Geology of Jersey
Encyclopedia
The geology of Jersey is characterised by the Late Proterozoic Brioverian volcanics, the Cadomian Orogeny
, and only small signs of later deposits from the Cambrian
and Quaternary
periods. The kind of rocks go from conglomerate to shale, volcanic, intrusive and plutonic igneous rocks of many compositions, and metamorphic rocks as well, thus including most major types.
s in the west, centre and south of Jersey, including at St Ouens Bay
, and St Aubins Bay. The Brioverian sedimentary rocks are all well bedded and were originally the mid and outer parts of a submarine fan. This constitutes the Jersey Shale Formation. The shale
is found at Gorey Harbour in the east coast, and La Belle Hougue Point, and Le Mont Mado granite has an occurrence. The shale is more easily eroded, and this has affected the shape of the island by the concave St Ouens's Bay and St Aubin's Bay. The deepest valleys (Valley of St. Peter and St. Lawrence) are cut into this soft rock. Other sediments associated with the shale are mudstone
, and fine grained sandstone
. Various sedimentary structures include flute and bounce castes, ripple lamination, graded bedding
, cross bedding, and boudinage
. The shale layers have been identified as Association IV in the submarine fan.
The sandstone is Association III of the submarine fan. Greywacke
is termed Association IV along with the shale. It is found in the same areas as the shale. In an analysis of the greywacke it has fragments with 70% quartz
, 10% to 15% plagioclase
and microcline
feldspar
, 2% dark iron containing minerals, with carbon flakes, in a matrix of 10 to 20% clay. The minerals contained are very diverse indicating a broad source area.
Conglomerate
, termed Association I from the upper parts of the fan, is found in lenticular bodies near St Peter's Valley, at Gargate Mill. The pebbles in the conglomerate are of multiple kinds of rock. The granite intrusions at the boundary of the formation have caused metamorphism and intrusion by dykes.
In the L'Êtacq area the metamorphis has produces spotted hornfels
with a grey colour. The spots are darker, containing cordierite
and biotite
. At St. Ouën's Bay the metamorphosis of greywacke has made hornblende-hornfels. Regional metamorphism has converted the shale to a low greenschist facies, where clay is converted to chlorite
.
The interpretation of this area is that is a part of the north facing continental slope of Armorica
facing a subduction
zone, where the Celtic oceanic plate was converging and descending in the trench. The sediments were carried by rivers from the Le Vast Arc, a strip of land oriented east north east, that lay to the south of Jersey. The sea at this time was termed l' Océan de la Manche.
from St. Saviour's Andesite Formation is in the centre and in the north at Les Rouaux. It is coloured grey and includes some basalt and pyroclastic fallout such as tuff
and agglomerate
. Porphyritic andesite is dark grey with white crystals of plagioclase feldspar embedded. Outcrops occur on the coast at Giffards Bay on the north coast and Vicard Point on the east coast. The volcanoes were to the north east and south. Vicard Tuff contains bombs. The Long Echet Tuff contains large crystals of quartz and feldspar and strands of quartz. Les Rouaux Agglomerate and L' Homme Mort Agglomerate contain fragments of the surrounding rocks including shale and andesite embedded in a feldspar matrix. The Les Rouaux Agglomerate also contains pumice
. This was eroded before the next phase of volcanism.
Porphoritic rhyolite
of St. John's Rhyolite Formation is found to the north east of the centre. It contains banded flows and ignimbrite. Totally it is 950 meters thick. It contains shards of glass and pumice. Different flows exits with sections called Bonne Nuit Ignimbrite followed by Frémont Ignimbrite on the north coast. They contain xenoliths of the andesite and shale found in Jersey. Also there are local mudstone and conglomerate deposited on the flows. The conglomerate is called L'Homme Mort Conglomerate.
The Jeffrey's Leap Ignimbrite is overlaid by the Anne Port Ignimbrite named after locations on the east coast. The Trinity Ignimbrite is a purple tuff near Les Grands Vaux, inland.
