CYNGOR CEFN GWLAD
CYMRU COUNTRYSIDE COUNCIL FOR WALES

SITE NAME: SGISTIAU GLAS YNYS MÔN

UNITARY AUTHORITY: ISLE OF ANGLESEY

The purpose of this note is to describe the nature and importance of a site, avoiding specialist terms, for the site owner and/or occupier. This note does not form part of the formal notification documents.

Sgistiau Glas Ynys Môn, Anglesey

This site consists of three individual localities which have been selected to illustrate key features associated with a rare type of rock called a blueschist. The blueschists on Anglesey are Precambrian in age and are amongst the oldest such rocks known in the world. They form a narrow NE-SW-trending belt across southern Anglesey, with the main area of blueschist centred around Llanfair P.G. and extending towards Llansadwrn. These rocks are of international importance and have been of interest to geologists since Reverend Blake first described them in 1888. Blueschist is a type of metamorphic rock, which means that it has been changed or altered in some way. Minerals crystallize at specific temperatures and pressures which provides geologists with important clues about how the rocks were formed. In this case, the sediments and basalts on an ancient ocean floor were buried quickly and subjected to high pressures, whilst remaining at relatively low temperatures. If the original sequence of rocks (protolith) was buried further the

temperature and pressure would increase, changing the mineralogy. This type of relatively rapid burial occurs at plate margins when oceanic crust slips underneath continental crust. On Anglesey this process occurred some 590-580 million years ago and can be compared to processes that are currently operating along active plate margins such as those along western North America. Other examples of ancient blueschists from around the world can found in California, Japan and Corsica and, as in the case of the blueschist on Anglesey, normally occur in long linear zones that define ancient plate subduction boundaries. The preservation of such ancient blueschists is a rare occurrence and the Sgistiau Glas Ynys Môn SSSI is an internationally important site providing an excellent opportunity for geological research. The rock exposures allow geologists to study the mineralogy of the blueschists, in addition to contributing to the understanding of the tectonic evolution of Anglesey and Northwest Wales.