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.
The Brioverian sedimentary rocks are all well bedded and were originally the mid and outer parts of a submarine fan.
The shale is found at Gorey Harbour in the east coast, and La Belle Hougue Point, and Le Mont Mado granite has an occurrence.
In an analysis of the greywacke it has fragments with 70% quartz, 10% to 15% plagioclase and microcline feldspar, 2% dark iron containing minerals, with carbon flakes, in a matrix of 10 to 20% clay.
Conglomerate, termed Association I from the upper parts of the fan, is found in lenticular bodies near St Peter's Valley, at Gargate Mill.
In the L'Êtacq area, the metamorphism has produced spotted hornfels with a grey colour.
It is coloured grey and includes some basalt and pyroclastic fallout such as tuff and agglomerate.
Porphyritic andesite is dark grey with white crystals of plagioclase feldspar embedded.
The Trinity Ignimbrite is a purple tuff near Les Grands Vaux, inland.
These contain flow banding and spherulites up to 10 cm and eutaxitic and fiamme textures.
Le Havre de Fer Beach has an outcrop of columnar rhyolite that has been separated by faulting from the rest of the units.
In this orogeny the rocks were first crushed and folded with East West pressure, then later they were compressed in the north south direction.
This rock can be observed to transition into diorite by changes in the mineral content and texture.
The diorite is emplaced at and near Sorel point, and at several places along the south coast east of Saint Helier.
Other parts more rich in volatiles have formed pegmatites featuring milky quartz and pink orthoclase.
This is mostly made from fine grained quartz and Perthite, but is coloured red yellow and brown.
Apart from the main outcrop, the conglomerate is also found in smaller areas at Les Hurets Valley, west of the Bouley Bay and at La Pierre de Fételle.
Hornblende lamprophyre (spessarites) contain plagioclase, coloured light brown and are oriented north–south.
A hornblende lamprophyre dyke in the Rozel Conglomerate is dated at 435 million years ago.
La Cotte de St Brelade has a Neanderthal rock shelter which was inhabited 200000 years ago by hunters of woolly mammoth and rhinoceros amongst other animals.
Head occurs at the foot of cliffs along the north, north east and south west sides, and can also be found beneath wind blown sand at the bays of St. Ouen, St. Aubin, St. Clement and the Royal Bay of Grouville.
The thickest parts of loess are five metres deep at St Clements and at La Hougue Bie on the eastern plateau.
At Quetivel Mill in St. Peter's Valley there is peat dated to 7600 BC which contains pollen from boreal forest.
The island was only separated from the continent of Europe by rising sea levels about 5000 BC during the new Stone Age.
This cave has a tunnel going in from the beach at the base of the cliff, and a crater shaped opening with steep rocky sides above.
At Le Pulec there is a small deposit of lead and zinc that was mined in the late 19th century, but the venture was unsuccessful.
There are many other disused or abandoned quarries that have been used for stone to make the buildings, castles, and piers on the island.
Ground water is irregular, with some perched aquifers and vertical screens preventing underground flow.
Earth tremors occur more commonly near Jersey that other areas in the Channel Islands, with magnitudes up to 3.
The tidal range is high at 13 meters, exposing a wide rock platform around Jersey.
