GeoMôn Geosite : Llanlleiana
Just along the coast to the east of Cemaes is a GeoMôn Geosite : Llanlleiana – the remains of porcelain works nestling in a deep valley. As well as being an fascinating former industrial site, a very scenic location and an exhilarating walk along the Coastal Footpath, this is an interesting geological location.
Porcelain – or at least ceramics – have been made at various locations along the north Anglesey coast. There are disused brickworks close by at both at Porth Wen and Cemaes. All three locations are utilising the same geo-resource.
Most of the north coast is early Cambrian green schist, or at least metamorphosed sediments within the green schist mineral facies. There are some more massive sandstones, and to the west is the famous Gwna melange. But the headland by Llanlleiana is actually underlain by Ordovician sedimentary rocks. If you have walked from Llanbadrig you will have seen the unconformity at Ogof Gwynfor. If you have walked from Porth Wen you may have noticed the unconformity in the cliffs immediately to the seaward of the brickworks where steeply dipping bright purple conglomerates overlie the green schists.
It is these Ordovician rocks which were being utilised – some of the shales had been re-hydrated back to clay and it was these that were excavated and crushed for making bricks. However the proximity of pure quartzites amongst the Cambrian metamorphic rocks offered another possibility – making firebricks. these were made at both Porth Wen and Holyhead.
At Llanlleiana, clay was excavated from the opposite hillside to the buildings – to the right in the photo below and by the tin shed. Quartzite also outcrops on the hillside above the buildings [see following image] where there is evidence of quarrying.
There is another interesting dimension to this site : the landscape. The wide, deep flat floored valley curves round and back out to the coast at Porth Cynfor. It’s not a river valley at all, it’s actually a glacial channel, either cut by ice or more likely cut by meltwater flowing between the ice sheet and the land and finding a weakness in the rocks. The flat, boggy floor is infill with sediments and peat.
View this YouTube aerial video of Llanlleiana