Basement
The pre Cadomian basement rocks consist of Orosirian or Paleoproterozoic Icart Gneiss dated at close to 2,018 million years ago. These are exposed on Guernsey and at Cape La Hague in Brittany. This is termed the Icartian succession. Near Tregor the gneiss in the Cadomian Perros-Guirec complex has been U-Pb dated to 1,790 million years ago. Sark also has a gneiss basement. Vidal et al. studied Strontium isotope ratios and concluded that the basement is not widely present below the Cadmonian orogeny, and Rabu et al. argued that the existing surface gneiss are fragments detached from the African craton. However it is possible that a basement of gneiss exists at depth below Normandy and Brittany. This gneiss was formerly known as Pentevrian. However it is now known that the Pentevrian type rocks do not predate the Cadomian orogeny, and this name is no longer appropriate for the Icart Gneiss.
Read more about this topic: Cadomian Orogeny