The
geology of Alderney includes similarities in its rock to the neighbouring
NormandyNormandy is a geographical region corresponding to the former Duchy of Normandy. It is situated along the English Channel coast of Northern France between Brittany and Picardy and comprises territory in northern France and the Channel Islands.Normandy is divided between French and British...
and
GuernseyGuernsey has a geological history stretching further back into the past than most of Europe. The southern part is constructed of Icart Gneiss. The Icart Gneiss is an augen gneiss of granitic composition containing potassium feldspar. This was formed from a granite dated at using U-Pb dating on...
. Although
AlderneyAlderney is the most northerly of the Channel Islands and a British Crown dependency. It is part of the Bailiwick of Guernsey. It is long and wide. The area is , making it the third largest island of the Channel Islands, and the second largest in the Bailiwick...
is only five kilometers long, it has a geological history spanning half of the life of the earth. It is part of the
ArmoricaArmorica 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...
n massif.
Relics of sediments appear as xenoliths in
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
s. However the earliest dated rock is the grey coloured Western
GranodioriteGranodiorite is an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite...
from in the
PaleoproterozoicThe Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
. As its name suggests it is found in the west end of Alderney.
The
geology of Alderney includes similarities in its rock to the neighbouring
NormandyNormandy is a geographical region corresponding to the former Duchy of Normandy. It is situated along the English Channel coast of Northern France between Brittany and Picardy and comprises territory in northern France and the Channel Islands.Normandy is divided between French and British...
and
GuernseyGuernsey has a geological history stretching further back into the past than most of Europe. The southern part is constructed of Icart Gneiss. The Icart Gneiss is an augen gneiss of granitic composition containing potassium feldspar. This was formed from a granite dated at using U-Pb dating on...
. Although
AlderneyAlderney is the most northerly of the Channel Islands and a British Crown dependency. It is part of the Bailiwick of Guernsey. It is long and wide. The area is , making it the third largest island of the Channel Islands, and the second largest in the Bailiwick...
is only five kilometers long, it has a geological history spanning half of the life of the earth. It is part of the
ArmoricaArmorica 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...
n massif.
Geological history
Relics of sediments appear as xenoliths in
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their...
s. However the earliest dated rock is the grey coloured Western
GranodioriteGranodiorite is an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite...
from in the
PaleoproterozoicThe Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
. As its name suggests it is found in the west end of Alderney. The xenoliths in it are dark ellipses that demonstrate that the rock has been squashed. This granite in turn was intruded by the Telegraph Bay Granite in the southernmost part of the island. This granite contains 50 mm
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
crystals.
ApliteAplite 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 continued from the same magma. The final stage of intrusion was a microgranite forming many dykes. Feldspar in the pink microgranite is only 2 mm big.
The next stage of geological history was the intrusion of the Central Diorite Complex that makes up the north and centre of the island. This belongs to the
Cadomian OrogenyThe 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...
time at . Embedded in the diorite are a couple of large
gabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
inclusions, as well as a picrite on the east of Braye Bay. Some of the diorite has orbicular structure, concentric spheres of
plagioclasePlagioclase is a very 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
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende 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. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
rich zones form balls up to 20 cm in diameter. A pale coloured granite intruded on the north: the Bibette Head Granite. This contains many xenoliths. Sodium rich dykes then were intruded.
In the next stage the terrane was uplifted, and eroded. Fine grained sand that formed
quartziteQuartzite 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...
was deposited. Further weathering ensued, with most of this deposit removed and
lateriteLaterite is a surface formation in hot and wet tropical areas which is rich in iron and aluminium and develops by intensive and long lasting weathering of the underlying parent rock. Nearly all kinds of rocks can be deeply decomposed by the action of high rainfall and elevated temperatures...
formed. Next a stream channel formed over the land, dumping coarse sand with feldspar. This formed a pink sandstone.The flow came from the northwest, with particles derived from granite and
gneissGneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large...
. Initailly this filled in the hollows in the underlying granites, but soon overflowed into a braided channel. Flood plain conditions caused layers of silt to form between the sand. These sediments deposited in the Cambrian are probably the final stage of the Cadomian Orogeny.
In the
Variscan OrogenyThe 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:...
folding and faulting affected all the rocks. Dolerite (or diabase) and
lamprophyreLamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...
dykes intruded. These are probably from the
CarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Ma , to the beginning of the Permian period, about 299.0 ± 0.8 Ma ....
period.
In the
PleistoceneThe Pleistocene is the epoch from 2.588 million to 12 000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
varying sea levels caused raised beaches to form 8, 18 and 30 meters above the current sea level. As in
JerseyThe 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...
,
loessLoess is an aeolian sediment formed by the accumulation of wind-blown silt and lesser and variable amounts of sand and clay. Loess sometimes refers to these deposits and the soil derived from them.-Properties:...
blew in as dust from the bare ground in the near glacial conditions in the
ice ageThe general term "ice age" or, more precisely, "glacial age" denotes a geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in an expansion of continental ice sheets, polar ice sheets and alpine glaciers. Within a long-term ice age, individual...
s. Head also formed in the periglacial circumstances by breaking off rock fragments and mixing with dirt.