Surrey lies on the north-western side of a large and eroded dome of rocks known as the Wealden anticline. This structure and the rocks comprising it, strongly influence the local landscape and many of the prominent landforms of Surrey and adjacent counties.
This is perhaps nowhere better experienced than the sweep through the central part of the County of the escarpment of the North Downs, which is at its most dramatic where the River Mole has cut through the chalk below Box Hill. Running south and parallel to the North Downs are the greensand hills which rise to almost 1,000 feet at Leith Hill, to form the highest point in south-east England. The presence and character of these hills has been strongly influenced by the underlying geology with the Wealden anticline providing the structure and the greensand giving rise to acid, sandy soils which support heathland and open woodland.
Rocks of Cretaceous (65-140 million years old) and Tertiary age (65-2 million years old) make up the Wealden anticline and dominate the County. The slope of the rocks is to the north, with the consequence that increasingly younger rocks are exposed the further north you travel. During the Ice Age, no glaciers reached Surrey, but the County was under the influence of very cold tundra-like conditions at the edge of the ice sheets. Processes of erosion and deposition during this period have contributed significantly to the formation of the present landscape.
The beginning of the Cretaceous Period in Surrey is represented by the non-marine Hastings Beds and Weald Clay. The sediments of these rocks were originally deposited in a fresh water to brackish-water embayment (which covered Surrey) opening out into marine areas to the north (East Anglia) and south (into France). The main outcrop in Surrey is the Weald Clay, which comprises shales and mudstones that are often finely banded.
As time passed, marine conditions were established as the connection of the embayment with the sea became stronger and sediments were deposited in a variety of shallow water and nearshore environments. Initially, offshore muds (now shales and mudstones) of the Atherfield Clay were deposited followed by shallow marine sands of the Hythe, Sandgate and Folkestone Beds. When fresh, the rocks commonly have a green colour due to the presence of glauconite, hence the name “Greensand”. These rocks provide the underlying structure to the picturesque Leith Hill district of Surrey, where the hills form the highest part of the Lower Greensand outcrop of the Weald. The top part of Leith Hill is formed of resistant chert, which protect the underlying sands of the Hythe Beds, which overlie the Atherfield and Weald Clay, which without protection are easily eroded. A line of springs runs around the hills at the sand clay junction. The springs have a pronounced effect on topography as they rapidly remove sand from the Hythe Beds and cause collapse of the rocks above, leaving spectacular vertical cliffs in places.
The Hythe and Folkestone Beds are mainly sandstone, whereas the Sandgate Beds are more variable and comprise sandstones and mudstones. West of Dorking, the Sandgate Beds become bands of calcareous-rich sandstone known as the Bargate Beds. All the sand formations show evidence for strong tidal influence in the form of large-scale structures that represent the migration of waves of sand over the sea floor. The Folkestone Beds contain phosphatic and iron-rich nodules, which locally yield a rich fossil fauna of marine shells.
A further rise in sea-level led to the formation of a larger and deeper sea and deposition of the Gault Clay and the Upper Greensand. The Gault Clay contains phosphate-rich nodules in discrete bands and has a rich marine fauna with abundant ammonites, bivalves and gastropods. The Upper Greensand comprises a variety of sediments with fine silts at the base giving way upwards into sandstones.
The beginning of the Upper Cretaceous, some 90 million years ago, is marked by a major phase of land subsidence and the establishment of a relatively deep, tropical sea in which the Chalk was deposited. The Chalk is white limestone comprising over 95% calcium carbonate. It contains thin beds of marl and nodules of flint, either scattered or in bands. The North Downs extending from Farnham to Dover (Kent) are formed by the Chalk. The steep, south-facing slope of the North Downs is a striking feature of Surrey and can be well observed from the M25 Motorway between Reigate and Westerham. The comparatively hard Chalk forming the North Downs is everywhere underlain by the softer Gault Clay. Erosion has cut down to these soft beds, which gradually undermines the harder chalk beds, thereby producing a steep slope.
At the end of the Cretaceous there was a major fall in sea-level. The Weald area of Sussex and Kent became a low-lying land area with a shallow sea to the north. The deposition of marine sandstones (Upnor Beds) in this sea, overlying the eroded surface of the Chalk, marked the beginning of the Tertiary in Surrey. Following a further drop of sea-level, the entire area became land and the reddish and mottled clays of the Reading Beds were deposited by a large river sand delta system.
Later, a rise in sea level, around 50 million years ago, led to the widespread deposition of the London Clay across the County. The London Clay is a bluish-grey marine clay resting with a sharp contact on the Reading Beds. Fossils are generally scarce, but common in certain horizons. Calcareous-rich layers within the clay have yielded a large number of fossils including starfish, crabs, lobsters, fish, turtles, bivalves and gastropods. The youngest part of the London Clay is known as the Claygate Beds and occurs widely in Surrey. These rocks are composed of sand and represent a transition between the deeper water London Clay and the succeeding shallower water, possibly estuarine, Bagshot Sand.
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. The Ice Sheets did not reach as far south as Surrey, but the influence of the alternating cold and warm phases can still be seen in the landscape.
Deposits from this time typically include gravels deposited in river floodplains, which form terraces at various heights on the valley sides. These are the remnants of successive floodplains, the highest terrace being the oldest and the lowest the youngest. The most prominent terraces mark the former levels of the Thames in north Surrey. Elsewhere in the County, a deposit known as head represents the main sediment of Quaternary age. Head comprises angular pieces of rock and soil derived locally from the extensive frost-shattering of rocks and the subsequent movement of this material down valley slopes. Large areas of clay-with-flints, derived from the weathering of material overlying the present day Chalk, occur across the North Downs. One particular suite of sediments that occur in the Guildford area is known as the Headley Formation and comprises structureless masses of gravel and sand on top of the chalk. These sediments contain marine fossils and were probably derived from erosion of the Greensand and Tertiary rocks during the early part of the Quaternary.
In addition to reptile remains, the Weald Clay is also an important source of insect fossils, which are otherwise rare in Lower Cretaceous rocks. A number of claypits in Surrey have yielded the well preserved bodies and wings of insects. At some locations, concentrations of remains occur, which can be attributed to several insect families, including dragonflies, crickets, cockroaches, flies and beetles.
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.