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Geology of Berkshire

Posted on November 7th 2007

Local geology of Berkshire

Berkshire lies on the north west side of the London Basin and the Berkshire Downs formed from the Upper Cretaceous Chalk mark the northern edge of this basin. The chalk outcrop slopes gently away to the south and is succeeded by sands and clays of Tertiary age which form the central part of the basin.

The oldest rock in Berkshire, the Oxford Clay, is a Jurassic sediment which was laid down about 150 million years ago and was produced during a long period of muddy seas. It contains fossils of ammonites and the bones of large terrestrial dinosaurs like Plesiosaurus and Pliosaurs. The clay mainly outcrops in the upper reaches of the Thames Valley between Lechlade and Oxford but, close to the river, the clays are overlain by alluvium. It is very popular for brick making and has been used for this over many years.

There are small exposures of the Lower Chalk in dry valleys near Lambourn and Streatley.

The Middle Chalk forms a scarp and terrace crossing the county from Ashdown Park to Streatley and is made up of almost pure calcium carbonate.

The Upper Chalk forms a wide terrace of high ground with its corners at Hungerford, Lambourn, Bradfield and Basildon but much of this chalk is covered by drift. The landscape is dominated by rolling hills and dry valleys of the Downs. In places on the Downs, areas of chalk grassland with characteristic plants such as orchids, thyme and harebell still survive. The Upper Chalk also outcrops between Henley, Maidenhead Thicket and Cookham.

The Chalk was laid down in a shallow sea during the Upper Cretaceous (beginning about 100 million years ago). The rock is made up of the calcium carbonate remains of mollusc shells, foraminifera and coccoliths. The absence of any terrestrial input of mud or sand suggests that the chalk was laid down a long way from the contemporary shore line. The chalk dips gently to the south with its exposed upper (northern) edge forming the ridge which runs from White Horse Hill at Uffington to the Thames north of Goring. This long escarpment is the most striking physical feature in the county and is indented by numerous winding valleys, most of which are dry and produced by terrestrial erosion during the Ice Age. The chalk passes south under the valley of the Kennet and then reappears at the surface to form a second scarp under Inkpen Beacon. The chalk forms the bedrock in the east of the county and Windsor Castle stands on a small folded dome of chalk.

In many areas the surface of the chalk is covered by clay with flints. This represents what was left after the prolonged erosion and weathering of the chalk during the Tertiary period.

South of a line from Hungerford, via Reading to Windsor, are Tertiary deposits of clays and sands laid down in shallow marine, coastal and fluvial environments. The Tertiary sediments belong to the Lambeth Group and comprise sands (the Upnor Formation) deposited in a shallow sea overlain by the reddish-brown clays of the Reading Formation. These latter sediments were deposited on marshy mudflats crossed by river channels and outcrop in a narrow strip between the Chalk and the overlying London Clay.

Sarsen stones are found over the surface of the Chalk forming the Berkshire Downs and these are isolated remnant blocks of weathered Tertiary sandstone with a hard silica cement. These probably represent outlying deposits of sands withing the Reading Formation and indicate that these Tertiary sediments formerly extended well beyond their present outcrop. Cementation of these sands probably occurred just below the ground surface under an arid or semi-arid climate, perhaps 5-10 million years ago or during the Ice Age. The surrounding uncemented sediments have since been eroded away. These stones have been used as building material for thousands of years including in the construction of Wayland’s Smithy and Windsor Castle. They can be seen over most of the Berkshire area serving as corner stones, gate posts, stepping stones or supplementing other building materials in walls.

The London Clay, a dark bluish to brownish sediment, forms a narrow band marking the southern boundary of the county from West Woodhay to Mortimer and across to Windsor. This was deposited in a shallow sub-tropical sea and contains the fossils of many marine animals such as bivalves, gastropods and sharks teeth.

In the Ascot-Bracknell-Wokingham area and to the south and east of Newbury the London Clay is overlain by the sandy Bagshot Formation. This is exposed on the scarps of many small ridges along the Kennet Valley although most of this is overlain by Plateau Gravel. The best exposures are in a band extending from Finchampstead Ridges to Virginia Water in south east Berkshire. The Bagshot Beds were laid down as a consequence of the shallowing of the sea which formed the London Clay and comprise of red sands, pipe clays and a coarse, pebbly, current-bedded sandstone. A patchwork of later river gravels lies on top.

Bracklesham Sand occupies a small area to the east of Sandhurst, Crowthorne and Tower Hill and consists of acid sands and clays.

During the Quarternary period, over the last 2 million years, the climate of Britain has varied considerably with Ice Ages interspersed with period of temperate climate conditions. Ice did not reach Berkshire although tundra like conditions would have prevailed and it was under these arctic conditions that the dry valleys of the Chalk probably formed when permafrost made the ground impermeable, allowing gradual erosion in river valleys.

Within the floodplain of the Thames there are wide expanses of terraced river sands and gravels with the majority of the gravel being flint derived from the Chalk. These terrace deposits represent successive levels of the river which, during each phase of downcutting, migrated further to the south. These sand and gravel deposits are economically important and have been excavated over many years for use in the aggregates industry.