Probably few people will be aware that a Local Geological Site has recently been designated within our parish, - or more accurately astride the parish boundary between Brightwell and Little Wittenham. It is a small outcrop at the passing point inside the bend in the sunken lane which leads up towards the Wittenham Clumps. The former pit is largely invisible from the road, obscured by nettles and the overhanging branches and roots of a large hornbeam tree. The site belongs to the Northmoor Trust.
The sedimentary rock exposed here belongs to the Upper Greensand, deposited as marine sediment some 100 million years ago, and lying stratigraphically above the Gault Clay and below the Chalk, the latter now forming the crest of the Clumps. The Upper Greensand underlies much of our village, but in this area is neither green nor particularly sandy. It is a creamy-grey rock, known historically as Malmstone, which weathers to a sticky, pale-grey soil. Although it resembles a greyish chalk it is in fact largely made up, not of calcium carbonate material, but from tiny particles of silica derived from the spicules of ancient sponge organisms. It contains few fossils. A slight greenish tinge is due to the presence of glauconite, a sedimentary mineral which contains potassium. This is believed to act as a natural fertilizer,-which may explain why the belt of Upper Greensand soils between Brightwell-cum-Sotwell, Milton Hill and Harwell proved so suitable for fruit growing.
The Malmstone is a permeable rock, in contrast to the underlying Gault Clay, so downward percolating groundwater emerges at the surface contact between the two. This determines the line of springs running through our village.
A Local Geological Site (formerly known as a Regionally Important Geological Site) is designated to indicate a site worthy of active conservation. A panel of county geologists and environmentalists made the recommendation on the basis that this is perhaps the only publicly-accessible site in the county where the Upper Greensand is exposed. There are a total of 42 Local Geological Sites across Oxfordshire.
It is not clear when this small pit was actively quarried, or how the stone was used. The rock is probably too soft to make a walling stone, but could have been used for some agricultural purpose. I would be pleased to hear from anyone who may know more about the origins and use of this pit.
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