Fine grained rhyolite of Bouley Rhyolite Formation occurs even further to the north east. This is 430 meters thick. At Giffard Bay are Gifford Rhyolite, Gifford Andesite, Gifford Ignimbrite and Gifford Tuffs. Les Platons Rhyolite top this off. These rocks contain xenoliths and tuff deposits from lakes and streams. The Les Platons Rhyolite contains spherulites. Spherulites are also found at Les Hurets on Bouley Bay. At Bouley Bay there are three units of ignimbrite: the Lower Bouley Ignimbrite, Middle Bouley Ignimbrite and Upper Bouley Ignimbrite. These contain flow banding and spherulites up to 10 cm and eutaxitic and fiamme textures. The Anne Port Rhyolite is the bottom unit on the east coast. This is massive but columnar jointed. It shows flow banding. It resembles the Giants Causeway at La Crête Point.
Le Havre de Fer Beach has an outcrop of columnar rhyloite that has been separated by faulting from the rest of the units. North of Archirondel Round Tower there are three more flows stacked on top of each other called Archirondel Ignimbrite, Dolmen Ignimbrite and St. Catherine's Ignimbrite. These are separated from each other by layers of tuff, and are coloured maroon by hematite
. They consist of pumice, quartz crystals and feldspar in a feldspar rich matrix.
between . In this orogeny the rocks were first crushed and folded with East West pressure, then later they were compressed in the north south direction. Gabbro is found at Sorel Point. This rock can be observed to transition into diorite by changes in the mineral content and texture. Layered gabbro has also been changed into diorite at Le Nez point, the southernmost point of the main island. The diorite is emplaced at and near Sorel point, and at several places along the south coast east of Saint Helier
. It also is found on the south east tidal rock platform.
Granite intrudes into the north west, south west and south east. The south west granite also includes porphyries and some fine grained sections.
The south west granites are 550 - 480 mya, and has three intrusions. The intrusions are called Corbière Granite, La Moye Granite, and Beau Port Granite. They are coloured pale to deep red. The north west area was intruded with four intrusions. One of these intrusions contained the gabbro and diorite. These granites are coloured grey-pink and orange, with the gabbro a dark grey, and the diorite speckled grey.
There are also pink and red aplite
veins in the igneous complexes up to 5 cm wide. The aplite is the lowest melting remanent of the granite magma, that is poor in volatiles. The aplite minerals include tourmaline
, topaz
and fluorite
. At St Brelade's Bay there are two aplogranites: Beau Port Granite and La Moye Granite. This is coloured yellow brown to pink and contains fine crystals of perthite
, and oligoclase
and quartz
, coloured by limonite
. Other parts more rich in volatiles have formed pegmatite
s featuring milky quartz and pink orthoclase
.
The north west area was intruded with four intrusions. The magma came from a volcanic arc. One of these intrusions contained the gabbro and diorite. The granites are coloured grey-pink and orange, with the gabbro a dark grey, and the diorite speckled grey. The main intrusion is the St. Mary's Granite . One other intrusion was an aplogranite called Mont Mado granite, or Red Granite solidified . This is mostly made from fine grained quartz and Perthite, but is coloured red yellow and brown. A third porphyritic granite is .
The south east granites moved into the south east about the same time. The components here are called Fort Regent/Elizabeth Castle Granophyre, Dicq Granite, Longueville Granite, and La Roque Granite.
The plutonic rocks are not foliated, showing that the orogeny was near completion at the time they formed.
. The pebbles consist of andesite, rhyolite, granite and shale. The pebbles range from 1 cm to 60 cm, and are quite irregular. The conglomerate bed is above a bed of red sandstone and mudstone which has pits, probably being rain drop marks, and also has polygonal shaped cracks. Apart from the main outcrop, the conglomerate is also found in smaller areas at Les Hurets Valley, west of the Bouley Bay and at La Pierre de Fételle. The conglomerate is estimated to be Ordovician in age.
The Variscan Orogeny
left a smaller print on the island, with the intrusion of some dykes, and some folding and jointing that affected the island's rocks including the Rozel Conglomerate. During the Mezozoic and Tertiary
, Jersey was part the north west edge of Armorica
. The dykes included a swarm of dolerite dykes, also lamprophyre
, feldspar
porphyry and aplite.
