Fig. 1. Geology of Southwest Wales
Some of the most spectacular coastline in all Wales lies in the southwest, much of it in the Pembroke Coast National Park. Bedrock outcrops are few and poor in the sheep country inland, but sea cliff exposures are excellent and the coastal scenery is magnificent. The Pembrokeshire Coastal Path provides easy access to the cliff exposures that show splendid views of the folded rocks, and of the effects of the ice age glaciers.
Geologic History
Southwest Wales lay on the southern shores of the Welsh Basin from Late Precambrian to Silurian time on the margin of the small continent of Eastern Avalon (roughly England, Wales, southern Ireland, and Belgium) that lay close to Gondwana (the collection of the modern southern continents). Volcanic rocks some 650 million years old and some two kilometres thick make the continental basement beneath southwest Wales. These latest Precambrian rocks are mostly flinty looking rhyolite, and green andesite lavas with many layers of ash and volcanic bombs. Several slivers of these Precambrian rocks poke through younger rocks both north and south of St. Brides Bay; the best exposures are at Caerfai Bay near St. David's and on the coastal path west of Haverfordwest.
During Cambrian time, a world-wide rise in sea level flooded the Precambrian shoreline of Eastern Avalon, reworking the weathered rock and soil into a thick layer of gravel, now solidified into conglomerate. Sand and mud accumulated above the gravel as sea level continued to rise. Good exposures of those Cambrian rocks exist at Caerfai Bay, south of St. David's.
The Welsh Basin deepened to the north during Ordovician and Silurian time and great thicknesses of turbidite sand and mud collected in the basin. In late Silurian time the Iapetus Ocean closed and the North American plate collided with the Eastern Avalon plate, compressing the mud and sand in the Basin and raising hills to the north.
Eastern Avalon moved north away from Gondwana during Ordovician time, as the Iapetus Ocean that separated Eastern Avalon from North America began to close. The oceanic plate sank beneath an oceanic trench near the Solway Firth in Scotland and plunged beneath the Eastern Avalon continent. Volcanoes erupted above the sinking slab of ocean floor to build islands. Some of those rocks survive in the Lake District, the peaks of Snowdonia, along the coast between St. David's Head and Fishguard, and in the Preseli Hills east of Fishguard.
When the last of the Iapetus ocean floor slid under the continental crust at the end of Silurian time, the North America block, Laurentia, and Eastern Avalon collided to make the Caledonian mountains in Scotland. The colliding continents crushed the older Palaeozoic rocks of central and northern Dyfed, crumpled them into folds, and changed the mud to slate.
The southern part of Dyfed, around Milford Haven, did not feel the effects of the collision. Here the layers of Silurian rocks still lay flat when the red mud and sand of the Devonian Old Red Sandstone washed down from the Caledonian hills to the north. The oldest of the Old Red Sandstone beds are red mudstone, followed by sandstone when the growing Caledonian hills shed their coarser debris.
As these hills eroded to a plain, sluggish rivers spread more mudstone across the older sandstone. Later mudstone built out into a shallow sea. By Carboniferous time, sea level had risen, animals flourished in the clear warm waters and died to accumulate thick layers of limestone. The best outcrops of Carboniferous Limestone are in the Pembroke peninsula, southwest of Tenby, now an army gunnery range.
Meanwhile, Gondwana had been moving north. In Carboniferous time, it collided with the continent of Avalon and North America. This was the beginning of the Hercynian collision. Land rising to the north shed sand and mud south towards the sea. These sediments spread out as deltas and swamps to make the late Carboniferous Millstone Grit and the Coal Measures. The coal beds are the remains of lush forests. Good examples of the Coal Measures of the Pembrokeshire Coalfield lie north of Tenby, along the coast at St. Brides Bay, and at Saundersfoot and Amroth.
The climax of the Hercynian collision at the end of Carboniferous time assembled most of the earth's continental crust into the supercontinent of Pangaea. It raised a mountain range that stretched from the Americas through northern Europe, including Southwest England and South Wales.
The collision shoved the rocks of southern Dyfed northwards, crumpling them into folds, spectacularly exposed near Milford Haven, and breaking them along faults. The northern edge of the resulting Hercynian mountain front was south of the A40 between Swansea and Haverfordwest. A large fault, the Johnston thrust fault, near this boundary, pushed Precambrian volcanic rocks to the surface near Johnston. The effects of the Hercynian earth movements are minor north of this line.
Like their predecessors, the Hercynian mountains eroded shedding their debris into valleys. In Permian and Triassic time, Pangaea began to split into the modern continents. For the next 200 million years Wales lay far from any mountains near the shore of the growing Atlantic Ocean, but there are no rocks younger than Carboniferous in SW Wales. Then, about 200,000 years ago, glaciers advanced several times from the north to cover southwest Wales. Their southern edges lay near the coast of south Wales. Glacial meltwater eroded deep channels east of Fishguard, and around the Preseli Hills.
Descriptions
of specific
areas:
Fishguard to St. David's, the North
Pembrokeshire Coast ; St. David's to Carmarthen South Pembrokeshire Coast ; Pembroke
Peninsula ; A40
and other inland roads