Lockdown evenings are doing wonders for progressing my cross-stitched Geo-SEW-ence map of the British Isles! This month I completed stitching Wales, and this blog edition will delve into some of the geology of Wales as well as reminisce on past geo-adventures in this beautiful part of the World.
The Map
Last year, in a moment of madness, I decided to convert my tatty poster of the bedrock Geology of the British Isles into a cross-stitched piece of art. Little did I know (having never done any needlework before) how long this would take me to achieve! But, after months of stitching, I have now completed Scotland, Ireland and Wales, and have thoroughly enjoyed learning about the geology of the UK along the way. Now just England to complete!
The Geology of North Wales
The North of Wales is largely made up of deformed Lower Paleozoic sediments that were deposited into the ancient Iapetus Ocean (purples), and of volcanics (pink) formed during the closure and subduction of this ocean. However, the northwest of Wales shows a large array of rocks deposited at different times. Anglesey is considered to be a 'classic' area of British geology. It’s complex geology was not so much fun the stitch, but shows a remarkable array of rocks concentrated over a very small area.
It is here that the most ancient rocks in Wales are found (green), dating back to the Neoproterozoic, or over 542 million years ago. These ancient rocks have been highly deformed and metamorphosed over millions of years, but still show evidence for their original deposition in a shallow sea pierced by volcanic islands. Plate tectonic reconstructions place North Wales close to the South Pole at this time, on the edge of the Gondwana supercontinent, adjacent to a subduction zone.
The Cambrian and Ordovician (542 - 444 million years ago) saw a deepening of the ocean, and deep marine and deltaic sediments were deposited, along with early fossils. Volcanics still played a very important role, with underwater eruptions and volcanic islands across the region. The famous mountains within Snowdonia National Park are formed by these hard volcanic rocks (pink).
Much of Wales is made up of Silurian strata (dark purple) which represents a reduction in volcanic activity, and the deposition of shallow marine carbonates and clastics across the so-called Midland Platform, and deep marine turbidites within the adjacent Welsh Basin (where the Snowdonia National Park is situated today). The closure of this ocean has since deformed and folded these sediments. The resulting structures are well exposed along the Pembrokeshire Coast, in South Wales.
The Geology of South Wales
The Pembrokeshire Coast of southwest Wales is a popular geological field trip location, and I was lucky enough to visit one very cold winters weekend with the BSRG (British Sedimentological Research Group) during my PhD. Steeply dipping strata (deformed during the Variscan Orogeny) allows geologists to see rocks from PreCambrian to Carboniferous in age along the sea cliffs here.
Following the closure of the Iapetus Ocean in the Silurian, Wales lay close to the equator. Between 416 and 359 million years ago, huge river systems deposited the Devonian Old Red Sandstones (see adjacent picture) that can be seen at Pembrokeshire and across much of the the southeast of Wales (indicated in brown on the cross stitch map).
An incredibly important part of Wales' history lies in the extraction of coal from the South Wales Coal Field (grey on the cross stitch map). These coals are formed of Carboniferous rocks, deposited 359 to 299 million years ago. In the early Carboniferous, a shallow sea deposited limestones (light blue). Falling sea level halted limestone deposition, and south of Wales became a landscape of dense damp forests, broken up by rivers, lakes and peat bogs. It is the decay and burial of these forests that has gone on to form coal across the South Wales Coal Field, and led to the development of the industry in this region.
The final major rock type of note in Wales is found around the Cardiff region, along the Bristol Channel. Here, Triassic sediments (deposited 251 - 200 million years ago) were laid down by river systems (reds on the cross stitch map). At this time, Wales and the rest of the UK sat in the centre of the palaeocontinent of Pangea, where the climate was very hot, with highly seasonal flash flooding. These spectacular Triassic sediments will be described in more detail when I write up the English Geo-SEW-ence!
It's been really fun getting to know Wales, and reminiscing about my (all to to brief) previous trips there. I cannot wait to get back there when it is safe to do so. In the meantime, back to stitching, only Southern England to finish now!
--- Keep Exploring ---
