Blueschist - copyright R Roberts

Highly deformed blueschist © R. Roberts, CCW

What is ‘special’ about the geology at Sgistiau Glas Ynys Môn SSSI?

 Sgistiau Glas Ynys Môn SSSI has one special feature:

·      Precambrian blueschist exposed in disused quarries, inland outcrops and road cuttings.

Sgistiau Glas Ynys Môn is of special scientific interest because the rocks contain a rare assemblage of blue amphibole-bearing minerals which were formed under high-pressure and low-temperature. These rocks, which are Precambrian in age (over 550 million years old), include the best and most accessible exposures of blueschist in Great Britain and are one of the oldest known examples of this rock type in the world. Blueschists are typically formed at oceanic plate margins where one plate slides beneath another, a process known as subduction. Rocks in the cold descending plate are buried quickly and subjected to very high pressure whilst still remaining at relatively low temperature. These physical conditions are responsible for formation of the amphibole minerals and which in turn provides important information concerning the geology of the area.

What do we want Sgistiau Glas Ynys Môn SSSI to look like?

The following is a description of how we would like to see the feature at Sgistiau Glas Ynys Môn SSSI:

Our aim for this site will be focussed on maintaining the current high level of rock exposure throughout the site, as well as keeping all rock faces visible and accessible. In seeking to achieve our aim, we will thereby ensure that the Sgistiau Glas Ynys Môn SSSI will continue to provide a key locality for research into Precambrian geology.

What management is needed at Sgistiau Glas Ynys Môn SSSI and why?

Sgistiau Glas Ynys Môn SSSI is an internationally important locality for studies of Precambrian geology and sites such as this are vital for further geological research. However it will only remain so if the necessary management continues and CCW’s priority is to work with you to ensure that this management is carried out.

What does this mean in practice?

There are a number of different factors that could damage the special feature at Sgistiau Glas Ynys Môn SSSI if they are not properly managed. These are the ones we regard as most important:


Any activities that might obscure the natural rock exposures, for example civil engineering works such as rock netting or shotcrete, the dumping of earth, spoil or other waste materials, and the construction of farm buildings on or against rock outcrops, must be avoided. Operations which might be undertaken for safety reasons could also result in the concealment of these important exposures. Therefore, a close liaison will be maintained between CCW and the landowners.


Periodically, students and researchers may need access to study the rock exposures at this site. However, there is no public access to the crags at Castellior Farm or the cutting on the A55. Therefore, access is entirely at the discretion of the landowners. Even though the crags surrounding the Marquess of Anglesey’s Column form part of a visitor attraction, all necessary permissions should be sought before undertaking any research on the rock exposures.

 Agricultural practices

Current agricultural practices, namely animal grazing, have little or no effect upon the feature of geological interest.


Our knowledge and understanding of geology is continually improving.  It is possible that new issues may arise in the future, whilst other issues may disappear.  This statement is written with the best information we have now, but may have to change in the future as our understanding improves. Any information you can provide on the geology of your site, its management and its conservation would be much appreciated.

If you would like to discuss any aspect of your SSSI, or have any concerns about your SSSI, please contact your local CCW office.


Llys y Bont

Ffordd y Parc



LL57 4BN

Telephone:      01248 672500

Fax:                 01248 679259

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