Buckinghamshire
Buckinghamshire has a varied landscape ranging from the Thames and Colne valleys in the south, through the chalk hills and valleys of the Chilterns, the low-lying clay Vale of Aylesbury interspersed by the limestones of the Midvale Ridge, the Greensand Ridge in the east, the Ouse Valley and the Whittlewood Ridge in the north.
This diversity of landscape reflects the varied geology of Buckinghamshire, which although it is difficult to observe, has clearly influenced both the landforms and its habitats.
Rocks of Jurassic age occupy the northern half of the county and younger Cretaceous rocks dominate in the southern half, except in the south-eastern corner, where they are covered by Tertiary sediments. Overall the rocks slope gently to the southeast, with the oldest rocks in the north, succeeded continuously by younger rocks to the south.
Jurassic
The oldest rocks present belong to the Lias (Lower Jurassic; approximately 190 million years old) and consist of an alternating sequence of clays and limestones. The succeeding limestones and clays of the Middle Jurassic Great Oolite provide evidence for a time when a warm, shallow sea was present, which was occasionally shallow enough for shoreline muds (the Blisworth Clay) to be deposited. The overlying Cornbrash, Kellaways Beds and the Oxford, Ampthill and Kimmeridge Clays complete the Jurassic succession and reveal a steady deepening over time of the sea in which they were deposited. The Cornbrash occurs in a narrow band through Tingwick, Buckingham, Berehampton and Newport Pagnell, while the Oxford Clay and Kimmeridge Clay lie in the Vale of Aylesbury. These clays give rise to poorly draining soils that support pasture and slow flowing brooks.
Uppermost Jurassic rocks (the Portland Limestone) forms part of the Midvale Ridge, which runs from Abingdon through to Aylesbury. The Portland Limestone, of which Buckinghamshire is the most northerly source of this famous building stone, was deposited in a shallow tropical sea.
Cretaceous
Much of the early Cretaceous (140-65 million years ago) succession is missing from Buckinghamshire. This is the result of about 40 million years of erosion, when the area now occupied by Buckinghamshire emerged from the sea as a land area across which large rivers flowed. The Whitchurch Sands are the only remaining evidence of these conditions. The succeeding Lower Greensand occupies the area around Leighton Buzzard and occurs as small outcrops atop the ridge to Thame. This Greensand ridge provides a strong contrast with the clay vales either side and supports woodland, pasture and mixed arable farmland. To the south a broad band of the bluish, stiff Gault Clay extends from Towersey across the county in a north-easterly direction which in turn is overlain by the Upper Greensand.
In the south-east of the County, the Chalk outcrop rises abruptly from the undulating plain to form the Chiltern Hills. The Chalk determines the form of the whole of the hilly district round Chesham, High Wycombe and the Chalfonts. This very pure limestone was deposited in a warm shallow tropical sea around 70-100 million years ago. In many areas, the scarp of the Chilterns is cloaked by calcareous grassland that supports many orchids and other rarities such as the Chiltern gentian.
Tertiary (comprising Neogene and Palaeogene)
Sediments of Tertiary age (65-2 million years ago), the Reading Beds (mottled clays and sands approximately 60 million years old), overlie the Chalk in the south-east of Buckinghamshire and these are in turn covered by the London Clay, which is exposed on the slopes around Stoke Common and Iver.
Quaternary
More recent sediments, often collectively known as drift, cover much of the underlying geology of Buckinghamshire. Thick deposits of clay-with-flints, derived from erosion of the Chalk, occur over most of the high ground in the southern corner of the county, while much of the northern part of the county is covered by boulder clay (also known as till). This blanket of sediments as well as the sands and gravels present in many of the river valleys (particularly the Thames and the Ouse) were deposited by a massive ice sheet during the Anglian glacial period which ended about 400,000 years ago. This ice sheet would have been several kilometres thick and its erosive actions were vital to the formation of the famous Chilterns landscape.
Geological highlights:
- The characteristic Portland Limestone is an excellent building stone and it can be seen in a large number of the older buildings, especially the churches of mid- to south Bucks (e.g. St Mary's, Princes Risborough, St Botolph's in Bradenham and St Mary and Holy Cross in Quainton). The outer walls of the Hartwell Estate are also constructed from the Portland Limestone and some blocks contain remains of the distinctive large ammonite Titanites giganteus.
- The village of Whitchurch gives its name to the Whitchurch Sandstone of Lower Cretaceous age. The distinctive ironstone and red sandstone has been used in the construction of many of the village houses and walls and gives the village its distinctive feel.
- Sands of Lower Cretaceous age (Lower Greensand) have been quarried at Stone near Hartwell for the production of high grade sand for use in the glass making industry.
- The sediments of the Reading beds are frequently hardened into silica-rich masses, known locally as Sarsen Stone and Hertfordshire or Bradenham Puddingstone (made up of rounded pebbles giving a pudding-like appearance). These silica-rich masses probably formed during the tropical conditions of the Tertiary. Many blocks were extracted locally at Denner Hill and Walters Ash, where examples can still be seen to litter the woodland and roadside verges.
- Sarsen stone is suffused with legends. Farmers called it 'growing stone' because no matter how much they removed more surfaced in their fields. Blocks of Puddingstone are visible at the base of many Buckinghamshire churches. Two massive sarsens support St. Dunstan's Church at Monks Risborough, which is the oldest recorded parish in the country, with a charter dating from the year 903. At Bradenham, in a paddock uphill from the church can be seen the largest blocks of all, the biggest being 4mx3m with six others over 3m and thirty around 2m. An Ice Age sarsen stone, unearthed in Great Close Field, has been erected by Bradenham Parish Council on the Green, to commemorate the dawn of the third Millennium.
Local sites
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.
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