Chapter 5
GLAMORGAN AND GWENT-- SOUTHEAST WALES.The M48/M4
Chepstow (354 194) to Carmarthen (241 220). 90 miles (144 km)
Fig. 30.
Geology of the
M4
The M4 skirts the southern boundary of the South Wales Coalfield. Between Chepstow and Carmarthen the motorway repeatedly crosses the contacts between the older rocks of Devonian Old Red Sandstone and Carboniferous Limestone, and the younger rocks of Triassic New Red Sandstone and Jurassic limestone. You need to watch carefully to tell the sequences apart because both contain limestone and red sandstone. The Carboniferous Limestone is battleship grey, in layers a metre or so thick; the Jurassic limestone is yellowish brown, and in layers ten to twenty centimetres thick. The Old Red Sandstone has thick reddish-brown sandstone beds; the New Red Sandstone has thin beds of bright red mudstone. The motorway passes few road cuttings so it is difficult to see the rocks.
The
old
West of the old Severn Bridge, grey Carboniferous Limestone pokes through the Triassic cover in road cuttings near Exit 2 to Chepstow. This limestone is in the hinge of a downfold that continues northeast into the Forest of Dean. You can see a few more cuttings in the limestone as far west as Exit 23a, east of Newport, where the road crosses back onto the cover of red mudstone of Triassic age. The mudstones do not make solid outcrops. They weather to a bright red soil.
The M48 joins the M4 at Exit 23 of the M4.
When the Bristol Channel began to open in Jurassic time, the skeletons of microscopic animals living in the warm shallow water settled to make limestone layers. Blocks of this limestone grace the lawns in front of the hotels at Exit 24, but do not reach the M4, which crosses back and forth between the Devonian Old Red Sandstone and younger New Red Sandstone. Road cuttings at the abutments at Exit 26 are in thick beds of the Old Red Sandstone. These maroon outcrops are on the southern edge of a large fold that brings the older, Silurian, rocks to the surface to the north, around Usk.
Few bedrock outcrops exist between Exit 26 and the area west of Exit 29. But the ploughed fields are the dark reds typical of soils developed on the Old Red Sandstone. The road cuttings provide a few glimpses of red sandstone.
Cardiff (318 175)
At Cardiff, the M4 skirts the base of the plateau to the north capped by Pennant Sandstone. The M4 crosses lowlands eroded into Triassic and Jurassic rocks. Carboniferous Limestone bounds the southern edge of the Coalfield, and dips north beneath the Coal Measures.
It is difficult to keep track of the rocks between exits 29 and 32 because the layers are nearly horizontal and offset along many faults. In places thin beds of yellow Jurassic limestone or grey Carboniferous Limestone poke through the soil. In other places the road crosses red soils developed on the Triassic mudstone.
The road west of Cardiff crosses the red soils developed on the Old Red Sandstone. At Exit 32, the road crosses the Taff Valley, with a large quarry in Carboniferous Limestone to the north. The M4 west of Exit 33 drops off a low saddle into the Ely Valley, one of the rivers draining the coalfield.
The road west of Exit 34 leaves the Old Red Sandstone and climbs through road cuttings of thick layers of grey Carboniferous Limestone. The limestone about three miles west of Exit 34 is stained red in places. The red pigment is rust that leached down from the red Triassic rocks above. A long waste tip east of Exit 35 is the only sign that the road passed up into the Coal Measures.
Several small faults west of Exit 35 break grey Carboniferous Limestone, black Triassic mudstone, and yellow Jurassic limestone. Road cuttings between exits 35 and 36 are largely in Triassic black mudstone that contains a few thin beds of limestone. They are the first hints of the deepening of the sea as we go up the stratigraphic column from the mostly muddy rocks of Triassic age to the limestone of Jurassic age.
The black mudstone continues for about half a kilometre west of Exit 36. Then there are no more outcrops until about six kilometres west of Exit 36 where thin beds of Jurassic limestone appear in the cuttings. About a kilometre farther west, the road drops back down into the thicker beds of grey Carboniferous Limestone. Finally the road drops onto the coastal plain at Exit 37.
