Presented at “A Welsh Journey”, Powerhouse, Pont-Tyweli, Carmarthenshire, 9th February 2020.

1. Taith ddaearegol ar hyd arfordir de Ceredigion
A geological journey along the
south Ceredigion coast
Professor Simon K. Haslett
Coastal and Marine Research Group, University of Wales
Presented at “A Welsh Journey”, Powerhouse, Pont-Tyweli, 9th February 2020

2. Discovering
Ceredigion Geology
• British Geological Survey
maps, memoirs and
guidebooks.
• Academic books, journal
articles and publications.
• My own geological learning
at university and field trips.

3. Geological
Periods and
Time
• Holocene (10 ka), including ‘Anthropocene’
• Pleistocene (1.6 Ma)QUATERNARY
• Neogene (23 Ma)
• Palaeogene (65 Ma)
TERTIARY (or
CENOZOIC)
• Cretaceous (145 Ma)
• Jurassic (208 Ma)
• Triassic (245 Ma)
MESOZOIC
• Permian (290 Ma)
• Carboniferous (362 Ma)
• Devonian (408 Ma)
UPPER
PALAEOZOIC
• Silurian (439 Ma)
• Ordovician (510 Ma)
• Cambrian (570 Ma)
LOWER
PALAEOZOIC
• Proterozoic (2500 Ma)
• Archean (3800 Ma)
• Hadean (4600 Ma)
PRECAMBRIAN

4. Geology of
the southern
Ceredigion
coast
• Holocene
• Coastal deposits: beach sand, dunes, salt marshes, peat
• Pleistocene
• Ice-age deposits: glacial till, glacial lake sediments, and
periglacial head (soil flow deposits in a tundra
environment)
Quaternary
• Llandovery Series
• Including the Aberystwyth Grits Group
Silurian
• Ashgill Series
• Yr Allt Formation (mostly in the Hirnantian Stage)
• Nantmel Mudstones Formation
• Caradoc Series
• Dinas Island Formation
Ordovician

5. Earth 430 million
years ago
• LAURASIA – a group of large
continental landmasses straddling
either side of the equator.
• AVALONIA – part of Laurasia
with southern Britain.
• GONDWANA – a large continent
situated over the South Pole.
• PANGAEA - a super-continent
formed of Laurasia and Gondwana
when they came together.
• IAPETUS OCEAN – separated
Scotland from southern Britain. (Credit: Fama Clamosa - CC BY-SA 4.0)

6. Cemaes Head: tectonic folding in the Dinas Island Formation (Caradoc Series, Ordovician)

7. Folding in Llandovery Series (Silurian) rocks,
Castell Bach, nr Cwmtydu

8. Folding in Llandovery Series (Silurian) rocks,
Castell Bach, nr Cwmtydu

9. Faulting and quartz veins in the
Yr Allt Formation (Ashgill Series), Ynys Lochtyn

10. Quartz veins in Yr Allt Formation (Ashgill Series), Ynys Lochtyn

11. Ordovician-Silurian Palaeo-geography of
southern Ceredigion
• Land was unvegetated (before land plants had evolved)!
• No plant cover meant that rocks exposed on land easily eroded.
• Rivers brought large quantities of mud and sand onto a narrow shelf.
• Sediment stacked high and was unstable on continental shelf.
• Earthquakes would be common due to the subduction zone nearby.
• Many submarine ‘landslides’ down onto a ‘slope apron’ and beyond,
often as slumps and lobes.
• Earthquakes and submarine slides likely to have caused numerous
tsunamis that hit the coast.

12. Nantmel Mudstone Formation (Ashgill Series), Mwnt

13. Yr Allt Formation (Ashgill Series), Llangranog

14. Large slumped beds in Yr Allt Formation
(Ashgill Series), Ynys Lochtyn

15. Ripple marks in Yr Allt Formation (Ashgill Series), Ynys Lochtyn

16. De-watering structures in Ashgill Series rocks, Penbryn

17. The Yr Allt Formation
Our local window into the global past!
• The Yr Allt Formation dates to the Hirnantian,
which is an international Stage named after rocks
at Cwm Hirnant in North Wales.
• Lasted about 1.4 million years, from 445 to 443 Ma
(million years ago).
• A length of day approximately 22.37 hours.
• Approximately 392 days in a year.
• An Earth–Moon distance of 375,330 km (vs.
384,000 km today) during the Late Ordovician (445
Ma).
• Tidal ranges larger than today creating wider
intertidal zones at the coast.
• Sea’s would rush in and out on the tide and tidal
bores would be more common.
• (some figures from Zhong et al., 2020. Late
Ordovician obliquity-forced glacio-eustasy
recorded in the Yangtze Block, South China.
Palaeogeography, Palaeoclimatology,
Palaeoecology, 540, 109520)
The Hirnantian ‘Ice-age’
• The Hirnantian glaciation was the first major global
ice-age that life experienced on Earth (previous
ice-ages were before life evolved).
• Gondwana had ice-sheets at the South Pole.
• Mass extinction of up to 85% of species.
• The estimated duration for the worldwide
Hirnantian glaciation ~830 kyr (in comparison the
most recent ice-age has lasted 1.6 Ma).
• Sea-level fell by around 100m exposing the
continental shelf to erosion.
• Late Ordovician ice-ages may have been triggered
by mega-volcanic eruptions causing a nuclear
winter.
• All this represented in southern Ceredigion by the
Yr Allt Formation …….

18. From Llangranog to Llandysul and beyond …..
• Yr Allt Formation outcrop (Hirnantian Stage age, global ice-age conditions).
• Sea-level fall, coastline moves seaward exposing shelf around Llandovery
and Llandeilo.
• Shallower sea, seabed affected by waves/currents forming ripple marks
• Rapid deposition of sand and mud as turbidites and slumps.
• Dewatering structures, sand volcanoes.
• Accumulating too fast for most marine animals to survive on the seabed -
buried alive!
• Fossils are extremely rare.

19. Yr Allt Formation (Ashgill Series) at Llandysul

20. Quartz vein (Yr Allt Formation) in River Teifi, Llandysul

21. Aberystwyth Grits Group (Llandovery Series), Newquay

22. Aberystwyth Grits (Llandovery Series),
Newquay and Aberystwyth

23. Glacial deposits (Quaternary Period), Newquay

24. Diolch yn fawr - Thank you