There is a known acid dykes of rhyolitic composition on the east side of Noirmont headland. This is 1 meter wide and coloured light grey to pink. It has an east-west strike. Porhyritic microgranite dykes occur below Le Saut Jeffroi, and south of Le Mont Orgueil. These have a NNE strike.
The aplite dykes are one to two meters wide.
There are two sets of lamprophyre
dykes. Biotite
lamprophyres (minettes) are oriented in the north west direction. They are coloured brown, and are up to 1 meter wide. They contain orthoclase. Hornblende
lamprophyre (spessarites) contain plagioclase, coloured light brown and are oriented north-south. A hornblende lamprophyre dyke in the Rozel Conglomerate is dated at .
The Jersey Main Dyke Swarm consists of dolerite dykes intruded into the South East Igneous complex. Some dolerite dykes have a porphyry centre. The strike is north west, and the direction is vertical. Augite and labradorite are found in these dolerites.
In the north west there are sills of dolerite.
During the Tertiary the surface of the island was a plain at 60 to 130 meters above sea level.
These raised beaches are found at Portelet Bay, Giffard Bay, and Belcroute Bay. Flint
is found amongst the pebbles on these beaches, and this is derived from chalk
deposits only found underwater.
La Cotte de St Brelade has a Neanderthal
rock shelter which was inhabited 200000 years ago by hunters of woolly mammoth
and rhinoceros amongst other animals.
During the Quaternary
Devensian glaciation, loess
was deposited, blown in by wind from the west. The loess has formed thick deposits on the island interior and combined with periglacial
frost shattered rock fragments sliding down the cliffs to form head which have themselves been eroded to form cliffs from 3 to 12 meters high. At Belcroute there is a more complex deposit of loess head on a raised beach deposit elevated at 8 meters, that sits on another loess deposit. Head occurs at the foot of cliffs along the north, north east and south west sides, and can also be found beneath wind blown sand at the bays of St. Ouen, St. Aubin, St. Clement and the Royal Bay of Grouville. The thickest parts of loess are five meters deep at St Clements and at La Hougue Bie
on the eastern plateau.
Peat occurs in the valleys. At Quetivel Mill in St. Peter’s Valley there is peat dated to 7600 BC which contains pollen from boreal forest.
The island was only separated from the continent of Europe by rising sea levels about 5000 BC during the new stone age.
There are several sea caves. A notable cave is the Le Creux du Vis. This cave has a tunnel going in from the beach at the base of the cliff, and a crater shaped opening with steep rocky sides above.
The only fossils date from the Quaternary, and are found in raised beaches, peat and clay. Mammuthus primigenius bones have been found at La Cotte de St. Brelade along with deer bones. Mollusk shells are found in the Loess and beach deposits. A submerged forest in St. Ouën's Bay between L'Ouzière slipway and Le Port Slipway has stumps of Birch and Alder (Betula sp and Alnus sp) from 1980 BC. Legends record that the sea encroached on the forest in the fifteenth century.
, molybdenite
and haematite.
Rocks have been used in Neolithic
times to build dolmen
s. These are found at La Hougue Bie in St. Saviour and Le Mont Ubé dolmen, St. Clément and La Pouquelaye de Faldouet and Le Couperon.
China clay quarries used to be in what is now Handois reservoirs. Bricks have also been made from clay from St. Saviour. La Société Jersiaise is making a brick archive.
Western Jersey sedimentary rocks are underlain by shallow granite, and in eastern Jersey there is a high in the gravity field near Grande Charriere, which could be due to gabbro.
Cadomian Orogeny
The Cadomian Orogeny was a tectonic event or series of events in the late Neoproterozoic, about 650-550 Ma, which probably included the formation of mountains. This occurred on the margin of the Gondwana continent, involving one or more collisions of island arcs and accretion of other material at a...
, and only small signs of later deposits from the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
and Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
periods. The kind of rocks go from conglomerate to shale, volcanic, intrusive and plutonic igneous rocks of many compositions, and metamorphic rocks as well, thus including most major types.
Brioverian sediments
The Brioverian rocks were formed between 900 and 700 mya. They were named after Briovère, the native name for St. Lô, in Normandy, which is the first area these rocks were described from. They start with turbiditeTurbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
s in the west, centre and south of Jersey, including at St Ouens Bay
Saint Ouen, Jersey
-Cueillettes:Unlike the other parishes of Jersey, the subdivisions of this parish are not named vingtaines, but cueillettes . Vingteniers are still elected, however, in the cueillettes.*La Petite Cueillette*La Grande Cueillette...
