Thegeology of Surrey is dominated bysedimentary strata from theCretaceous, overlaid byclay andsuperficial deposits from theCenozoic.
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During theEarly Cretaceousepoch (from about 145 to about 100 million years ago) Surrey alternated between a fresh-to-brackish water embayment depositingHastings Beds andWeald Clay, comprising shales and mudstones that are often finely banded. Offshore muds (now shales and mudstones) of theAtherfield Clay were deposited followed by shallow marine sands of theHythe, Sandgate and Folkestone Beds. Where not eroded to lower heights, there is then a marine layer of the sands of the Hythe Beds topped by chert seen on today's remainingGreensand Hills. Instead of themudstone and sandstone-producing three beds mentioned before Hythe Beds, west ofDorking the marine Bargate Beds made of calcareous (chalk andlimestone-rich)sandstone were deposited. The Folkestone Beds contain phosphatic and iron-richnodules, which locally yield a richfossil fauna of marine shells.
Then under even deeper seas,Gault Clay and the UpperGreensand were deposited. The Gault Clay contains phosphate-rich nodules in discrete bands and has a rich marine fauna with abundantammonites,bivalves andgastropods. The Upper Greensand comprises a variety of sediments with fine silts at the base giving way upwards intosandstones.
90 million years ago theNorth Downs hard chalk was deposited, a whitelimestone which is over 95%calcium carbonate. It contains thin beds ofmarl and nodules offlint, either scattered or in bands. The North Downs extending fromFarnham toDover, Kent are formed by this chalk. They now have an often white, almost vertical south-facing slope.[1]
Just before thePaleogene, which included the mass-extinction event of the non-aviandinosaurs, sea levels dropped, exposing Sussex and Kent, and the marineUpnor Beds were deposited in Surrey. In the Paleogene, Southern England rose slightly, and the seas retreated, 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, caused widespread deposition, until 2 million years ago, of the London Clay across the County. The London Clay is a bluish-grey marine clay with isolated pockets offossils especially where chalkier. The youngest part of the London Clay is known as theClaygate Beds and occurs widely in Surrey. This even sandier material represents a transition between the deeper water London Clay and the succeeding shallower water, possibly estuarine,Bagshot Sand.[1]
Major climate changes in Britain causing sea level changes in thelast 2.58 million years, with miniIce Ages, the ice sheets did not extend to Surrey but sand and gravel deposits swept towards the fledglingRiver Thames were spread in all lower parts. Gravel terraces at various heights on the valley sides 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. Along tributary slopes, a deposit, head, forms the main sediment of latest 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 ofclay-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 theGuildford vale is known as the Headley Formation and comprises 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 Paleogene.[1]
A large geologicalfold, theWealden anticline, is the major rock structure in the county.[1] This anticline was formed by theAlpine orogeny about 25 to 12 million years ago.
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The northeast of the county, such as the north ofTandridge (district), is in the wide part of theNorth Downs. Thus from the east,Tatsfield has two western pockets of slightly acid, loamy soils with free drainage otherwise has the expected shallow, lime-rich soil over chalk or limestone of the escarpment with lower parts of the escarpment summit here, where that topsoil has eroded, having slightly acid, loamy and clayey soils with impeded drainage soil. Westward, the shallow lime is found all the way along the North Downs to the western border, pastGuildford only a few hundred metres narrow toFarnham Castle and evenDippenhall, the latter accompaniment is found on both sides only toBuckland, well beforeDorking. A horseshoe of land including the rise north ofGodstone of Godstone Hill that leads to the escarpment is free draining lime-rich loamy soil (i.e.alkaline). Continuing with the Downs, in a broad band, the far northern type soil slopes such as nearCroydon come across the Surrey border atBanstead andEwell, which is free draining, slightly acid but base-rich soils, producing extremely fertile pastures and deciduous woodland, which becomes the edging for both sides of the high escarpment atBox Hill and is just found for a brief while in a middle section inPolesden Lacey then throughout the northern slope carrying on westward.Chipstead,Banstead andTadworth have the first free draining slightly acid loamy soil that tops the wider downs to Guildford and is found around Dorking and the across theVale of Holmesdale north of the Greensand Hills.
Equally it is this topsoil north of the Thames across Spelthorne (such as aroundAshford) and west of the Thames in the east ofRunnymede (such as aroundThorpe): here the land is flat flood plain, mostlysilt mixed with lime-rich London Clay.[2]
Between the Thames and the North Downs the land is overall slightly lower than south of the Downs but is less in the current flood plain, drained by the tributaries mentioned. There is moreloam persisting the further from thealluvial plain of the Thames and tributaries; from the southbank atThames Ditton (nearHampton Court southwest to Ripley, Send and Old Woking is still more free draining slightly acid loamy soil. Impeded drainage but rarely waterlogged soil features inAddlestone, northKnaphill and around Perry Hill,Worplesdon whileChobham lies in loamy soils with naturally high groundwater producing wet acid meadow and woodland edged by streamsidefen/peat marshy brooklands.[2]
Heath: inEsher,Oxshott,Weybridge,Wisley, all aroundWoking,Brookwood,Deepcut,Pirbright,Frimley,Lightwater,Camberley,Chobham Common,Virginia Water andOttershaw is naturally wet, very acid sandy and loamy soil which is just 1.9% of English soil and 0.2% of Welsh soil, which gives rise topines andconiferous landscapes, such as pioneered atWentworth and Foxhills estate (now spa, hotel, restaurant and golf club) by pro-American Independence statesmanCharles James Fox.[2]
South, but not beyond theNorth Downs and to the south of the Greensand Ridge is slowly permeable loamy/clayey slightly acid but base-rich soil forming a 3–4-mile-wide (5–6 km) band fromEwell andClaygate throughLeatherhead,Little Bookham, the north side ofGuildford toFarnham, whereas South of the Greensand is some 20–15 miles (30–25 km) thick (with some areas of free then poor drainage aroundEast Grinstead/Felbridge).[2] The Greensand ridge itself has the heath soil above and zones of slightly acid only freely draining sandy soils, which make those areas more densely wooded such asThursley,Brook,Churt,Seale,Runfold andPuttenham.