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Screwstones
In Derbyshire and elsewhere in the English Midlands, internal moulds of Carboniferous sea-lily stems are referred to as screwstones. This peculiar fossil preservation results from the dissolution, by percolating groundwaters, of the calcite skeleton of the stem itself, leaving a sediment-filled axial hole. The width of this hole expands and contracts along the length of the stem, being widest at the joints between the columnals and narrowest at their centres. The flanged structure resembles a screw but the flanges are separate and do not form a continuous helical thread like a true screw.
A totally different kind of screwstone, which is helical, is formed by the internal moulds of snail shells.
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