They are known to be pre-Jurassic, but whether they are Palaeozoic or Archaean is uncertain.
The northern portion, lying mainly between io and 15° N., consists of a huge mass of Archaean rocks with a mean height of from 7000 to 7500 ft.
The geological formations represented are the following in descending order: Recent or sub-recent Pleistocene Pliocene Miocene Lower Miocene Oligocene and Eocene Upper Cretaceous or Lower Eocene Upper Cretaceous Jurassic or Lower Cretaceous Carboniferous to Trias Archaean The latest movement to which the Gulf has been or is now being subjected is one of gradual elevation, of which traces are found in recent littoral concretes, now as much as 450 ft.
On the mainland, the north and east shores are of gneisses and granites of archaean age, with a broken and hilly surface rising in places to 600 ft.
The outcrops of these rocks succeed each other in order of age in roughly concentric belts, with the Archaean mass of the island of Anglesey as a centre, but the arrangement in detail is much disturbed and often very irregular.