Coast Road--Central Wales

The cliffs along the coast road between New Quay and Aberystwyth (Fig. 2) provide some of the finest exposures anywhere of Silurian sandstone laid down from turbidity currents. The currents flowed from south to north, building a fan of sediments on the floor of the Welsh Basin during Silurian time. The sediments near New Quay settled near the upper part of the fan, so the sandstone there is in thick layers. The thinner beds between Aberaeron and Aberystwyth collected farther from shore, in a deeper part of the basin. The cliffs farther north at Borth expose mudstone laid down in the deepest part of the basin, about 50 kilometres from where the turbidity currents began their journeys.

The cliffs at New Quay expose sandstone dropped along the slope down which the turbidity currents flowed. The currents dumped great masses of sand in beds up to one metre thick. The quarry at the end of the harbour road exposes beds of sandstone more than a half metre thick with thin layers of mud between them (Fig. 3). The mud settled quietly during the intervals between turbidity flows.

Fig. 3. Turbidite sandstone exposed in the quarry at the west end of the harbour at New Quay.

Fig. 4. Turbidite sandstone just west of the headland at New Quay. The thickest sand layer is one metre thick; the next current scoured its top.

The upper surfaces of thick beds exposed just around the headland from the quarry show gouged beds (Fig. 4). The next turbidity current made those as it roared like an express train over the still soft bed below. The Caledonian collision of late Silurian time tilted and folded these beds. Soft yellowish brown Irish Sea till now rests on the solid rock outcrops at the top of the cliffs.

The Silurian sedimentary rocks at Aberaeron were deposited farther offshore in the Welsh Basin than those at New Quay. Turbidity currents flowing down the flank of the basin had lost much of their load by the time they had spread out along its floor, so the sandy layers are thinner than those at New Quay. The cliffs about 300 metres south of the harbour contain especially good exposures of these turbidite sandstones, rotated to near vertical during the Caledonian collision (Fig. 5.). Irish Sea glacial till lies above them, at the tops of the cliffs.

Fig. 5. Turbidity currents laid down these sandstones, and mud settled between each flow. The field of view is 60 centimetres. South end of Aberaeron beach.

The bedrock shows hundreds of alternating layers of turbidite sandstone and mudstone ranging from a centimetre to about a metre thick. If you look carefully at the thicker beds, you can see the complete record of a single turbidity current. Each bed has coarse sand at its base, where the current dropped its largest grains. The current rippled the upper sand surface as it slowed. Mud settled as the current stopped to become the upper part of the layer. In some places, you can see where the currents ripped up the underlying bed and carried fragments of the bed down the slope.

The road between between Aberaeron and Aberarth follows a low terrace about a kilometre wide. The glacial till that plasters it appears in the cliffs along the beach north of Aberarth. This Welsh till is bluish grey when fresh. It is a mixture of particles of all sizes, including many boulders as much as one metre across (Fig. 6).

The cliffs about two hundred metres along the shore north of Aberarth provide a magnificent display of turbidites. Some layers are full of broken pieces that the fast currents ripped from older deposits. Other beds are highly contorted and twisted (Fig. 7), so presumably they were soft enough to flow like golden syrup over a suet pudding when the next current rushed by. Look carefully at the bottoms of the beds. Some show elongate gouge marks rasped out by the moving sand.

Fig. 6. Left: Sea cliffs in glacial till at Aberarth. The sea undercuts the easily eroded till.

Fig. 7. Right: Ripped up and contorted turbidite layer in a sequence of graded turbidites. Cliff outcrop about 200 metres north of Aberarth. The thickest bed is about a metre thick.

Banded turbidites similar to those at Aberaeron appear in many road cuttings between Aberaeron and Aberystwyth.

The striking feature of the region is the narrow coastal platform, about ten metres above sea level, carved into the hills that rise 300-metres to the plateau above. The narrow coastal platform is an old wave-cut bench. Old shorelines exist at this height in many places around the British Isles, and indeed along stable coasts all over the world. They record a higher stand of sea level during a time between ice ages, when the climate was warmer than now, and the melting ice sheets released their trapped water back into the oceans.