, and St Aubins Bay. The Brioverian sedimentary rocks are all well bedded and were originally the mid and outer parts of a submarine fan. This constitutes the Jersey Shale Formation. The shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
is found at Gorey Harbour in the east coast, and La Belle Hougue Point, and Le Mont Mado granite has an occurrence. The shale is more easily eroded, and this has affected the shape of the island by the concave St Ouens's Bay and St Aubin's Bay. The deepest valleys (Valley of St. Peter and St. Lawrence) are cut into this soft rock. Other sediments associated with the shale are mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
, and fine grained sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. Various sedimentary structures include flute and bounce castes, ripple lamination, graded bedding
Graded bedding
In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones...
, cross bedding, and boudinage
Boudinage
thumb|Boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as a bed of sandstone, is stretched and deformed amidst less competent surroundings...
. The shale layers have been identified as Association IV in the submarine fan.
The sandstone is Association III of the submarine fan. Greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
is termed Association IV along with the shale. It is found in the same areas as the shale. In an analysis of the greywacke it has fragments with 70% 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,...
, 10% to 15% plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
and microcline
Microcline
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, 2% dark iron containing minerals, with carbon flakes, in a matrix of 10 to 20% clay. The minerals contained are very diverse indicating a broad source area.
Conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, termed Association I from the upper parts of the fan, is found in lenticular bodies near St Peter's Valley, at Gargate Mill. The pebbles in the conglomerate are of multiple kinds of rock. The granite intrusions at the boundary of the formation have caused metamorphism and intrusion by dykes.
In the L'Êtacq area the metamorphis has produces spotted hornfels
Hornfels
Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...
with a grey colour. The spots are darker, containing cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
. At St. Ouën's Bay the metamorphosis of greywacke has made hornblende-hornfels. Regional metamorphism has converted the shale to a low greenschist facies, where clay is converted to chlorite
Chlorite
The chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.-Oxidation states:...
.
The interpretation of this area is that is a part of the north facing continental slope of Armorica
Armorica
Armorica or Aremorica is the name given in ancient times to the part of Gaul that includes the Brittany peninsula and the territory between the Seine and Loire rivers, extending inland to an indeterminate point and down the Atlantic coast...
facing a subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zone, where the Celtic oceanic plate was converging and descending in the trench. The sediments were carried by rivers from the Le Vast Arc, a strip of land oriented east north east, that lay to the south of Jersey. The sea at this time was termed l' Océan de la Manche.
Brioverian volcanics
These sediments were uplifted and are overlaid by volcanic rocks of the Jersey Volcanic Group around 530 Mya. The magma came from the subduction zones below and to the north of the island. AndesiteAndesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
from St. Saviour's Andesite Formation is in the centre and in the north at Les Rouaux. It is coloured grey and includes some basalt and pyroclastic fallout such as tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
and agglomerate
Agglomerate
Agglomerates are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs...
. Porphyritic andesite is dark grey with white crystals of plagioclase feldspar embedded. Outcrops occur on the coast at Giffards Bay on the north coast and Vicard Point on the east coast. The volcanoes were to the north east and south. Vicard Tuff contains bombs. The Long Echet Tuff contains large crystals of quartz and feldspar and strands of quartz. Les Rouaux Agglomerate and L' Homme Mort Agglomerate contain fragments of the surrounding rocks including shale and andesite embedded in a feldspar matrix. The Les Rouaux Agglomerate also contains pumice
Pumice
Pumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
. This was eroded before the next phase of volcanism.
Porphoritic rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
of St. John's Rhyolite Formation is found to the north east of the centre. It contains banded flows and ignimbrite. Totally it is 950 meters thick. It contains shards of glass and pumice. Different flows exits with sections called Bonne Nuit Ignimbrite followed by Frémont Ignimbrite on the north coast. They contain xenoliths of the andesite and shale found in Jersey. Also there are local mudstone and conglomerate deposited on the flows. The conglomerate is called L'Homme Mort Conglomerate.
