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Hertfordshire puddingstone is a conglomerate sedimentary rock composed of rounded flint pebbles cemented together by a younger matrix of silica quartz. The distinctive rock is largely confined to the English county of Hertfordshire but small amounts occur throughout the London Basin.[1] Despite a superficial similarity to concrete, it is an entirely natural silcrete. A fracture runs across both the pebbles and the sandy matrix as both have equal strength unlike concrete where the pebbles remain whole and a fracture occurs only in the matrix. Like other puddingstones, it derives its name from the manner in which the embedded flints resemble the plums in a pudding.[1][2] It forms the local base of the Upnor Formation of the Lambeth Group.
Geological origin
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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 known as the Paleocene–Eocene Thermal Maximum 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 rarely found in situ in the strata but its hardness has preserved loose rocks and boulders commonly found in river beds, and less frequently exposed at the surface.[2][3] A well-researched outcrop lies at Colliers End near Ware.[1] Another large piece lies at the bottom of the trout lake in Fisheries Road, Hemel Hempstead. This was discovered by contractors deepening the lake in 1975. It is 30 ft x 20 ft by 3–4 feet thick.[4]
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 density of flint inclusions shows notable variation between specimens.[1][2]
Rock similar to the sand matrix of Hertfordshire Puddingstone, and with similar silica cement, but lacking the pebbles, occurs further west in Southern England, and is called Sarsen stone.[5] Most of the stones at Stonehenge and Avebury are sarsens.
In folklore
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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. In living memory a piece of Pudding Stone was given to a bride and groom, possibly as a fertility symbol.[7] Its supposed magical powers gave it the names of woe stone, hag stone or witch stone. It was also called grow stone or breeding stone because of a related belief that it could multiply itself.[4]
See also
- Geology of Hertfordshire
- Hertfordshire for a general description of the county.
- Roxbury puddingstone
- Woolwich-and-Reading Beds
References
- Hertfordshire RIGS Group, Herts County Council (2003). "A Geological Conservation Strategy for Hertfordshire" (PDF). Archived from the original (PDF) on 27 September 2011.
- Hertfordshire Archives and Local Studies, parish records
- ^ a b c d Lovell, Bryan; Jane Tubb. "Ancient Quarrying of Rare in situ Palaeogene Hertfordshire Puddingstone" (PDF). Mercian Geologist (2006 16 (3)): 185. Archived from the original (PDF) on 25 April 2012.
- ^ a b c Lovell, Bryan; Jane Tubb. "Hertfordshire Puddingstone". Hertfordshire Geological Society. Archived from the original on 25 July 2011. Retrieved 9 November 2011.
- ^ Puddingstone song, Mike Excell singing the Puddingstone Song for Hertfordshire Geological Society, June 2009
- ^ a b Hands, Roger and Joan; Davis, Eve (1989). The Book of Boxmoor (1994 ed.). Barracuda Books. ISBN 0-86023-544-0. page 14
- ^ Hertfordshire Puddingstone Archived 5 July 2008 at the Wayback Machine East Herts Geology Club, Dr. Steve Perkins , August 2005.
- ^ Quern Tastic!, Shelley, Dominic
- ^ Grovehill Chronicle , Ruth Clinch, 2010. Accessed Oct 2011