Guernsey has a geological history stretching further back into the
past than most of Europe. The majority of rock exposures on the Island
may be found along the coastlines, with inland exposures scarce and
usually highly weathered. There is a broad geological division between
the north and south of the Island. The Southern Metamorphic Complex is
elevated above the geologically younger, lower lying Northern Igneous
Guernsey has experienced a complex geological evolution
(especially the rocks of the southern complex) with multiple phases of
intrusion and deformation recognisable.
1 Geological History
The southern part if the island is largely composed of Icart Gneiss.
Gneiss is an augen gneiss of granitic composition containing
potassium feldspar. This was formed from a granite dated at
2,061 million years ago using U-Pb dating on zircon grains. A
foliated Perelle quartz diorite (also called Perelle Gneiss), occurs
in the centre and west of the island. This is a calc-alkaline
tonalitic rock. The foliation was formed at around 600 million
years ago during the Cadomian Orogeny. Rafts of metamorphosed
sediments, older than the Foliated Perelle
Diorite are embedded
The Pleinmont Formation consists of metamorphosed sediments is of
unknown age, although it has been proposed that these belong to the
Brioverian group that outcrop on nearby
Jersey It is named after
Pleinmont Point on the south west tip of the Island.
The older rocks were deformed during the Cadomian Orogeny. The Icart
Gneisses formed the basement to an outboard terrane that was
subsequently accreted to Armorica.
The Cobo Granite was formed at approximately 570 million years
ago, named after Cobo Bay on the mid west coast. The north end of
the island is an unfoliated calc-alkaline pluton of the Bordeaux
Diorite Complex consisting of diorite, tonalite, and granodiorite.
This is also dated at 570 million years ago.
On the central east coast around
Saint Peter Port
Saint Peter Port is the St Peter Port
Gabbro containing layers with olivine, hornblende and two kinds of
pyroxene. The igneous plutonic intrusion is 2.5 km from north to
south and is 0.8 km thick. It dips shallowly to the west. The
lower and upper portions are layered on the scale of a meter, while
millimeter scale layering is found on the uppermost parts. This is
also dated at 570 million years ago. Near Vale Castle the rock is
of a type called bojite with interlocking hornblende and plagioclase
Quaternary Devensian glaciation, loess was deposited, blown
in by wind from the west. The island was only separated from the
continent of Europe by rising sea levels at about 5000 BC during the
new stone age.
Tight folds at the contact between Icart
Gneiss (light) and
mylonitised quartz diorite (dark), western end of
Many of the rocks present in the south of
Guernsey may also be
Lihou Island. At the western coast of the island, a shear
zone is exposed at the contact between the Perelle Foliated Quartz
Diorite and the Icart Gneiss. The younger quartz diorite is
mylonitised, where field evidence suggests that it was most likely
deformed synchronously with its intrusion. The contact between the two
rocks is tightly folded, as are the mylonitic fabrics in the two
Dykes are abundant on the Island, largely of doleritic composition.
Guernsey has had an active quarrying industry over the years, largely
removing rock for use as building materials. Many local houses are
constructed of either blue-grey Bordeaux Diorite, or red-brown Cobo
Granite. The only remaining active quarry is Les Vardes on the west
coast of the Island, operated by Ronez. Here the Bordeaux
extracted and crushed on site to produce aggregate.
Many of the disused quarry sites have been allowed to fill with water,
such as St. Andrews Reservoir, now used by the States of Guernsey
water board. Mont Cuet is another former quarry, now used as a
landfill site to dispose of the majority of the Island's
^ Gibbons, Wes; Harris, Anthony Leonard; London, Geological Society of
(1994). A Revised Correlation of
Precambrian Rocks in the British
Isles. Geological Society of London. p. 102.
Eldridge M. Moores; Rhodes Whitmore Fairbridge, eds. (1997). "Great
Britain: Channel Islands". Encyclopedia of European and Asian regional
geology. London: Chapman & Hall. pp. 276–277.
Renouf, John (May 1985). "Geological excursion guide 1:
Guernsey, Channel Islands". Geology Today. 1 (3): 90–93.
Geology of Europe
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Isle of Man
Bay of Biscay
basins and grabens
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European Cenozoic Rift System
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