The Jeffrey's Leap Ignimbrite is overlaid by the Anne Port Ignimbrite named after locations on the east coast. The Trinity Ignimbrite is a purple tuff near Les Grands Vaux, inland.
Fine grained rhyolite of Bouley Rhyolite Formation occurs even further to the north east. This is 430 meters thick. At Giffard Bay are Gifford Rhyolite, Gifford Andesite, Gifford Ignimbrite and Gifford Tuffs. Les Platons Rhyolite top this off. These rocks contain xenoliths and tuff deposits from lakes and streams. The Les Platons Rhyolite contains spherulites. Spherulites are also found at Les Hurets on Bouley Bay. At Bouley Bay there are three units of ignimbrite: the Lower Bouley Ignimbrite, Middle Bouley Ignimbrite and Upper Bouley Ignimbrite. These contain flow banding and spherulites up to 10 cm and eutaxitic and fiamme textures. The Anne Port Rhyolite is the bottom unit on the east coast. This is massive but columnar jointed. It shows flow banding. It resembles the Giants Causeway at La Crête Point.
Le Havre de Fer Beach has an outcrop of columnar rhyloite that has been separated by faulting from the rest of the units. North of Archirondel Round Tower there are three more flows stacked on top of each other called Archirondel Ignimbrite, Dolmen Ignimbrite and St. Catherine's Ignimbrite. These are separated from each other by layers of tuff, and are coloured maroon by hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
. They consist of pumice, quartz crystals and feldspar in a feldspar rich matrix.
Cadomian Orogeny
Intrusions were formed during the Cadomian OrogenyCadomian Orogeny
The Cadomian Orogeny was a tectonic event or series of events in the late Neoproterozoic, about 650-550 Ma, which probably included the formation of mountains. This occurred on the margin of the Gondwana continent, involving one or more collisions of island arcs and accretion of other material at a...
between . In this orogeny the rocks were first crushed and folded with East West pressure, then later they were compressed in the north south direction. Gabbro is found at Sorel Point. This rock can be observed to transition into diorite by changes in the mineral content and texture. Layered gabbro has also been changed into diorite at Le Nez point, the southernmost point of the main island. The diorite is emplaced at and near Sorel point, and at several places along the south coast east of Saint Helier
Saint Helier
Saint Helier is one of the twelve parishes of Jersey, the largest of the Channel Islands in the English Channel. St. Helier has a population of about 28,000, roughly 31.2% of the total population of Jersey, and is the capital of the Island . The urban area of the parish of St...
. It also is found on the south east tidal rock platform.
Granite intrudes into the north west, south west and south east. The south west granite also includes porphyries and some fine grained sections.
The south west granites are 550 - 480 mya, and has three intrusions. The intrusions are called Corbière Granite, La Moye Granite, and Beau Port Granite. They are coloured pale to deep red. The north west area was intruded with four intrusions. One of these intrusions contained the gabbro and diorite. These granites are coloured grey-pink and orange, with the gabbro a dark grey, and the diorite speckled grey.
There are also pink and red aplite
Aplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
veins in the igneous complexes up to 5 cm wide. The aplite is the lowest melting remanent of the granite magma, that is poor in volatiles. The aplite minerals include tourmaline
Tourmaline
Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors...
, topaz
Topaz
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO42. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces.-Color and varieties:...
and fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
. At St Brelade's Bay there are two aplogranites: Beau Port Granite and La Moye Granite. This is coloured yellow brown to pink and contains fine crystals of perthite
Perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...
, and oligoclase
Oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
and 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,...
, coloured by limonite
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
. Other parts more rich in volatiles have formed pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s featuring milky quartz and pink orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
.
The north west area was intruded with four intrusions. The magma came from a volcanic arc. One of these intrusions contained the gabbro and diorite. The granites are coloured grey-pink and orange, with the gabbro a dark grey, and the diorite speckled grey. The main intrusion is the St. Mary's Granite . One other intrusion was an aplogranite called Mont Mado granite, or Red Granite solidified . This is mostly made from fine grained quartz and Perthite, but is coloured red yellow and brown. A third porphyritic granite is .