West of Exit 37, the road turns northwest. To the east are steep hills eroded in sandstone, part of the Coal Measures. Their seaward slopes were sea cliffs during higher stands of sea level between the ice ages. Sand dunes to the west are the most obvious feature of the coastal plain. The wind swept them up during the later stages of the ice age, when sea level was lower and broad areas now under water were exposed. Dune grass now stabilizes the dunes. When people disturb that rather tenuous cover, the sand again blows before the wind.
To the north, the road follows the edge of the Coalfield, passing cuttings in the resistant sandstone of the upper Coal Measures, the Pennant Sandstone. The largest exposures are in the cuttings near the bridge across the River Neath east of Swansea.
The road around Swansea crosses the Coal Measures, with spectacular cuttings in sandstone at the roundabout just west of the bridge at Britton Ferry. Slag heaps rise west of the road north of Exit 47. The nearby road cuttings reveal horizontal layers of black mudstone and a few thin beds of sandstone. These rocks were probably laid down in marshy lakes and backwaters between active river channels flowing northwards from a land source in Devon. The conditions may have resembled those that exist today in the Ganges and Mississippi deltas where forests flourish between marshlands.
The M4 crosses the estuary of the Loughor at Exit 48. The territory you can see from the road is certifiably free of bedrock outcrops. The concealed beds beneath the surface dip gently down to the south, so you cross onto older rocks as you drive north. The low ridge northwest of Cross Hands is the Lower Carboniferous Limestone. No road cuttings expose it, but you can see limestone quarries lining the ridge southwest of the road. The road west of this ridge passes below the Carboniferous Limestone into the underlying Old Red Sandstone of Devonian time. The rock here is red mudstone laid down on an ancient floodplain.
The M4 becomes the A48 at Exit 49. It continues to Carmarthen across soft sandstone and mudstone of the Old Red Sandstone, but there are no outcrops.
The
M4 motorway between
Like the modern rift valleys of
East Africa, these fault bounded Triassic valleys of Wales developed
desert
lakes, or playas, on their floors. The streams that ran in from the
hills
evaporated in those playas, laying down their loads of sediment in
thick
deposits of mud on the valley floors. These red muds are the same age
as the
dune sands that lie farther north. The mud and sand are part of the New
Red
Sandstone.
As the new
Fig. 31. Geologic map of the Glamorgan coast between Cardiff and Swansea.
Lavernock Point (319 168)
Lavernock Point is best known as the place where Guglielmo Marconi made the first overwater radio transmission in 1897. It may also become the site of Britain's first major tidal power station, the Severn Barrage.
Park by the church built of the local light grey Jurassic limestone with a fine Welsh slate roof. Look for the fossils in the limestone blocks in the churchyard wall.
The Jurassic limestone is exposed in the sea cliffs. The limestones are yellowish on weathered surfaces, light grey on a freshly broken surface. They are beautifully layered. The layers of limestone are ten to twenty centimetres thick; thin layers of dark grey mudstone separate them. Some geologists think the alternating layers of limestone and mud record regular changes in climate. Periods of a wetter climate increase the plant cover, which decreases the rate of erosion. Rivers would have flowed clear then, and pure limestone accumulated in the clear, warm sea. When the climate was dry, plant cover decreased, the soil eroded more rapidly and rivers carried mud into the sea.
Fig. 32. Jurassic limestone at Lavernock Point.
Clams, oysters, and other fossils crowd many of the rock layers (Fig. 33). If you have a strong magnifying lens, you can also see the remains of a wide variety of minute animals. The warm seas that deposited these rocks teemed with life.
Fig. 33. Oyster fossils crowd many of the limestones, though most limestone is a collection of animals too small to see without a magnifier.
The beds dip gently down to the south, so the rocks become progressively older to the north. The older rocks are red and green layered mud deposited during Triassic time. They continue north to the Cardiff estuary. The constant erosion of the mud in the cliffs colours the sea red. There are good exposures of these limy mudstones in road cuttings north of Penarth on the A4232, just north of the exit to the Cardiff docks.
