Geology of London
Encyclopedia
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
upon which London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
is built.
Oldest rocks
The oldest rocks proved through boreholes to exist below LondonLondon
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
are the old, hard rocks of the Palaeozoic. These consist of Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
mudstones and sandstones, generally overlain by Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
strata which are largely of Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
. The Devonian rocks are absent in parts of South London. The Palaeozoic rocks dip southwards and are more than 1,000 metres below the English Channel
English Channel
The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...
. Above this is a 60 metre thick layer of impermeable
Permeability (fluid)
Permeability in fluid mechanics and the earth sciences is a measure of the ability of a porous material to allow fluids to pass through it.- Units :...
Gault clay
Gault Clay
Gault is a clay formation of stiff blue clay deposited in a calm, fairly deep water marine environment during the Lower Cretaceous Period...
s. These clays are relatively young, only going back to the early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
which began around 144 million years ago.
On top of these clays is a non-contiguous layer of Upper Greensand
Greensand
Greensand or Green sand is either a sand or sandstone, which has a greenish color. This term is specifically applied to shallow marine sediment, that contains noticeable quantities of rounded greenish grains. These grains are called glauconies and consist of a mixture of mixed-layer clay...
above which lies a rolling bed of white chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
about 200 metres thick. In the Lower Chalk and in the lower region of the Middle Chalk there are abundant fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ized shell fragments, especially Inoceramus
Inoceramus
Inoceramus is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus Pteria....
clams. In places these form the greater part of the rock but they decrease in amount upwards in the succession. Flint
Flint
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...
s are abundant in the Upper Chalk.
These bands of chalk form the basis of the London Basin
London Basin
The London Basin is an elongated, roughly triangular sedimentary basin approximately long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea...
, a v-shaped syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
bounded by chalk outcrops forming hills: the North Downs
North Downs
The North Downs are a ridge of chalk hills in south east England that stretch from Farnham in Surrey to the White Cliffs of Dover in Kent. The North Downs lie within two Areas of Outstanding Natural Beauty , the Surrey Hills and the Kent Downs...
to the south of London, and the Chilterns to the north. The chalk is a soft white limestone that is different in appearance to other limestones because it is porous and earthy whilst others are compact and crystalline.
Tertiary period
The chalk basin has been infilled with a sequence of clays and sandSand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
s of the more recent Tertiary Period (1.6 to 66.4 million years old). Most significant is the stiff, grey-blue London Clay
London Clay
The London Clay Formation is a marine geological formation of Ypresian age which crops out in the southeast of England. The London Clay is well known for the fossils it contains. The fossils from the Lower Eocene indicate a moderately warm climate, the flora being tropical or subtropical...
, a marine deposit which is well known for the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s it contains and can be over 150 metres thick beneath the city. This supports most of the deep foundations and tunnels that exist under London.
Also in this area are the Lambeth Group
Lambeth Group
The Lambeth Group is a stratigraphic group, a set of geological rock strata in the London and Hampshire Basins of southern England. It comprises a complex of vertically and laterally varying gravels, sands, silts and clays deposited between 56-55 million years before present during the Ypresian age...
(formerly known as the Woolwich and Reading Beds) and Thanet sands. Most of these sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and clays were deposited 60-50 million years ago during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. Southern England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
at this time was covered by a warm tropical sea: this is shown by the fossil evidence. The sands contain animals that lived in both estuaries and freshwater. Some species burrowed into the underlying chalk.
Quaternary Period
Above this is the subsoilSubsoil
Subsoil, or substrata, is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc., to produce true soil...
which consists of deposits of gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
up to 10 metres deep. This was deposited during the last ice-age ½ million years ago when the River Thames
River Thames
The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...
was diverted to its present position. While establishing its new path, the river eroded its valley, creating a series of sand and gravel terraces. These terraces are named after the area they are best known in, for example: Dartford
Dartford
Dartford is the principal town in the borough of Dartford. It is situated in the northwest corner of Kent, England, east south-east of central London....
Heath Gravel, Swanscombe
Swanscombe
Swanscombe is a small town, part of the Borough of Dartford on the north Kent coast in England. It is part of the civil parish of Swanscombe and Greenhithe.-Prehistory:...
, Orsett
Orsett
Orsett is a village and ecclesiastical parish located within Thurrock unitary district in Essex, England, situated around 5 km north-east of Grays...
Heath, Corbets Tey, Mucking
Mucking
Mucking is a hamlet and former Church of England parish adjoining the Thames estuary in southern Essex, England. It is located approximately 2 miles south of the town of Stanford le Hope in what is now Thurrock unitary authority.-Early history:...
, West Thurrock
West Thurrock
West Thurrock is a traditional Church of England parish and town in Thurrock, Essex, England, located 17.5 miles east south-east of Charing Cross, London.-Location:...
, Kempton Park
Kempton Park, Surrey
Kempton Park is a locality in the Spelthorne district of Surrey, which is the location of Kempton Park Racecourse.Kempton Park appears on the Middlesex Domesday Map as Chenetone. It was held by Robert, Count of Mortain...
, Shepperton
Shepperton
Shepperton is a town in the borough of Spelthorne, Surrey, England. To the south it is bounded by the river Thames at Desborough Island and is bisected by the M3 motorway...
, Staines
Staines
Staines is a Thames-side town in the Spelthorne borough of Surrey and Greater London Urban Area, as well as the London Commuter Belt of South East England. It is a suburban development within the western bounds of the M25 motorway and located 17 miles west south-west of Charing Cross in...
and Tilbury
Tilbury
Tilbury is a town in the borough of Thurrock, Essex, England. As a settlement it is of relatively recent existence, although it has important historical connections, being the location of a 16th century fort and an ancient cross-river ferry...
Gravels.
The sand and gravel terraces are made up of pebbles with flint, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and quartzite
Quartzite
Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
. In places, there are deposits of brick-earth, which is a mixture of clay and sand that has supported London's long-standing brick-making
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...
industry. Atop these natural layers are the deposits of hundreds of years of human occupation. In the oldest parts the City of London
City of London
The City of London is a small area within Greater London, England. It is the historic core of London around which the modern conurbation grew and has held city status since time immemorial. The City’s boundaries have remained almost unchanged since the Middle Ages, and it is now only a tiny part of...
and the City of Westminster
City of Westminster
The City of Westminster is a London borough occupying much of the central area of London, England, including most of the West End. It is located to the west of and adjoining the ancient City of London, directly to the east of the Royal Borough of Kensington and Chelsea, and its southern boundary...
this layer can be up to 6 metres deep.