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

Normandy

Normandy is a geographical region corresponding to the former Duchy of Normandy. It is in France.The continental territory covers 30,627 km² and forms the preponderant part of Normandy and roughly 5% of the territory of France. It is divided for administrative purposes into two régions:...

 and Guernsey

Geology of Guernsey

Guernsey 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 Alderney

Alderney

Alderney is the most northerly of the Channel Islands. It is part of the Bailiwick of Guernsey, a British Crown dependency. 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 Armorican Massif

Armorican Massif

The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers...

.

Geological history

Relics of sediments appear as xenoliths in granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

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

Granodiorite

Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...

 from in the Paleoproterozoic

Paleoproterozoic

The 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 feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 crystals. 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 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

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...

 time at . Embedded in the diorite are a couple of large gabbro

Gabbro

Gabbro 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 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 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....

 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

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...

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

Laterite

Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...

 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

Gneiss 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.-Etymology:...

. 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

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:...

 folding and faulting affected all the rocks. Dolerite (or diabase) and lamprophyre

Lamprophyre

Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions...

 dykes intruded. These are probably from the Carboniferous

Carboniferous

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...

 period.

In the Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans 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 Jersey

Geology of Jersey

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...

, 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...

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

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

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