Two walkers on the coast at Speeton in North Yorkshire, England, have discovered a series of posts eroding out of a cliff that are thought to have been part of an nineteenth century mine. Terri-Leigh Broadhead contacted local archaeologist Trevor Brigham after discovering the posts while walking on the beach with her father. Brigham at first thought they might have been associated with an eighteenth century clay mine, but upon further investigation decided the posts are more likely to have been part of a nineteenth century 'coprolite' mine, which excavated phosphorus nodules for use in the fertilizer industry.
Phosphate 'coprolites' were mined at a number of sites around the UK in the nineteenth century, following the discoveries that phosphorus (initially obtained from Bird guano in South America) was beneficial in improving the fertility of soils, and then that this could be obtained by treating phosphate nodules with sulphuric acid. At the time all such nodules were widely believed to be coprolites (fossil dung), following publications on the subject by the geologists William Buckland and John Henslow, although, while these mines did produce coprolites and other fossils, the majority of the material would have been non-coprolitic in origin.
Phosphate nodules form when phosphorus-rich organic material, such as Animal dung or bone, is buried in shallow marine settings, where sulphur-oxidising Bacteria will tend to break down other tissues, but phosphorus becomes trapped as phosphate. In environments where these settings are subjected to repeated intervals of sedimentation and erosion, this will lead to an unconformity (part of a geological sequence where there is an erosional surface marking a period of time not recorded) topped by a gravel bed with phosphate nodules. In some settings a sequence of these phosphate nodule-rich gravel beds can build up that is thick enough to be worth mining.
These phosphate nodule accumulations are often fossil-rich, as the remains of the Animals from which the phosphates are derived become trapped in the nodules, and some extremely famous fossil deposits are formed in this way, such as the Kem Kem Beds of Morocco. Victorian palaeontologists were aware of this, and like palaeontologists today, were very interested in phosphate nodules, but the bulk of the material recovered from phosphate mines such as Speeton would have been ground up and treated with acid to recover their phosphorus, which would then have been used as fertilizer to help feed the growing populations of Britain's new industrial cities.
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