West Berkshire (including Reading, Wokingham, Bracknell Forest, Windsor and Maidenhead, and Slough)
This part of central southern England sits on the north-west side of the London Basin, a large geological 'dish' that dominates the geology of the Home Counties.
The rolling, broad expanses of the Berkshire Downs, formed from the Upper Cretaceous Chalk, mark the northern edge of the basin. As the outcrop of the chalk slopes gently away to the south it is succeeded by sands and clays of Tertiary age which form the central part of the London Basin. These sediments were deposited in shallow marine, coastal and terrestrial environments and reflect changing sea-levels and periods of land uplift over the 20 million year period that they were laid down. The generally free-draining sands give rise to acidic soils which typically support heathland vegetation, although much of the original heathland in the Bracknell-Wokingham area is now fragmented and extensive conifer plantations have been planted.
Over the last two million years the area was not directly impacted by the repeated advances and retreats of the great ice sheets of the Ice Age. However, the arctic conditions that prevailed during glacial periods led to the formation of the characteristic dry valleys of the chalk downs. Throughout this period, the River Thames and its various tributaries transported vast amounts of sediment through the area. This was deposited as terraces of sand and gravel along the sides of the Thames valley. Today the gravels form an important economic resource and have been extensively quarried for the production of aggregate. The large lakes left following extraction now provide important areas of open water for recreation and habitat for wildlife.
Cretaceous
During the Cretaceous (142-65 million years ago) much of what is now southern Britain was covered, to varying degrees, by a shallow sea leading to the deposition of marine, coastal and sometimes terrestrial sediments. It was not until the Late Cretaceous that a major rise in sea-level across Europe led to a significant deepening of the Cretaceous sea. Without the influence of sediments washed in from nearby landmasses a very pure marine limestone was deposited in the warm, relatively deep sea. This is the Chalk, which may constitute up to 98% calcium carbonate, and which today forms the rolling and open expanses of the Berkshire Downs. Much of the chalk outcrop forms an elevated gentle, southerly dipping plateau, dissected by a network of dry valleys, with the southerly edge of the outcrop running roughly in a line between Newbury and Maidenhead. In places on the Downs, areas of chalk grassland with characteristic plants such as orchids, thyme and harebell still survive. Away from the main Chalk outcrop there is a very small folded dome of Chalk forming the prominent hill upon which Windsor Castle is located.
Tertiary (comprising Neogene and Palaeogene)
Sediments of Tertiary (65-2 million years ago) age occupy the central part of the London Basin and outcrop in this area southwards of a line running from Hungerford to Windsor.
These clays and sands were laid down in shallow marine, coastal and fluvial (river) environments. The oldest 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. Named after the Reading area, these largely fossil-poor clays outcrop in a narrow strip between the Chalk and the overlying London Clay. The dark bluish to brownish London Clay outcrops in a broad swathe between Windsor and Burghfield and along the valley of the River Kennet towards Newbury. This was deposited in a shallow sub-tropical sea and contains the fossils of many marine animals such as bivalves, gastropods and sharks teeth. The remains of many species of plants, such as the seeds of palms, climbers, magnolias and mangroves can also be found in the London Clay and were probably washed out to sea by large rivers.
In the Ascot-Bracknell-Wokingham area and to the south and east of Newbury the London Clay is overlain by the sandy Bagshot Formation. These free-draining sands give rise to nutrient-poor acidic soils which support heathland vegetation and extensive conifer plantations. The original heathlands are now largely fragmented and degraded but the landscape still maintains its open and `heathy' character.
Sarsen stones, isolated remnant blocks of weathered Tertiary sandstone with a hard silica cement, can be found over the surface of the chalk. These probably represent outlying deposits of sands within the Reading Formation and indicate that these Tertiary sediments formerly extended well beyond their present outcrop. Cementation of the sands probably occurred just below the ground surface under an arid or semi-arid climate, perhaps 5-10 million years ago, and the surrounding uncemented sediments have long since been washed away.
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, and has probably been formed over many millions of years. The clay-with-flints often gives rise too more acidic soil conditions which contrasts with the calcareous soils derived directly from the chalk bedrock.
Quaternary
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
Ice did not reach the Berkshire area during the Quaternary, although tundra-like conditions would have prevailed. Under these arctic conditions the dry valleys of the chalk probably formed. These features occur on what is a very permeable rock which does not generally support surface drainage systems. However, during the various glacial periods, deep permafrost would have made the ground impermeable and allowed gradual erosion of the frozen soil surface to occur, particularly during summer thaws. Today, the now permeable chalk downs are cut by dry valleys (where rivers once flowed) which were formed by this erosion.
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. A whole suite of gravel terraces can be traced on the north slopes of the Thames valley between Reading and Maidenhead. The deposits were formed under the variable climatic conditions of the last half a million years. The fossils present within the sands and gravels, particularly plant pollen and the snail shells, can be used to document the changing environments. The sand and gravel deposits of the Thames Valley are economically important and have been excavated for many years for use in the aggregates industry. Many of the old quarries are now flooded and the large areas of open water provide valuable wetland habitat for wildlife, notably ducks and geese during the winter, and also for recreational activities such as sailing and other watersports.
Geological highlights:
- Sarsen stones from the Berkshire Downs have been used as a building material in the area for thousands of years. They have been used in the construction of Wayland’s Smithy, Ashbury, a Neolithic long barrow located just off the Ridgeway on the Berkshire Downs. Here, a series of gigantic sarsen stones, the tallest standing 10 feet high, are arranged in a line at the entrance to the south end of the barrow. This is one of many prehistoric sites associated with Wayland, the Norse god of blacksmithing. Sarsens are also found over most of the Berkshire area serving as corner-stones, gate-posts and stepping stones or supplementing other building materials in walls. They were also extensively used in the building of Windsor Castle.
- Former gravel workings in the Furze Platt area, north of Maidenhead, have produced large numbers of Palaeolithic implements from one of the River Thames terraces (the Lynch Hill Terrace). Most of the implements are flint hand axes and include the largest hand axe discovered in the British Isles. At 32cm in length it has been suggested that this hand axe could have been of ceremonial value rather than for actual use. By the end of the 1930’s over 1600 hand axes had been recovered from Furze Platt. The dating of the gravels and the implements has proved problematic, but it is now thought that the deposits are about 300,000 years old.
- The Reading area gives its name to a succession of reddish-brown clays and associated sands of Tertiary (Palaeocene) age that were first described here. The sediments of the Reading Formation provide evidence that a shallow sea spread over this area some 60 million years ago, before retreating and being replaced by riverine conditions. Within the sands and clays which were deposited in these rivers is found a well preserved fossil flora, including, at some locations, well-preserved leaves and occasional fossil insects. Some of the fossil leaves even show evidence of the activity of contemporary leaf-mining insects.
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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