Relics of sediments appear as
xenoliths in
granites. However the earliest dated rock is the grey coloured Western
Granodiorite from 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
feldspar crystals.
Aplite veins 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 Orogeny time at . Embedded in the diorite are a couple of large
gabbro inclusions, as well as a
picrite on the east of Braye Bay. Some of the diorite has
orbicular structure, concentric spheres of
plagioclase and
hornblende rich 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
quartzite was deposited. Further weathering ensued, with most of this deposit removed and
laterite formed. 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. initially 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. In the
Variscan Orogeny folding and faulting affected all the rocks.
Dolerite (or diabase) and
lamprophyre dykes intruded. These are probably from the
Carboniferous period. In the
Pleistocene varying sea levels caused raised beaches to form 8, 18 and 30 meters above the current sea level. As in
Jersey,
loess blew 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. ==References==