Bucks Geology Group
Grid reference: SP 650 140.
Ordnance Survey map: Aylesbury and Leighton Buzzard Sheet 165.
Geological maps: 1:50,000 series Thame sheet 237.
Bucks County Council administration area: Aylesbury Vale.
Owned by: Open access areas around the town and windmill.
Area of site: ha.
Interest Summary: The geological interest in the form of local building materials, can be seen in the building stones of the village, which incorporate a large collection of locally sourced stone including Whitchurch Sandstone and Portland Limestone. Geologically, Brill is one of several villages in the area which are situated directly on the Whitchurch Sandstone.
Access, location and parking: Brill is easy to reach being NW of Thame and south of the A41, off the B4011. There are numerous areas for parking in the village. There are many facilities and amenities with the village including toilets, cafes and pubs.
Brill is one of a number of outlier hills which are capped by harder more resistive sansdtones and limestones. These include Whitchurch, Stone, Chilton, Waddesdon and Ashendon (see geology sketch below). The hills are only present because of the resistant capping provided by the Whitchurch Sandstone and the of equally hard Portland Stone underlying the Whitchurch Sandstone. Together they produce a hard layer which is resistant to erosion. The underlying clays around these hills have been eroded down to form the lower clay vales which are a distinctive a feature of this part of Buckinghamshire. The geomorphological aspect of this combination of rock type and erosion has produced a highly aesthetic landscape.
Sketch geology map showing the relationship between the harder Whitchurch and Portland beds and the underlying clays.Sketch geology cross-section through Brill Hill.
The Whitchurch Sandstone (above) is lowermost Cretaceous (Valanginian in age, that is 138-131 million years old). At Brill, the Whitchurch Sand has been recorded as upto 18m thick. The lower section was silty with clay beds, while the upper section was mainly ferruginous sandstone with ironstones. The more cemented lithologies, generally red and iron-rich sandstones, have been used in many of the older buildings and numerous walls in Brill.
In the 1830's a fossil tree 12m long was discovered. The account by Thomas Knight in the Provincial Medical Journal- August 1842, gives the account 'On the top of Brill Hill, thick beds of a bright yellow of ochre also exist. On removing a thick bed of it a short time ago, a fine trunk of a fossil tree was exposed, and laid bare the whole length of the pit; its surface was completely covered with metal firmly attached, and though for a time it retained its silvery appearance hardness, till oxygen of the air in a few weeks its frail nature, and resolved it into green crystals of sulphate of iron. The bed of green sand underneath this cuiriosity had great quantities of pyritic formations, in the shape of irregular balls weighing several ounces.'
The windmill at Brill sits in an area which has been heavily quarried. Much of the quarrying was probably for the Whitchurch sands and Portland limestones. A near by borehole (right) drilled in 1986 by the British Geological Survey suggests these deposits are around 24m thick below the top of the hill.
As well as the Whitchurch Sandstones there are other local building materials identifiable in the village - for instance the Portland Stone (the pale blocks in building above), including ammonites, and locally produced bricks. The latter was often very local - the open land all around the windmill area is pock-marked with small pits. Further down the hill the Kimmeridge Clay has been exploited for brick making with evidence for kilns going back to the 13th Century.