging to the Upper Jurassic Period. The inclusions or nodules are chert (like flint, made mostly of silica). This rock type is visible in various place along the Dorset coast but particularly on the Isle of Portland and between Durlston Head and Chapmans Pool. These beach boulders have fallen from near the top of the adjacent cliff (was at one time a hill) known as Houns-tout. The way in which the silica bands and nodules were formed is quite complex. There is a detailed account of the process in the paper by Gorman et al (1993) who summarise their investigation by saying:
Chert bands are the product of chemical segregation of silica which was originally dissolved in the pore waters of the sediment and derived from the dissolution of the siliceous sponge spicules, the original carbonate having been dissolved from the chert areas concurrent with the precipitation of silica.
Gorman, I., Hart, M. B., Williams, C. L. (1993) Chert formation in the Portland Limestone Formation (Upper Jurassic) of the Dorset Coast: a preliminary investigation, Proceedings of the Ussher Society 8, pp181 – 185.
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