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 COMPLEX.
* 1 Geological History
The southern part is largely composed of ICART GNEISS. The Icart
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
The older rocks were deformed during the Cadomian Orogeny. The Icart
Gneisses formed the basement to an outboard terrane that was
subsequently accreted to
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 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 crystals.
Many of the rocks present in the south of
Dykes are abundant on the Island, largely of doleritic composition.
Many of the disused quarry sites have been allowed to fill with
water, such as St. Andrews Reservoir, now used by the States of
* ^ citation needed
* 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.
* John Renouf: Geological excursion guide 1:
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