The south east granites moved into the south east about the same time. The components here are called Fort Regent/Elizabeth Castle Granophyre, Dicq Granite, Longueville Granite, and La Roque Granite.
The plutonic rocks are not foliated, showing that the orogeny was near completion at the time they formed.
Rozel Conglomerate
The land here was uplifted and eroded in the Cambrian. A coarse conglomerate known as Rozel Conglomerate was washed in by a flash flood flowing from somewhere to the north of the island during the Cambrian to Ordovician time period. This conglomerate is on the north east cape at RozelRozel
Rozel may refer to:*Le Rozel, Manche, Basse-Normandie, France*Rozel, Kansas, United States...
. The pebbles consist of andesite, rhyolite, granite and shale. The pebbles range from 1 cm to 60 cm, and are quite irregular. The conglomerate bed is above a bed of red sandstone and mudstone which has pits, probably being rain drop marks, and also has polygonal shaped cracks. Apart from the main outcrop, the conglomerate is also found in smaller areas at Les Hurets Valley, west of the Bouley Bay and at La Pierre de Fételle. The conglomerate is estimated to be Ordovician in age.
Variscan Orogeny
Sinistral tear faults have affected the rock.The Variscan Orogeny
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
left a smaller print on the island, with the intrusion of some dykes, and some folding and jointing that affected the island's rocks including the Rozel Conglomerate. During the Mezozoic and Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
, Jersey was part the north west edge of Armorica
Armorica
Armorica or Aremorica is the name given in ancient times to the part of Gaul that includes the Brittany peninsula and the territory between the Seine and Loire rivers, extending inland to an indeterminate point and down the Atlantic coast...
. The dykes included a swarm of dolerite dykes, also lamprophyre
Lamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
, feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
porphyry and aplite.
There is a known acid dykes of rhyolitic composition on the east side of Noirmont headland. This is 1 meter wide and coloured light grey to pink. It has an east-west strike. Porhyritic microgranite dykes occur below Le Saut Jeffroi, and south of Le Mont Orgueil. These have a NNE strike.
The aplite dykes are one to two meters wide.
There are two sets of lamprophyre
Lamprophyre
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
dykes. Biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
lamprophyres (minettes) are oriented in the north west direction. They are coloured brown, and are up to 1 meter wide. They contain orthoclase. Hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
lamprophyre (spessarites) contain plagioclase, coloured light brown and are oriented north-south. A hornblende lamprophyre dyke in the Rozel Conglomerate is dated at .
The Jersey Main Dyke Swarm consists of dolerite dykes intruded into the South East Igneous complex. Some dolerite dykes have a porphyry centre. The strike is north west, and the direction is vertical. Augite and labradorite are found in these dolerites.
In the north west there are sills of dolerite.
Tertiary
During the Eocene limestone was deposited in the sea around Jersey, but none is actually on the island.During the Tertiary the surface of the island was a plain at 60 to 130 meters above sea level.
Quaternary
The high surface has been eaten into by cliffs dropping down to 8 meters. These have been formed during the Quaternary when the sea level was around 8 meters above the current sea level. There are also raised beaches at 18 and 30 meters above current sea level. During the glacial periods valleys were eroded deeply below the sea levelThese raised beaches are found at Portelet Bay, Giffard Bay, and Belcroute Bay. 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...
is found amongst the pebbles on these beaches, and this is derived from 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....
deposits only found underwater.
La Cotte de St Brelade has a Neanderthal
Neanderthal
The Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of western and central Asia...
rock shelter which was inhabited 200000 years ago by hunters of woolly mammoth
Woolly mammoth
The woolly mammoth , also called the tundra mammoth, is a species of mammoth. This animal is known from bones and frozen carcasses from northern North America and northern Eurasia with the best preserved carcasses in Siberia...
and rhinoceros amongst other animals.
