What is a septarian nodule? Well, basically, it is a big boulder containing a three dimensional jig-saw of smaller angular pieces of the same rock – and all the pieces are bound together with white crystalline calcite. I can’t do better than to quote the definition given in The Oxford Dictionary of Earth Science:
A concretion, roughly spheroidal in shape, usually of clay ironstone, and characterised by an internal structure of angular blocks separated by radiating mineral-filled blocks. The mineral filling the cracks is usually calcite. The structure results from the formation of a hard exterior to the nodule due to the development of an aluminous gell on the exterior, followed by dehydration of the colloidal mass in the interior, leading to cracking and subsequent infilling of the radiating pattern of cracks.
The British Regional Geology Series for the area indicates that the Ringstead Waxy Clays, which are virtually at the top of the Corallian Beds of the Upper Jurassic strata, comprise about 5 metres of clay with thin seams of clay ironstone that are nodular in places. It seems very possible that the septarian nodules are from this source. The Ringstead Waxy Clay is also the deposit in which numerous fossil oysters, Deltoideum (Liostrea) delta, are found [mentioned elsewhere in Jessica’s Nature Blog and also on the sister site Oysters etc.]
Oxford Dictionary of Earth Sciences, Edited by Michael Allaby, Oxford University Press, first published 1990, third edition 2008, ISBN 978-0-19-921194-4
The Hampshire Basin and adjoining areas, R. V. Melville and E. C. Freshney (1982), British Regional Geology Series, Fourth Edition, Institute of Geological Sciences, HMSO, ISBN 0-11-884203-x.
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