SW Wales N Pembroke

This section covers the coastal areas from New Quay on the north coast to Aberieddy.

Fig. 2. The north coast of the St. David’s Peninsula. Modified after the Geological Map of Wales, 1994, British Geological Survey.

New Quay (Cei Newydd) (238 260) -- St. David's (Tyddewi) (175 225). 50 miles (80 km)

The A487 between New Quay and Cardigan crosses a grassy plateau with deep tree-filled valleys running down to the coast. The road builders wisely stayed inland to avoid the ravines, so you must take side trips to Llangrannog and Aberporth and to see the Ordovician mudstone and siltstone that lie beneath the Silurian turbidite sandstone typical of Central Wales.

Rocks exposed in the cliffs at Llangrannog show regularly alternating layers of mudstone and sandstone. Submarine slumps created fast-moving slurries, turbidity currents, that carried the sand into the deepest parts of the Welsh Basin. Wonderfully contorted rocks in the northern headland are like snow shoveled off your driveway, and the cause is about the same. A later turbidity current ploughed into these beds, contorting them as it pushed them across the sea floor.

Fig. 3. Ordovician sand and mud contorted by submarine slides. The view is about two metres high. Cliffs at Llangrannog.

The Caledonian collision of Late Silurian time created the complex faults and folds visible in the south headland. The force of the collision converted the mudstone into slate with good cleavage surfaces that cross the original sedimentary layering at an angle of about 45 degrees.

A layer of igneous dolerite about three metres thick intruded between layers of mud soon after they settled in the floor of the basin. Like most dark igneous rocks, the dolerite contains a small amount of the strongly magnetic mineral magnetite that retains memory of the earht’s magnetic field at the time the dolerite cooled. Measurements of that fossil magnetic field show that these rocks were about 50 degrees south of the equator when they intruded into the mud.

Fig. 4. Ordovician mudstone and thin layers of sandstone at Aberporth. View is about one metre wide.

The low cliffs near the car park at Aberporth are of fine slate, a slightly metamorphosed mudstone (Fig. 4). If you examine them very closely, you can find some very thin sandy layers that record the original bedding. The slaty cleavage cuts across them at a steep angle. The original mud was deposited deep in the Welsh Basin during Ordovician time.

Cardigan to Fishguard

The valley of the River Teifi was a major drainage channel under the glacier during the last ice age. The valley has a cross section shaped like a trough. It was reamed out by the rapidly flowing meltwater. The low cliffs northwest of Cardigan expose gravel deposited from torrents of glacial melt water, lying on glacial till. Glacial till from the Irish Sea ice rests on Ordovician slate north of the hotel at Gwbert (216 250), and at the lifeboat station on the west side of the estuary.

From Cardigan (218 246), the A487 climbs the plateau before dropping to the Afon Nyfer about six kilometres east of Newport (206 239); the valley is a broad trough, a relic of the rushing glacial meltwater flowing beneath the ice. High on the slopes of the valley are some precariously perched banks of sand with no obvious source. These are small deltas deposited at the edge of a lake dammed by the Irish Sea lobe of glacial ice.

Frost shattered the slate on the low cliffs at Newport during the ice ages. Geologists call such shattered piles of broken flowing rocks "head". The glacial till that lies on the head shows that the glacier advanced over the frozen ground. Otherwise, it would simply shoved that loose rubble aside.

The moors of the rolling Preseli Hills dotted with tors dominate the views between Newport and Fishguard. Easily weathered Ordovician slate lies beneath the moors. The tors are more resistant igneous rocks. They rose as molten magma into faults along the edge of the Welsh Basin during Ordovician time. These igneous intrusions supplied the massive bluestones in the stone circle of Stonehenge on the Salisbury Plain. The builders moved them about two hundred kilometres east.
Fig. 5. The pile of gravel is an esker, the remains of a river that flowed on or within a glacier. Survey point on the B4582 west of Cardigan.

The B4582 is an alternative route from Cardigan to Newport. The road passes a viewpoint (2125 2415) on a pile of gravel that appears to have been deposited by a stream. This is the highest point around, the top of a hill, so it is startling to think of a river here. In fact the river was flowing even higher, on or perhaps within a glacier. As the ice melted at the end of the last ice age, the melting ice ever so gently set the gravel onto the ground surface. Such gravel deposits typically cross the landscape without regard to the local topography; geologists call them eskers.

A side trip to the shore at Moylgrove (211 246) shows banded Ordovician slate and some sandstone lying beneath a cover of glacial till. Folds exposed in the headland have an unusual chevron pattern. They were tightly crumpled during the Caledonian collision.

Dinas Head is a glacially smoothed headland. South of the headland, a valley shaped like a trough crosses from sea to sea. No stream has flowed through it since the end of the last ice age, when it was a melt water channel beneath the ice. (See the Fishguard Glacial Meltwater Channels section below for other examples.) The best place to see the dry channel is at the beach on the west side of the headland (Figs. 6 & 7). The rocks along the shore are typical Ordovician slate with very thin sandy laminations every centimetre or so.

Large blocks of dark rock sticking up out of the grass and heather make the tors of the Preseli Hills west of Dinas Head. The rock is dolerite, its surfaces weathered to a pale grey. It was part of the igneous activity of Ordovician time, when the floor of the Iapetus Ocean was plunging beneath what is now

England

and

Wales

. Similar igneous rocks extend from the Lake District in northern

England

, through Snowdonia, and south to St. David's Peninsula.

