Bryn Celli Ddu – the Mound in the Dark Grove – is probably the best-known and best-preserved prehistoric monument on Anglesey, and is one of the most evocative archaeological sites in Britain. The monument was built in the later Neolithic around 5,000 years ago, and consists of a long passage that leads to a polygonal stone chamber, and covered by an earthen mound.
A pillar of stone stands at the centre of this monument, which has been the subject of debate for the last decade or so. This post argues that it is made from blueschist, a metamorphic rock for which Anglesey is internationally famous.
Bryn Celli Ddu sits on an island famous for its geology. Originally described and mapped by Edward Greenly, Anglesey was the subject of its own book, surprisingly named: The Geology of Anglesey (1919). Ever since Greenly wrote this book, Anglesey has been described as the Mona Complex of Precambrian rocks.
These Precambrian rocks are fault-bound which create terranes across the island, collectively known by geologists today as the Monian Composite Terrane. These terranes are made up of smaller crustal fragments known as the Monian Supergroup (South Stack group, New Harbour group), the Coedana Complex (Coedeana gneisses, Gwna roup, Coedana granite hornfles) and the Blueschist Belt. These are covered by Phanerozoic volcanic and sedimentary rocks.
The map below shows where the Precambrian rocks are located in terms of the three main terrane boundaries.
The Anglesey blueschist belt
The yellow area on the map shows the region of the Anglesey blueschist belt. This area includes the Marquess of Anglesey’s column, a famous site for viewing the best blueschists for geologists looking to study the rock. The blueschists of Anglesey are some of the oldest known in the world, and geologists flock to see them as they are some of the most accessible and remarkable blueschists in the world.
Blueschist is an unusual metamorphic rock that is produced by subduction of oceanic crust. The occurrence of this blueschist is rare. It is created through a process of metamorphosis.
The only way that rocks can be metamorphosed to blueschist, is to be quickly be shoved down to those extreme depths in the earth and then rapidly brought back up before the rocks have time to heat up completely. That’s exactly what happens where two tectonic plates are colliding. It is this exceptional occurrence which can be seen clearly across the landscape of Bryn Celli Ddu, on Anglesey.
The blueschist at Bryn Celli Ddu
The blueschist that we see at Bryn Celli Ddu is part of this amazing geological belt. It’s incredible to think that Bryn Celli Ddu, along with a series of prehistoric monuments, including a Bronze Age cairn and 8 cupmarked rock art outcrops, were all built on this rare blueschist belt.
The question is, did Bryn Celli Ddu get built from blueschist?
To identify a rock as blueschist, geologists look for certain qualities in the rock:
- Blueschist, as its name suggests has a blueish tinge.
- It is also called glaucophane schist with the name coming from the presence of the predominant minerals glaucophane and lawsonite in the rock.
- The structure of the rock can appear folded.
- The layers in schist can become intensely contorted even on a small scale.
All these characteristics can be found on the Bryn Celli Ddu stone pillar. It is a very unusual rock, cylindrical in its form, which appears folded and intensely contorted. But more than that, the whole of the Bryn Celli Ddu monument is made from several blueschist rocks.
As part of the Bryn Celli Ddu Public Archaeology project this June, we are working with Anglesey Geopark and Natural Resources Wales geologists, who will be on hand at our Open Day on the 17 of June, to answer any questions about the rocks and blueschist at Bryn Celli Ddu.