Hertfordshire puddingstone

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Specimen of Hertfordshire puddingstone

Hertfordshire puddingstone is a puddingstone, a type of conglomerate sedimentary rock, which is largely confined to the English county of Hertfordshire. Superficially, it resembles concrete, but it is entirely natural, comprised of rounded flint pebbles bound with a younger matrix of silica quartz. Its name derives from a resemblance to Christmas pudding.

The flints were eroded from the surrounding chalk beds some 56 million years ago in the Eocene epoch and were transported by water action to beaches, where they were rounded by wave erosion and graded by size. A lowering of sea levels and general drying during a brief arid period drew out silica from surrounding rocks into the water immersing the flint pebbles. Further drying precipitated the silica which hardened around the pebbles, trapping them in the matrix. Puddingstone is commonly found in river beds, and is less frequently exposed to the surface.

Oxides of iron were also trapped in the silica matrix, giving rise to many different hues when the puddingstone is examined closely. From a greater distance, puddingstone is generally brown or ginger in colour, although pink is possible.

The silica is very hard, which led to use being made of puddingstone as an auxiliary building material supplementing flintstone buildings such as St Mary's Church, Stocking Pelham; as a decorative feature or waymark in Hertfordshire villages, such as at Watton-at-Stone; or, during Roman times, for grinding corn.

Hertfordshire puddingstone was credited in local folklore with several supernatural powers, including being a protective charm against witchcraft. Parish records from the village of Aldenham relate that in 1662 a woman suspected of having been a witch was buried with a piece of it laid on top of her coffin to prevent her from escaping after burial.