Ordovician slate appears below the viewpoint just east of old Fishguard. The slate by the old fort is finely laminated, typical of the sediments deposited in the depths of the Welsh Basin. Across the river, to the west, is a thick sill of black dolerite that intruded the slate when it was molten magma. It would be a tor if it were on the hills.

Fig. 7. Meltwater channels near Fishguard. Water from melting ice carved these channels beneath the ice in the directions of the arrows. Modified from Fig. 1, D.Q.Bowen, Pleistocene deposits and fluvioglacial landforms of North Preseli, op. cit. p.289-295; in Bassett, M.G. (ed.), 1982. Geological Excursions in Dyfed, Southwest Wales; National Museum, Cardiff.

Melting glaciers dump enormous amounts of water, much of it into channels beneath the ice. The torrents scoured the channels into a distinctive shape that makes them easy to recognize. Their steep sides and flat bottoms look like giant cattle troughs. The insignificant streams that now flow in those valleys are not, by any stretch of the imagination, capable of having eroded them. They are misfits in the landscape.

The Afon Gwaun was one of the melt water drainage channels that flowed beneath the ice sheet that covered the Preseli Hills. The B4313 southeast from Fishguard follows the old meltwater trough for about ten kilometres. Many of the dry valleys leading into the Gwaun Valley were part of a system of lesser drainage channels. Other fine examples of these glacial meltwater channels occur about three kilometres southeast of Fishguard, where the B4313 crosses a dry valley, and four kilometres south of Fishguard, where the A40 crosses a similar dry valley at a picnic area.

Fig. 8. Water flowing under glaciers on the Preseli hills carved these troughs during the ice age. View east from above Scleddau, four kilometres south of Fishguard on the A40.

Southwest of Fishguard on the A487, near Mathry (188 232), the road runs along the north side of the Afon Cleddau. This river once flowed north from Mathry. During the ice ages, glacial till blocked its valley, forcing the river east and south to Haverfordwest and Milford Haven.

The A487 crosses the plateau of the St. David's Peninsula southwest of Mathry. A few hills rise above the plateau in the north. The rocks beneath the plateau are Precambrian volcanic rocks, overlain by Cambrian sandstone and Ordovician slate and igneous rocks of the Welsh Basin. The Precambrian rocks are not exposed in convenient places on the peninsula, but you can see them at Treffgarne, along the A40 north of Haverfordwest.

Side trips to the coast at Strumble Head and Abereiddy are therefore necessary if you want to see good exposures of the Ordovician rocks.

Strumble Head is an ideal place to see outcrops of Ordovician igneous rocks. Tors stretch across the peninsula from east to west; a car park (190 239) on a C-class road provides ready access to several tors. Lichen covers the rocks, but a few broken surfaces show that the rock is a green andesite. It was medium grey when it was new, but chemical changes have since produced the mineral chlorite, which explains the dark green colour.

The best examples of the volcanic rocks are in the wall near the lighthouse at Strumble Head. The wall has blocks of volcanic ash, basalt with many gas bubbles, and various green andesites. Pale grey Carboniferous Limestone, brought in from the east, caps the wall.

The cliffs show massive, thick, volcanic flows. They stand almost on end because they were tightly folded during the Caledonian collision. Fractures break the flows into neat blocks.

Ordovician slate is common along the coast at Trefin (184 232) and Abercastle (185 234), but the best exposures are in the cliffs north of Abereiddy Bay, about four kilometres north of the A487 (Figs. 2 & 11). The car park for the bay is in a glacial meltwater channel.

Fig. 11. The geology of Abereiddy Bay.Modified from Fig. 1, Hughes, C.P., Jenkins, C.J., and Rickards, R.B., 1982, Abereiddi Bay and the adjacent coast, op. cit. p.51-64; in Bassett, M.G. (ed.), 1982. Geological Excursions in Dyfed, Southwest Wales; National Museum, Cardiff.

Waves carved Abereiddy Bay out of soft Ordovician slate. The Caledonian collision threw the Cambrian rocks into a large down-fold, or syncline. The best exposures are north of the bay, along the Pembrokeshire Coastal Path. The path crosses a series of headlands of hard volcanic rocks that separate bays etched in the softer slate. From south to north, rocks in the headlands are: volcanic flows and ash beds of the Llanvirn Volcanic Formation; a black intrusive igneous rock with large grains--gabbro; thinly bedded sandstone of Cambrian age; and a repeat of the gabbro.

Fig. 12. Ordovician volcanic ash beds alternating with softer slate. View from headland north of the slate quarry at Abereiddy bay.

South of the first headland, a limestone rims the edge of a slate quarry that was worked from about 1850 to 1900. The quarry is now an artificial harbour for small boats. The slate has almost no sedimentary layering, which suggests the original mud collected in an extremely calm environment. If you look carefully in the slate pebbles on the beach, you might find the remains of floating animals, called graptolites that lived during Ordovician time. The fossils look like bits of thin black jigsaw blades laid on the bedding surfaces. You may collect from the beach screes but not from the solid rock.

THE ROADSIDE GEOLOGY OF WALES

Chapter 6

SOUTHERN DYFED--SOUTHWEST WALES

The North Pembrokeshire Coastal section