- Geology of Alderney
Alderneyis only five kilometers long, it has a geological historyspanning half of the life of the earth. Part of the Armorican massif, it has similarities in its rock to the neighbouring Normandyand Guernsey.
Relics of sediments appear as
xenolithsin granites. However the earliest dated rock is the grey coloured Western Granodioritefrom Ma|2220 in the Paleoproterozoic. 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 feldsparcrystals. Apliteveins 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 Orogenytime at Ma|600|500. Embedded in the diorite are a couple of large gabbroinclusions, as well as a picriteon the east of Braye Bay. Some of the diorite has orbicular structure, concentric spheres of plagioclaseand hornblenderich 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
quartzitewas deposited. Further weathering ensued, with most of this deposit removed and lateriteformed. 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. Initally 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.
Variscan Orogenyfolding and faulting affected all the rocks. Dolerite(or diabase) and lamprophyredykes intruded. These re probably from the Carboniferousperiod.
Pleistocenevarying sea levels caused raised beaches to form 8, 18 and 30 meters above the current sea level. As in Jersey, loessblew in as dust from the bare ground in the near glacial conditions in the ice ages. Head also formed in the periglacial circumstances by breaking off rock fragments and mixing with dirt.
*"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.
*N d'A Laffoley: "Geological excursion guide 2: Alderney, Channel Islands" in "Geology Today" volume 1 number 5 page 151 1985
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