The geology of Alderney includes similarities in its rock to the neighbouring

Normandy Normandy (; french: link=no, Normandie ; nrf, Normaundie, Nouormandie ; from Old French , plural of ''Normant'', originally from the word for "northman" in several Scandinavian languages) is a geographical and cultural region in Northwestern ...

and

Guernsey Guernsey (; Guernésiais: ''Guernési''; french: Guernesey) is an island in the English Channel off the coast of Normandy that is part of the Bailiwick of Guernsey, a British Crown Dependency. It is the second largest of the Channel Islands ...

. Although

Alderney Alderney (; french: Aurigny ; Auregnais: ) is the northernmost of the inhabited Channel Islands. It is part of the Bailiwick of Guernsey, a British Crown dependency. It is long and wide. The island's area is , making it the third-largest ...

is only five kilometers long, it has a geological history spanning half of the life of the earth. It is part of the Armorican Massif.

Geological history

Relics of sediments appear as

xenoliths A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in ign ...

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

s. However the earliest dated rock is the grey coloured Western

Granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

from in the

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

. 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

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

crystals. Aplite 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 Orogeny time at . Embedded in the diorite are a couple of large

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is che ...

inclusions, as well as a

picrite Picrite basalt or picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts (20-50%) and black to dark brown pyroxene, mostly augite. The olivine-rich p ...

on the east of Braye Bay. Some of the diorite has orbicular structure, concentric spheres of

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

and

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. Hornblende minerals are common in igneous and metamorphic rocks ...

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

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

was deposited. Further weathering ensued, with most of this deposit removed and

laterite Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...

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

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

. initially 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 Orogeny The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Variscan'', comes f ...

folding and faulting affected all the rocks.

Dolerite Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained ...

(or diabase) and

lamprophyre Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxid ...

dykes intruded. These are probably from the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

period. In the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

varying sea levels caused raised beaches to form 8, 18 and 30 meters above the current sea level. As in

Jersey Jersey ( , ; nrf, Jèrri, label=Jèrriais ), officially the Bailiwick of Jersey (french: Bailliage de Jersey, links=no; Jèrriais: ), is an island country and self-governing Crown Dependencies, Crown Dependency near the coast of north-west F ...

loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...

blew in as dust from the bare ground in the near glacial conditions in the

ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gree ...

s. Head also formed in the periglacial circumstances by breaking off rock fragments and mixing with dirt.

References

*''Great Britain: Channel Islands'' in ''Encyclopedia of European and Asian Regional Geology'' by Eldridge M. Moores, Rhodes Whitmore Fairbridge, Published 1997 by Springer pp 276–277. *N d'A Laffoley: ''Geological excursion guide 2: Alderney, Channel Islands'' in ''Geology Today'' volume 1 number 5 page 151 1985 {{Geology of Europe Geography of Alderney