The best outcrops are
along the western headland, at the main public beach at Barry Island. Fine exposures of the
Triassic beds
drape over ancient ledges of Carboniferous Limestone (Fig.
34). This is a
classic unconformity in which younger rocks lie on an irregular surface
eroded
across the older rocks beneath. This unconformity is especially easy to
see
because the layered rocks above the old erosion surface are horizontal,
while
the rocks beneath dip at a moderate angle to the south.
Fig.
34. Triassic scree, now lithified, lies flat on an old erosion surface
on
dipping Carboniferous Limestone. Cliffs
at west end of the main beach on
The Carboniferous Limestone, folded during the Hercynian collision, dips about 40 degrees to the south. The old erosion surface cuts irregularly across the tilted layers, just like the modern beach where rocks jut above the sand. The bluff at the west end of the main beach has a magnificent outcrop of limestone debris draped over the dipping Carboniferous Limestone. This debris collected at the base of a small cliff in Triassic time. Somewhat later the debris was then covered with red and green layers of Triassic mud that are also exposed in the cliff at the head of the beach. Some bedding surfaces show sun cracks in mud, evidence of periodic drying. These are the same muds that pass up into the Jurassic limestone at Lavernock Point.
Llantwit Major (297 168)
The A4226/B4265 between Barry Island and Llantwit Major runs along
the top of
the old coastal platform. A few road cuttings expose the yellow
Jurassic
limestone.
The waves at Llantwit Major are now eroding a new wave-cut bench across these Jurassic limestones (Fig. 35). At low tide, you can easily follow individual limestone beds. Prominent fractures break the beds into a brickwork pattern. This limestone is somewhat younger than the limestone at Lavernock. It was laid down in an environment that may have resembled the modern Red Sea or Gulf of Aden -- desert land surrounding warm seas with abundant marine life.
Fig. 35. Wave-cut platform and cliff cut into Jurassic limestone. Llantwit Major.
Fig. 36. An old erosion surface separates the white Jurassic limestone from the darker grey Carboniferous Limestone. Southerndown.
The B4524 continues to Southerndown where you can see the typical Jurassic limestone, pale grey on fresh surfaces, weathering to yellow on exposed surfaces. At the eastern headland (Fig. 36), the more massive Carboniferous Limestone, in shades of battleship grey, lies beneath the yellow Jurassic limestone. The lowest bed of the Jurassic limestone is an unusual white limestone, seven metres thick, that rests on top of and against an ancient cliff of grey Carboniferous Limestone forming an angular unconformity (see also Fig. 34). This is the Sutton Stone, used locally as a building stone, which collected as sand on the Jurassic beach, and later hardened to rock. The sea continued its invasion of the land and the Sutton Stone passes upward into the typical banded Jurassic limestone.
Fig. 37. Sharp folds in banded Jurassic limestone pushed up against the massive Carboniferous Limestone on the right. Southerndown.
The road between Southerndown and Ogmore-by-Sea (286 175) crosses the Jurassic limestone; watch for the distinctive yellowish colour. The underlying grey Carboniferous Limestone appears sporadically at beach level. Triassic rubbly breccia, similar to that at Barry, appears again in the breakwater at Porthcawl (282 176). Fine outcrops of Carboniferous Limestone appear on the foreshore at Porthcawl. Return to the M4 at exit 37.
Prominent waves of sand dunes march inland from the coast between Porthcawl and Swansea. The prevailing westerly winds blow the sand off the beach, which generally faces west, into the dunes. Fine examples of glacial till occur in the cliffs along Margam Burrows.
The A48 Cardiff to Bridgend
Outcrops are rare on the flat inland plain. A few road cuttings show the Jurassic limestone. The best is on the A4232, just south of Exit 33 from the M4. Carboniferous Limestone pokes through the soil in a number of places, showing that the bottom of the valley was quite irregular before the Triassic and Jurassic sediments filled it. You can find the Carboniferous Limestone easily by watching for the quarries worked for road metal.
Just north of Cowbridge (300 174) is an iron mine. The ore is red hematite (iron oxide). It replaced Carboniferous Limestone when fluids rose through fractures in the limestone during the Hercynian collision.