During the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
Devensian glaciation, loess
Loess
Loess is an aeolian sediment formed by the accumulation of wind-blown silt, typically in the 20–50 micrometre size range, twenty percent or less clay and the balance equal parts sand and silt that are loosely cemented by calcium carbonate...
was deposited, blown in by wind from the west. The loess has formed thick deposits on the island interior and combined with periglacial
Periglacial
Periglacial is an adjective originally referring to places in the edges of glacial areas, but it has later been widely used in geomorphology to describe any place where geomorphic processes related to freezing of water occur...
frost shattered rock fragments sliding down the cliffs to form head which have themselves been eroded to form cliffs from 3 to 12 meters high. At Belcroute there is a more complex deposit of loess head on a raised beach deposit elevated at 8 meters, that sits on another loess deposit. Head occurs at the foot of cliffs along the north, north east and south west sides, and can also be found beneath wind blown sand at the bays of St. Ouen, St. Aubin, St. Clement and the Royal Bay of Grouville. The thickest parts of loess are five meters deep at St Clements and at La Hougue Bie
La Hougue Bie
La Hougue Bie is a historic site in the Parish of Grouville, Jersey. Hougue is a Jèrriais/Norman language word meaning a "mound" and comes from the Old Norse word haugr. Bie is of uncertain origin...
on the eastern plateau.
Peat occurs in the valleys. At Quetivel Mill in St. Peter’s Valley there is peat dated to 7600 BC which contains pollen from boreal forest.
The island was only separated from the continent of Europe by rising sea levels about 5000 BC during the new stone age.
There are several sea caves. A notable cave is the Le Creux du Vis. This cave has a tunnel going in from the beach at the base of the cliff, and a crater shaped opening with steep rocky sides above.
The only fossils date from the Quaternary, and are found in raised beaches, peat and clay. Mammuthus primigenius bones have been found at La Cotte de St. Brelade along with deer bones. Mollusk shells are found in the Loess and beach deposits. A submerged forest in St. Ouën's Bay between L'Ouzière slipway and Le Port Slipway has stumps of Birch and Alder (Betula sp and Alnus sp) from 1980 BC. Legends record that the sea encroached on the forest in the fifteenth century.
Economic geology
At Le Pulec there is a small deposit of lead and zinc that was mined in the late 19th century, but the venture was unsuccessful. There are also some other small unmined mineral veins, such as ankeriteAnkerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca2. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron and manganese.The crystallographic and...
, molybdenite
Molybdenite
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...
and haematite.
Rocks have been used in Neolithic
Neolithic
The Neolithic Age, Era, or Period, or New Stone Age, was a period in the development of human technology, beginning about 9500 BC in some parts of the Middle East, and later in other parts of the world. It is traditionally considered as the last part of the Stone Age...
times to build dolmen
Dolmen
A dolmen—also known as a portal tomb, portal grave, dolmain , cromlech , anta , Hünengrab/Hünenbett , Adamra , Ispun , Hunebed , dös , goindol or quoit—is a type of single-chamber megalithic tomb, usually consisting of...
s. These are found at La Hougue Bie in St. Saviour and Le Mont Ubé dolmen, St. Clément and La Pouquelaye de Faldouet and Le Couperon.
China clay quarries used to be in what is now Handois reservoirs. Bricks have also been made from clay from St. Saviour. La Société Jersiaise is making a brick archive.
Quarrying
Granite quarrying still occurs in three quarries in the north west at Ronez, Gigoulande (by Granite Products company) and La Saline. The La Saline company produces decorative stone for building. The other two quarries produce aggregate for roads and concrete and a product called hoggin. There are many other disused or abandoned quarries that have been used for stone to make the buildings, castles, and piers on the island.Sand
Sand mining takes place at St. Ouen’s Bay in the west by Simon's Sand and Gravel, and formerly at Grouville in the east.Water
Ground water is found in rock fractures and to a limited extend in porous rocks, in the upper 40 meters. Ground water is irregular, with some perched aquifers and vertical screens preventing underground flow. The water underground is young (as determined by isotope analysis), replenished from the surface from local rain, and not by underground streams from Normandy.Earthquakes
Earth tremors occur more commonly near Jersey that other areas in the Channel Islands, with magnitudes up to 3. The epicentres are mostly on the sea floor to the west south and east.Study
La Société Jersiaise has a geology section that studies, presents talks, and tours of the Geology of the island.Geophysics
The tidal range is high at 13 meters, exposing a wide rock platform around Jersey.Western Jersey sedimentary rocks are underlain by shallow granite, and in eastern Jersey there is a high in the gravity field near Grande Charriere, which could be due to gabbro.