ROADSIDE GEOLOGY OF WALES

Chapter 5

THE GOWER

                The Gower peninsula stretches west of Swansea. Access is easiest from Exit 47 on the M4. Follow the signs to Mumbles.        

              The Gower is a plateau, an ancient erosional plain now about one hundred metres above sea level. The fairly flat upper surface cuts right across folded Devonian and Carboniferous rocks, making them very difficult to see, except in the sea cliffs. The most accessible exposures are at Mumbles Head in the southeast, Three Cliffs Bay at Penmaen along the south coast, and at Rhossili at the western end of the peninsula (Fig. 38).

Fig. 38. Geological map of the Gower.

        Mumbles Head (264 187)

                Mumbles Head and the stack offshore are made of massive, folded, Carboniferous Limestone. Look for the prominent anticline at the Head. The layers of rock make a large fold that looks like an arch, crumpled during the Hercynian collision. North of Mumbles Head, along the esplanade, the layers of limestone dip 65 degrees to the north; at the Head the dip varies across the nose of the fold; south of the Head the dip averages about 25-35 degrees to the south. From the car park west of Mumbles Head, you can see a broad synclinal trough folded in the layers of rock exposed on the beach. Look for the broken rock in a fault zone near the lifeguard's lookout. The red iron ore in the broken rock was once mined in the cliffs above.

                The southerly dip of the rock layers continues to the west, steepening to vertical in Langland Bay. A thick layer of glacial sediments lies on the limestone at Langland Bay (261 187).

        Three Cliffs Bay (254 188)

                Three Cliffs Bay is easy to miss. Follow the signs to the campground almost two kilometres west of Parkmill (255 189). Park in the campground car park, and follow the track through National Trust property to the beach.

                The view across the bay is impressive (Fig. 39). The flat surface of the old coastal platform lies about 100 metres above sea level, and waves are cutting into it to carve out a new platform at sea level. Remnants of a second wavecut platform lie at about seven metres above sea level. The waves probably cut it during one of the interglacial episodes of Pleistocene time, when melting glaciers raised sea level. Rocks beneath the surface are Carboniferous Limestone tilted during the Hercynian collision. You see them exposed in the prominent sea cliffs that give the bay its name.

Fig. 39. Three Cliffs Bay

                Carboniferous Limestone along the eastern side of Three Cliffs Bay contains several massive layers as much as ten metres thick with layered limestone between them. The limestones contain numerous fossils. The first massive limestone bed south of a castle is capped by a layer of yellow sandstone. The top of a second massive layer is weathered red, and contains large holes at right angles to the layering. Both of these features record times when the water shallowed. The thinner layers of limestone were laid down in deeper water, the massive layer in shallow water, and the sand was probably deposited near a beach. The limestones were even above sea level at times, and fresh water dissolved sink holes in the exposed limestone.

Fig. 40. Left: Three Cliffs Bay. Carboniferous Limestone in the shadow (top right) is broken up by being exposed to air and weathered soon after it was formed.

                This successive shallowing and deepening resulted from Gondwana advancing from the south and riding onto Avalon and North America, pushing crust down in front, deepening the shelf so only thin limestone collected. Continuing compression from the advancing Gondwana continent forced the sea floor up, the water shallowed and massive shallow water limestone built up. Eventually the limestones were pushed above sea level and weathered. Three such cycles of thin to thick to weathered limestone are exposed in the cliffs at Three Cliffs Bay. The final collision of Gondwana and Avalon and North America tilted the limestone to their present steep dip.

                Several glacial deposits lie on the limestone at the head of the bay. Frost shattered the upper part of the outcrops during the ice ages, and a scree of broken limestone, called head, spread over the surface. The glacial till that overlies the head records the glacier that finally flowed across the frozen ground. Wind blew sand inland into dunes. They moved again during a stormy period in the Fourteenth Century, and engulfed the village and castle to the north.

                The red fields north of the road at Penmaen indicate the presence of the Old Red Sandstone. A prominent ridge that stretches west and northwest from Penmaen to Reynoldston stands about 50 metres above the surrounding surface. The resistant rocks that hold it up are Devonian conglomerate full of white quartz pebbles as much as three centimetres across. Many local houses and walls are built of this attractive rock.

        Rhossili (241 188) and Worms Head (238 188)

                Take the B4247 to Rhossili. The coastal platform between Port Einon (247 185) and Oxwich (250 187) has a fossil cliff behind it. It was cut during an interglacial period about 125,000 years ago, when sea level was higher.

Fig. 41. Rhossili Beach. The bench at the foot of the cliff is an old river terrace covered by head that flowed from the hills above. The low area in the distance is a dune field.

                A beautiful sandy beach (Fig. 41) fringes the western end of the peninsula north of Rhossili. Westerly winds moved the sand up onto the beach from glacial debris deposited in the shallow sea. Hills of Old Red Sandstone rise in the east; cliffs of Carboniferous Limestone to the south. The bench above the beach is part of a glacial outwash delta now being eroded as waves cut into it.

                The cliff path south from Rhossili gives spectacular views of the Carboniferous Limestone in the cliffs and of the limestone on the bird sanctuary on Worms Head (Fig. 42).

Fig. 42. Worms Head, southwest corner of the Gower. Massive Carboniferous Limestone in the cliff beneath the Gower platform.

THE NORTH GOWER COAST

                The anticline seen at Mumbles brings the mudstone of the Millstone Grit and lower part of the Coal Measures to the surface along the north coast of the Gower. The Millstone Grit mudstone looks like the Coal Measures mudstone.

                The A484 connects Llanelli (253 200) with Carmarthen. The road crosses a downfold, or syncline, in the Coal Measures but you see almost no outcrops. A quarry produces road metal from a thick sandstone bed at Pwll. The ridge that runs northeast from Kidwelly (242 207) is made of the Carboniferous Limestone; it is extensively quarried, but no exposures are near the road.

                Near Carmarthen, the road passes through progressively older rocks into the Old Red Sandstone. It appears in road cuttings on the outskirts of Carmarthen.

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