We are the Local Environment Group for the area around Prestwood in Buckinghamshire, including Great Missenden,
The Hampdens, The Kingshills, North Dean and Speen
We aim to protect and enhance the quality of the natural environment through the involvement of local people.
The principal surface rocks of our area are “soft” ones like chalk and clay, so that there are no major geological sites. There are four aspects of interest. The use of some of the clays as brick-earth is mentioned under “Heathlands”. The other three are Denner Hill stone, chalk-rock and swallow-holes.
In places ancient rivers had cemented sand grains and pebbles with the chalk they had picked up, creating under the pressure of hundreds of feet of clay, super-hard boulders or sarsens, known locally as Denner Hill stone, from the site of one of the main aggregations of these stones, that were excavated, sometimes from many metres down, and worked into pavement slabs and building stone that survives still in the pavements of Windsor and town halls in Wycombe and Aylesbury. Such boulders, no longer of economic use, can be seen in many places around Prestwood (eg on the opposite side of the track passing Andlows Farm and on Prestwood Common). Some of the boulders, instead of being solely sand-grains cemented together, include whole pebbles. When broken these boulders show a beautiful pattern and are known as “puddingstone”. These are also quite common and some can be seen among the other stones by Andlows Farm.
The chalk-rock is a narrow bed of particularly hard chalk, almost a limestone, with huge flints, that separates the Middle and Upper Chalk deposits. It winds around the Chilterns high on the hills and can be noticeable because its greater durability than the surrounding chalk makes it stand out as a particularly steep section of slope, where roads often bend to reduce the gradient. The soil overlying the chalk-rock tends to be thin and highly calcareous, so that where grassland has been allowed to survive this tends to support a very special chalk flora, especially those plants that like rocky places, like common rock-rose. There are few exposures of this band, although there are many old quarries along it where it was once excavated for building purposes (particularly for barn floors, where it was durable but also permeable to allow animal urine to drain away). One of these quarries at Stony Green used to have a “classic” exposure of this band of rock, mentioned in the commentary on the first national Geological Map. It might be good to clear this exposure at some time, so that it could be made visible once again.
Swallow-holes occur most commonly in the hard limestones on the north and west of Britain, but similar phenomena occasionally manifest themselves in the chalk regions. They occur where underground water has worn channels through the chalk and the chalk above them has gradually weathered away, suddenly giving way so that the overlaying clay falls into a huge hole. These swallow-holes can be differentiated from quarries by the fact that their sides are steep all the way round, ie there is no graded access to cart out excavated material. A good example can be seen near the north edge of Atkins Wood. The large depression known as Cockpit Hole in Great Kingshill may also have had its origin in this way, as it is too deep and steep-sided to have ever been useful as a pond, and (contrary to legend) was never used as pit for cock-fighting.
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