Fig. 8. The cliff at the north end of the esplanade at Aberystwyth shows hundreds of sandstone beds, each carried by a turbidity current.

The north end of the promenade at Aberystwyth ends against a cliff, one of the most visited outcrops in Wales (Fig. 8). Geologists love its splendid display of layered Silurian sandstone, deposited from turbidity currents flowing into the Welsh Basin. These sandy layers were laid down more than 50 kilometres offshore. Experiments show that the currents must still have flowed at more than 15 kilometres per hour to carry the sand.

These beds record in detail the passage of the turbidity currents. The sand at the bottom of many shows regular light and dark banding (Fig. 9), where the rushing current smoothed all before it. A sand layer about 20-30 centimetres thick that lies on this banded rock collected as a slowing current dumped most of its load. It then rippled the upper surface of the bed. The mud at the top settled in quiet water before the next turbidity current arrived.

Fig. 9. Ten-centimetre-thick sandstone layers alternating with mudstone. Cliff at the northern end of the esplanade at Aberystwyth.

The underside of a turbidite bed is a very special place. The surface of the mud may lie undisturbed for many years between the turbidity flows, while animals colonize it. The undersides of beds are exposed in the small caves at the base of the cliff, and show tracks of animals that crawled about on the sea floor (Fig. 10). Each species leaves a distinctive track. Geologists try to infer from these tracks the kinds of animals that lived in this deep-sea environment. Were they marine worms? Shrimps? Sea slugs? Only their traces remain.

The bottoms of some sandy beds contain large and elongate ridges, flute casts (Fig. 11). They are depressions the passing turbidite flow gouged in the soft mud. They are deeper at the upstream end and tail out in the direction the current flows. These flute casts show that the currents flowed northeast.

Fig. 11. Scour marks on the underside of one of a sandstone bed. Tails on the scours show the turbidity current moved from top right to bottom left. North end of the esplanade at Aberystwyth.

The road north of Aberystwyth climbs into beds laid down in the deepest part of the Welsh Basin. The cliffs south of Borth contain slate (Fig. 12) that contrast strongly with the banded sandstone at Aberystwyth. The slate was originally fine mud, deposited in quiet water in the deepest part of the Welsh Basin, beyond where turbidity currents could carry sand. The Borth mudstone is the same age as the thick sandstone seen at New Quay. The turbidites here are thin mud layers, up to five centimetres thick, separated by finer muds, up to twenty centimetres thick, that settled slowly from above between each turbidite flow.

Fig. 13. Close up of the banded Silurian slate at Borth. Two centimetre high ripples on the upper surface of light sandy layers show the turbidity current had slowed. .

Borth is on the estuary of the River Dovey, with mudflats to the east and sand dunes behind the beach. An ice age glacier gouged the Dovey Valley to nearly one hundred metres below sea level, then filled it with sediment as it melted. Waves swept pebbles and sand eroded from the cliffs and from the glacial till north to build the sand spit north of Borth. Extreme low tides expose the remains of a forest about six thousand years old. The pine and birch trees flourished when the climate was warmer than at present.

Good slates of Silurian age, complete with colour banding and a steep cleavage are exposed in the quarry by Machynlleth Station, and in road cuttings about a kilometre west of the town (Fig. 14). Both outcrops show folds in the colour banding. Indeed, all the rocks in this part of central Wales are so intricately folded that many sections look like concertinas. The folds result from shortening of the layers during the Caledonian collision; this collision also made closely spaced fractures at right angles to the shortening to give the pervasive cleavage of the rocks. These outcrops around Machynlleth are typical.

Fig. 14. Colour-banded Silurian slate one kilometre west of Machynlleth on the A487 The view is about two metres across.

The continuation of the A487 from Machynlleth north to Dolgellau is described in the Cadair Idris section.

ROADSIDE GEOLOGY OF WALES

THE COAST ROAD The A487