Anglesey rocks!

For a number of years now it has been my privilege to lead a geological walk as part of Anglesey Marine Week.  I say “privilege” for two reasons: first, because Marine Week is a fantastic event reminding us of the importance of Anglesey’s coastal waters; and secondly, because it is a great feeling to represent the GeoMôn UNESCO Global Geopark in drawing attention to our magnificent geological heritage.

20160816_144448Jon (blog author) leading a group during Anglesey Marine Week. 


Almost all of Anglesey’s most important geological sites are on the coast.  This is in no small measure due to the relentless erosion caused by the waves lapping on to the seashore, exposing rock strata which have remained hidden for millennia.


But the sea is not just an agent of destruction.  Many of Anglesey’s geological sites bear witness to the role of the sea in creating the very rocks that are now exposed to its erosive power.  Three examples illustrate this well. 


The first is represented by the beds of limestone to be found all along Anglesey’s east coast.  Around 350 million years ago, the area we now call Anglesey was under water, overlain by a shallow tropical sea.  This sea was teeming with marine life, and as these creatures died their remains sank to the sea bed and piled up, layer after layer.  These limestone rocks frequently contain fossils of some of these creatures, including corals and molluscs.

 Limestone with fossils
Anglesey limestone with fossils


The second example is the “pillow lava” to be found at Llanddwyn Island.  These very distinctive rocks formed a result of underwater “smokers” spewing out lava during periods of eruption.  The seawater proved very effective at cooling the liquid lava, which quickly formed blobs, or “pillows”, on the sea floor.  As the eruptions continued the pillows would overlay each other, creating the structures we see at Llanddwyn today.

 Pillow lava
Pillow lava


My third example can be found in the cliffs around Castle Rock, in Red Wharf Bay.  At certain points along this coastline sea-smoothed pebbles can be found embedded some way up the cliff side.  This is clear evidence that at one time the sea level was significantly higher than we see today.

 Castle Rock
Castle Rock


These three examples underline the importance of the interaction between the sea and the land.  It’s fun to take a moment to imagine how this might be seen by our far-distant descendants, millennia from now.  Will the vast sandy area of Red Wharf Bay become a sandstone outcrop?  Will the Menai Strait have widened to become a vast sea between Anglesey and the mainland?  Will the whole landmass be covered by a mile-high layer of ice and permafrost? – it has happened before!


For me it makes good sense to include aspects of Anglesey’s geology in Marine Week.  Anglesey’s rocks are of global significance, and in recognition of this UNESCO have declared the island to be a Global Geopark.  This designation is on a par with “World Heritage Site”, probably the most widely recognised and appreciated aspect of UNESCO’s work.  Unlike World Heritage Site however, designation as a UNESCO Global Geopark does not last forever.  A review takes place every four years, and Anglesey underwent its latest review earlier this year.  Everyone associated with the Geopark is keeping everything crossed for a favourable outcome!  To find out more, find us on Facebook or visit our website,

Written by Jon Pinnington, GEOMON member and former guide.Jon Pinnington

About nature.bites.admin

Kathy is a wildlife enthusiast who loves nothing more than to inspire people about nature. She is the Trust Development Officer for the West Wales Rivers Trust, a researcher at Aden Productions and offers guided wildlife tours on Anglesey with the amazing Ken Croft. Outside of work, Kathy arranges Anglesey Marine Week in order to promote engagement in conservation. Please see the 'Wildlife Tours' page of the website for more details of how to join a tour.

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