OXFORDSHIRE ROCKS | Whitchurch Sand Formation
[133.9 - 139.4 Ma*]
Image: Concretion 'dogger' at Shotover (Ⓒ Crown Copyright)
* Ma is an abbreviation for million years
White, buff, yellow, orange and red to dark brown, generally unconsolidated fine- to medium-grained sand, with seams and irregular masses of limonite-cemented sandstone and rare siliceous or calcareous concretions, localized beds of grey to white silt and clay or mudstone, and rare ooidal limonitic ironstone.
In Oxfordshire base is an unconformable contact with sands of the Whitchurch Sand Formation resting on calcareous sandstone or calcareous mudstones or siltstones of the Kimmeridge Clay Formation.
The upper boundary is a discordant contact between the sands of the formation and overlying coarse gritty sandstones of the Lower Greensand Group, or calcareous mudstones of the Gault Formation. Mostly occurs as outliers without any succeeding strata.
Up to 20 m thick.
Formation has few fossils preserved, however non-marine bivalves and gastropods have been found that have been used to date the sediments as Valanginian in age (140-134 Ma).
The Whitchurch Sand Formation can be found capping the outliers in the Oxford area.
The Whitchurch Sand Formation can be found in Oxfordshire, Buckinghamshire and Wiltshire.
Shotover Hill is capped by the iron-rich Whitchurch Sand Formation (a former name for this formation was the Shooter Sand or the Shooter Iron Sand).
This geological unit is porous but is underlain by dense clay. Water stored in the sand, flows out at springs around the hill near the junction of the sand and clay.
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In the 17th and 18th century, the valuable pigment Yellow Ochre was mined from the Whitchurch Sand. Thin layers of fine clay embedded in the iron-rich sand, and thousands of years of soil chemistry, produced a material that needed little processing before it could be used by artists. Wheatley windmill was modified to grind the ochre, in addition to milling bread flour.
Casey, R and Bristow, C R. 1964. Notes on some ferruginous strata in Buckinghamshire and Wiltshire. Geological Magazine, Vol.101, 116-128.
Horton, A, Sumbler, M G, Cox, B M and Ambrose, K, 1995. Geology of the country around Thame. Memoir of the British Geological Survey, Sheet 237 (England and Wales).