Gault Clay
Encyclopedia
Gault is a clay formation of stiff blue clay
deposited in a calm, fairly deep water marine environment during the Lower Cretaceous
Period (Upper and Middle Albian
). It is well exposed in the coastal cliffs at Copt Point in Folkestone
, Kent
, England
, where it overlays the Lower Greensand formation,and is found in exposure on the south side of The North Downs
and the north side of the South Downs
. It is also to be found beneath the scarp
of the Berkshire Downs
, in the Vale of White Horse
, in Oxfordshire
, England, and on the Isle of Wight
where it is known as Blue Slipper. Gault underlies the chalk beneath the London Basin
, generally overlying eroded rocks of Jurassic
and Devonian
age; lower gault is present only below the outer parts of the basin and is absent under central London.
The clay has been used in several locations for making bricks, notably near Wye in Kent
.
Gault often contains numerous phosphatic nodules,some thought to be coprolites and may also contain sand as well as small grains of the mineral glauconite
. Crystals of the mineral selenite are fairly common in places, as are nodules of pyrite
.
Gault yields abundant marine fossil
s, including ammonite
s (such as Hoplites, Hamites, Euhoplites
, Anahoplites, and Dimorphoplites), belemnites
(such as Neohibolites), bivalves
(such as Birostrina and Pectinucula), gastropods
(such as Anchura), solitary coral
s, fish
remains (including shark
teeth), scattered crinoid
remains, and crustacean
s (such as the crab
Notopocorystes). Occasional fragments of fossil wood may also be found.
The Gault Formation consists of gault and the Upper Greensand. These represent different facies
, with the sandier parts probably being deposited close to the shore and the clay in quieter water further from the source of sediment; both are believed to be shallow-water deposits.
The Gault Formation represents a marine transgression following erosion of the Lower Greensand. It is subdivided into two sections, the Upper Gault and the Lower Gault. The Upper Gault onlap
s onto the Lower Gault. The Gault Formation thins across the London Platform and then terminates against the Red Chalk just to the south of The Wash
.
Gault exposure at Copt Point, which is the type locality
for the formation, is 40 m in thickness.
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
deposited in a calm, fairly deep water marine environment during the Lower 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...
Period (Upper and Middle Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
). It is well exposed in the coastal cliffs at Copt Point in Folkestone
Folkestone
Folkestone is the principal town in the Shepway District of Kent, England. Its original site was in a valley in the sea cliffs and it developed through fishing and its closeness to the Continent as a landing place and trading port. The coming of the railways, the building of a ferry port, and its...
, Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...
, 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...
, where it overlays the Lower Greensand formation,and is found in exposure on the south side of 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...
and the north side of the South Downs
South Downs
The South Downs is a range of chalk hills that extends for about across the south-eastern coastal counties of England from the Itchen Valley of Hampshire in the west to Beachy Head, near Eastbourne, East Sussex, in the east. It is bounded on its northern side by a steep escarpment, from whose...
. It is also to be found beneath the scarp
Escarpment
An escarpment is a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations.-Description and variants:...
of the Berkshire Downs
Berkshire Downs
The Berkshire Downs are a range of chalk downland hills in southern England, part of the North Wessex Downs Area of Outstanding Natural Beauty...
, in the Vale of White Horse
Vale of White Horse
The Vale of White Horse is a local government district of Oxfordshire in England. The main town is Abingdon, other places include Faringdon and Wantage. There are 68 parishes within the district...
, in Oxfordshire
Oxfordshire
Oxfordshire is a county in the South East region of England, bordering on Warwickshire and Northamptonshire , Buckinghamshire , Berkshire , Wiltshire and Gloucestershire ....
, England, and on the Isle of Wight
Isle of Wight
The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...
where it is known as Blue Slipper. Gault underlies the chalk beneath 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...
, generally overlying eroded rocks of Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
and 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...
age; lower gault is present only below the outer parts of the basin and is absent under central London.
The clay has been used in several locations for making bricks, notably near Wye in Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...
.
Gault often contains numerous phosphatic nodules,some thought to be coprolites and may also contain sand as well as small grains of the mineral glauconite
Glauconite
Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
. Crystals of the mineral selenite are fairly common in places, as are nodules of pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
.
Gault yields abundant marine fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, including ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s (such as Hoplites, Hamites, Euhoplites
Euhoplites
Euhoplites is an extinct ammonoid cephalopod from the Lower Creataceous, characterized bystrongly ribbed, more or less evolute, compressed to inflated shells with flat or concave rims, typically with a deep narrow groove running down the middle. In some, ribs seem to zigzag between umbilical...
, Anahoplites, and Dimorphoplites), belemnites
Belemnoidea
Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles...
(such as Neohibolites), bivalves
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
(such as Birostrina and Pectinucula), gastropods
Gastropoda
The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...
(such as Anchura), solitary coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
remains (including shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
teeth), scattered crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
remains, and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s (such as the crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
Notopocorystes). Occasional fragments of fossil wood may also be found.
Gault Formation
The Gault Formation consists of gault and the Upper Greensand. These represent different facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
, with the sandier parts probably being deposited close to the shore and the clay in quieter water further from the source of sediment; both are believed to be shallow-water deposits.
The Gault Formation represents a marine transgression following erosion of the Lower Greensand. It is subdivided into two sections, the Upper Gault and the Lower Gault. The Upper Gault onlap
Onlap
Onlap or overlap is the geologic phenomenon of successively younger rock strata extending progressively further across an erosion surface cut in older rocks. It is generally associated with a marine transgression. It is a more general term than overstep, in which the younger beds overlap onto...
s onto the Lower Gault. The Gault Formation thins across the London Platform and then terminates against the Red Chalk just to the south of The Wash
The Wash
The Wash is the square-mouthed bay and estuary on the northwest margin of East Anglia on the east coast of England, where Norfolk meets Lincolnshire. It is among the largest estuaries in the United Kingdom...
.
Gault exposure at Copt Point, which is the type locality
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
for the formation, is 40 m in thickness.