| London Clay Formation | |
|---|---|
| Stratigraphic range:Early Eocene,54–50 Ma[1] | |
 London Clay atReculver | |
| Type | Formation |
| Unit of | Thames Group |
| Underlies | Bagshot Formation (London Basin),Wittering Formation andPoole Formation (Hampshire Basin andEnglish Channel) |
| Overlies | Harwich Formation |
| Thickness | up to 150 m |
| Location | |
| Region | southernEngland |
 Geological map of theLondon Basin; the London Clay is marked in dark brown. | |
TheLondon Clay Formation is amarinegeological formation ofYpresian (earlyEocene Epoch, c. 54-50 million years ago)[1] age whichcrops out in the southeast ofEngland. The London Clay is well known for itsfossil content. The fossils from thelower Eocene rocks indicate a moderately warm climate, thetropical orsubtropical flora. Thoughsea levels changed during the deposition of the clay, thehabitat was generally a lush forest – perhaps like inIndonesia orEast Africa today – bordering a warm, shallow ocean.
The London Clay is a stiff bluishclay which becomes brown when weathered and oxidized. Nodular lumps ofpyrite are frequently found in the clay layers. Pyrite was produced by microbial activity (sulfate reducing bacteria) during clay sedimentation. Once clay is exposed to atmospheric oxygen,framboidal pyrite with a greatspecific surface is rapidly oxidized.Pyrite oxidation produces insoluble browniron oxyhydroxide (FeOOH) andsulfuric acid leading to the formation of relatively solublegypsum (CaSO4·2H2O,calcium sulfate dihydrate). This latter is more soluble and mobile thaniron oxides and can further recrystallize to form larger crystals sometimes calledselenite (coming from themoon, but not related toselenium, although theetymology is the same), or "waterstones".
Largeseptarian concretions, produced by microbial activity (oxidation oforganic matter) in the ancient seafloor during clay earlydiagenesis, are also common. These have been used in the past for making cement. They were once dug for this purpose at Sheppey, nearSittingbourne, and atHarwich, and also dredged, off theHampshire coast. The clay is still used commercially for makingbricks, tiles, and coarse pottery in places such asMichelmersh in Hampshire.
The London Clay is well developed in theLondon Basin, where it thins westwards from around 150 metres (492 feet) in Essex and north Kent to around 4.6 metres (15 feet) inWiltshire.[2] It is not frequently exposed as it is to a great extent covered by more recentNeogene sediments andPleistocene gravel deposits. One location of particular interest isOxshott Heath, where the overlying sand and the London Clay layers are exposed as a sand escarpment, rising approximately 25 metres (82 feet). This supported a thriving brick industry in the area until the 1960s. The London Clay is also well developed in theHampshire Basin, where an exposure 91 metres (299 ft) thick occurs atWhitecliff Bay on theIsle of Wight and around 101 metres (331 ft) is spread along 6 kilometres (4 miles) of foreshore atBognor Regis,West Sussex.[3]
The clay was deposited in a sea up to 200 metres (660 ft) deep at the eastern end. Up to five cycles of deposition (representingtransgression followed by shallowing of the sea) have been found, most markedly at the shallower, western end. Each cycle begins with coarser material (sometimes including roundedflint pebbles), followed by clay which becomes increasingly sandy. The final cycle ends with the Claygate Beds.[2]
The youngest part of the London Clay, known as theClaygate Beds orClaygate Member forms a transition between the clay and the sandierBagshot Beds above. This is shown separately on many geological maps, and often caps hills. It is up to 15 metres (49 ft) thick atClaygate, Surrey.[2] It is now believed to bediachronous, with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east.[4]
The presence of a thick layer of London Clay underneathLondon itself, providing a soft yet stable environment fortunnelling, was instrumental in the early development of theLondon Underground, although this is also the reason why London had no trueskyscraper buildings, at least to the same degree as many other cities throughout the world. Erecting tall buildings in London required very deep, large, and costly piled foundations. This has changed in recent decades due to the development of 'plunge piles'. London's skyscrapers float on rafts embedded in the clay.
London Clay is highly susceptible to volumetric changes depending upon its moisture content.[5] During exceptionally dry periods or where the moisture is extracted by tree root activity, the clay can become desiccated and shrink in volume, and conversely swell again when the moisture content is restored. This can lead to many problems near the ground surface, including structural movement and fracturing of buildings, fractured sewers and service pipes/ducts and uneven and damaged road surfaces and pavings. Such damage is recognised to be covered by the interpretation ofsubsidence in buildings insurance policies, and the periods of dry weather in 1976/77 and 1988/92, in particular, led to a host of insurance claims. As a result, many insurance companies have now increased the cost of premiums for buildings located in the most susceptible areas where damage occurred, where the clay is close to the surface.
London Clay is also used to line exhausted quarries. This is because old quarry holes are generally refilled with waste material and by lining it with London Clay (which is virtually impermeable) it prevents waste and hazardous substances from entering the groundwater.
London Clay is an ideal medium for boring tunnels, which is one reason why theLondon Underground railway network expanded very quickly north of theRiver Thames. However, south of the Thames, the stratum at tube level is composed of water-bearing sand and gravel (not good for tunnelling) with London Clay below, which partly explains why there are very few tube tunnels south of the Thames. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almostwaterproof, resulting in virtually no seepage ofgroundwater into the tunnel. It is over-consolidated, which means that it was once subject to an overburden pressure higher than it is subjected to today, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground.
Due to its impermeability especially when exposed by ploughing, London Clay does not make good agricultural soil.[citation needed] InMiddlesex, ploughing London Clay land so deep that it brings up clay has historically been called "ploughing up poison".[6]
It was also disliked for building houses on, though inevitably most ofGreater London is built on it.Jane Ellen Panton in herSuburban Residences and How To Circumvent Them (1896),[7] remarks: "I do not believe clay is or even can be fit for anyone to reside upon ... though roses flourished magnificently children didn't, and coughs and colds [lasted through autumn and winter, this atShortlands".[8]
Many London buildings have ultimately been constructed with London Clay. When compressed and burnt, London Clay can be fashioned into brick known asLondon stock, identifiable by its yellowish brown hue.[9]
Plant fossils, especially seeds and fruits, are found in abundance and have been collected from the London Clay for almost 300 years.[10] Some 350 named species of plant have been found, making the London Clay flora one of the world's most diverse for fossil seeds and fruits.[11] The flora includes plant types found today in tropical forests of Asia and demonstrates the much warmer climate of theEocene epoch, with plants such asNypa (Nipah palms) and otherpalms being frequently encountered.
Notable coastal exposures from which fossils can be collected are on theIsle of Sheppey inKent andWalton-on-the-Naze,Essex, in the London Basin, andBognor Regis in the Hampshire Basin.
