Precambrian
Late Precambrian rocks are widespread on Anglesey, Llŷn and Arfon with other, more restricted occurrences in north Pembrokeshire, Radnorshire and Carmarthenshire. The often intensive metamorphism which these originally volcanic and sedimentary rocks have been subjected to and their generally faulted relationship to neighbouring rocks has meant that geologists’ understanding of them has been limited. The majority lie on or close to the margins of terranes, blocks of the Earth’s crust which have had differing geological histories before assuming their present configuration. However in recent years there has been a progressive elucidation of the way in which Wales’ many terranes came together during the late Precambrian and the Palaeozoic era.
The Stanner-Hanter Complex on the English border comprises volcanic rocks around 700 million years old which puts them within the Cryogenian period.
Read more about this topic: Geology Of Wales