Chapter
4.
CENTRAL
WALES
Descriptions
of specific
areas:
The Coast Road The inland roads The Welsh Borderland
Fig. 1. Geologic map of central Wales.
Central Wales is a land of rolling hills covered by glacial sediment that hides the rocks beneath. The Welsh Basin underlies most of Central Wales, where siltstone and sandstone collected during Ordovician and Silurian time.
The Palaeozoic Welsh Basin
Muddy sandstone of Silurian age dominates central Wales, with a few similar looking Ordovician sediments wherever folds bring them to the surface. This was the deepest part of the Paleozoic Welsh Basin. The collection of basins that make up the Welsh Basin continued to sink while they filled with sand and mud eroded from the land to the north, south and east. Some of the basins collected as much as three thousand metres of this muddy sandstone. The Caledonian collision of Devonian time compressed the sediments into large folds.
The best places to see the Welsh Basin rocks are along the coast between New Quay and Aberystwyth (Fig. 1), a section famous for its turbidite sandstone with layers that grade from sand at the base to fine mud at the top. (The Geological Association Guide #54, The Aberystwyth District, describes these rocks in detail.) The rocks are banded sandstone, siltstone, and mudstone laid down on the basin floor as a large submarine fan. The weathered rocks are brown, but when freshly broken most of them are grey, and some are purple and green. Most outcrops show regular sedimentary banding, but submarine slumping while the sediments were still soft, produced some convolutions and broken beds.
Alternating sand and mud seem an odd combination of sediment. Sand needs strong currents to move the grains while mud settles only in quiet water. How can conditions in deep water change many times from fast currents to still water?
Earthquakes, storms, and the simple buildup of sediments triggered submarine slides along the southern and eastern edges of the basin. These slides stirred up more sand and mud as they roared down the slope at motorway speeds. This flowing mass of sand, mud, and water, called a turbidity current, poured down the slope, and spread out across the basin floor. Near the base of the slope the current slowed and dropped part of its load of sand to leave a thick sand layer on the basin floor. The cliffs at New Quay show fine examples.
The current continued to billow out across the deeper part of the basin, where the sea floor is flat. The sand and mud slurry began to settle, the larger grains first, smaller grains last; so the layers from each pulse of sediment typically grade from coarse at the bottom to fine at the top. Finely laminated sand commonly lies on the graded sand as the currents swept in the finer sands. As the current slowed, it rippled the upper layers of sand.
Then in the calm between turbidity currents, fine mud settled onto the sand, resulting in the spectacular alternations of the turbidite sand layers and thin mud layers. The cliffs at Aberystwyth show fine examples of these strongly banded rocks.
Currents strong enough to carry sand rarely reached the deepest parts of the basin. Here, a slow, steady shower of mud collected on the sea floor, with clouds of mud and silt drifting in from nearby turbidity currents from time to time. The resulting rocks are typically colour-banded silt and mud as in the cliffs at Borth.
Near the end of Silurian time the
Caledonian collision folded the rocks and pushed most of Wales above sea level. The
coastline retreated south to Devon.
In late Silurian to Devonian time, torrential rains washed sediment from the hills onto the broad plains. Those sediments compacted to sandstone and mudstone of the Old Red Sandstone formations. A stain of red iron oxide explains the colour of most of these rocks. The best outcrops of the Old Red Sandstone are in the Brecon Beacons.
Then, at the end of Carboniferous time, the collision of the southern continents of Gondwana with the Europe/North American block folded the rocks of South Wales, but merely raised Central Wales above sea level.