In Geological terms, Jersey is not a southward extension of Great Britain, but is rather a northward extension of Little Britain - Brittany. The fact that Jersey is a part of the "British Isles" is merely an accident of human history of very recent times, and a glance at the Atlas will show that the Channel Isles, of which Jersey is the largest, are over 70 miles from the south coast of England, but only a few miles off the French coast, in the Bay of St. Malo.
In terms of age, whilst it is generally true that the oldest parts of Britain are in the north west and the youngest in the south east, yet Jersey contains shales which are older than most of the rocks in Britain, once again defeating any notion of a continuity southwards.
The Island is a dissected plateau composed basically of four rock types, which occurred in the following order - shale, volcanic rock, gabbro/granite and conglomerate. The map shows their distribution, and each will be very briefly described in succeeding paragraphs. To give an idea of the age of the island, even the conglomerate, the last rock formation, was deposited in the Cambrian and early Ordovician periods, whilst the shale, the earliest rock, dates from the Precambrian - some two or three hundred million years before! By contrast, many of the superficial deposits come from relatively recent, glacial times. Thus we have an interval of some 500 million years with no deposition, although chalk and Eocene limestone occur off the north coast.
The oldest rock is the Shale which occupies the center and west of the island. It has been folded four times, but dips generally some 30 degrees to the south east and is eroded by the sea in St. Ouen's Bay to the west and in St. Aubin's Bay on the south. In it can be found mudstone, siltstone, greywacke and sandstone. In addition to the sea coast, there are some good exposures in old quarry sites in the middle of the island, illustrating the sedimentary layers and a variety of sedimentary structures.
The shale was succeeded conformably by a succession of andersite volcanic rocks including tuffs, ignimbrite and (mostly) rhyolite of a characteristic and unusual purple colour. Presumably Jersey was at that time over a subduction zone between an oceanic and a continental plate, but all trace of the volcano(s) has disappeared. Jersey even has its own Devil's causeway - an area of columnar jointing on the east coast. The volcanics run in a band running diagonally NW to SE from the north coast to the east coast.
There are some clear exposures of the junction between the shale and the three separate plutonic intrusions of granite, which emplaced between 570 and 480 million years ago. The junctions are steep and show evidence of thermal metamorphism. These granites contain a high proportion of pink feldspars, making them much prized as building stone, perhaps the best known example being the London Embankment. However as the map shows, and contrary to popular belief, they comprise only a quarter of the Island. Perhaps it is because these granites make some of Jersey's most spectacular cliffs that they are so well known. On the north and south east coasts there are small areas of gabbro, intruded before the granite, which have been altered to diorite by the intrusion of the granites.
Finally, the north east corner of the island was covered unconformably in the Cambrian and Ordovician periods by an area of conglomerate.
No more rocks were formed in Jersey, but in glacial times there was a deposition of head and later of loess. Ice cover did not reach this far south, but the loess was blown from the north east at a time when the English Channel was dry. Some peat is to be found, and its fauna and flora suggest deposition in a climate as cold as present day Scandinavia, perhaps 25,000 years ago. There is also an area of high sand dunes, some of which extend 80m above present sea level. And, in addition, the island has easily visible examples of raised beaches at three different levels, formed during the interglacial periods.
There are many cleanly exposed examples of minor intrusions, including dolerite and mica-lamprophyre dykes.
So in conclusion Jersey has a vast amount to interest a geologist, and it has many clean exposures of the rocks and structures so briefly described above. If you would like to visit, you can obtain further information from the BGS book in the series "Classical Areas of British Geology" (ISBN 0 11 884458 x) and from the 1:25000 IGS map "Jersey" (Channel Isles sheet 2). There is a field guide, No 41 by the Geologists' Association, Burlington House, Piccadilly, London W1 0JU and a booklet "The Minerals of Jersey" by Dr A.E.Mourant published by the Societe Jersiaise, 7 Pier Rd., St Helier, JE2 4XW.
The award-winning Jersey Museum has a small but interesting section on local geology; and its country offshoot, the Houge Bie Museum has a comprehensive and equally engaging display of rocks and minerals of the island, set out by Dr. John Renouf, by whose kind permission the map is reproduced.
The Jersey twig of the Wessex branch of the OU Geology Society is small, but welcomes geologists from afar who will doubtless show us our Island as seen through new eyes.