Fig. 29. Geologic map of the coastal region between Manorbier and St. Govan's Head.
The peninsula south of Pembroke crosses folded Devonian Old Red Sandstone and Carboniferous Limestone, but the only exposures are in the cliffs. The rocks were strongly folded during the Hercynian collision when Gondwana docked with the Avalon/North American block.
Freshwater East (202 198)
The valley leading to the shore at Freshwater East follows Ordovician rocks in the core of a large anticlinal fold, though no Ordovician outcrops appear on the shore. On either side of this fold --on the southern headland and the northern cliffs-- are Devonian Old Red Sandstone conglomerate.
Stackpole Quay (199 196)
Fig.
30. Thick-bedded Carboniferous Limestone dips north at Stackpole Quay
(Fig. 29).
The limestone thrust north over Devonian Old Red Sandstone beds, seen
in the
far cliffs.
Complex folds created during the Hercynian collision are well exposed at Stackpole Quay (Fig. 29). The harbour is in the core of an arch, or anticline, in Carboniferous Limestone. A set of fractures crossing the bedding opened when the rocks stretched around the crest of the fold.
South of the harbour, a fault separates this fold from the vertically tilted layers of limestone to the west. Farther south, the limestone shows spectacular large folds. But by Stackpole Head (Fig. 29), the layers of limestone are horizontal.
A fault north of the harbour separates the limestone from folded mudstone and sandstone deposited during Devonian time.
Fig. 31. Geologic map of west part of Pembroke peninsula.
St. Govan's Head, accessed by C-class roads, has fifty-metre-high cliffs of thick horizontal beds of Carboniferous Limestone. The limestone has nearly vertical joints that are the ideal arrangement for making high vertical cliffs. There are fine outcrops of the limestone along the path down to the chapel built in a cleft in the cliff. The massive limestone forms continuous cliffs northeast to Stackpole Head and Stackpole Quay. St. Govan's Head lies just south of a firing range; the army closes the road during military exercises.
Freshwater West (187 200)
Fig. 32. Geologic map of Freshwater West.
The
rocks at Freshwater West are Devonian mudstone with a few sandstone
layers, and
are mostly below high tide. Some are in the danger zone of a military
firing
range--keep
out.
At
the northern end of the wave-cut platform, the Old Red Sandstone rocks
lie unconformably
on folded lower Palaeozoic rocks. Beneath the barnacles and seaweed are
the
lowest beds of the Old Red Sandstone, quartz pebble conglomerate.
Farther
south, and above the
conglomerate, the rocks are mostly mudstone. They are green in the
lower layers
but gradually become red upwards before they reach the stack, called
Little
Furznip. Mudstone in the stack contains layers of blotchy white
calcite; this
is the same rock, caliche, you see at Llanstephan (235 210) (Fig. 42)
and
Manorbier (206 198) (Fig. 34). Further east, a fault breaks the rocks
between
the stack and the cliff. You can see its offset in the displacement of
the
white caliche. The Hercynian mountain building fractured the rocks,
giving them
a weak slaty cleavage cutting nearly vertically through the mudstone.
Rocks south of Little Furznip are mostly mudstone, but they include many sandstone beds as much as two metres thick. The sandstones are cross-bedded. It is easy to imagine floods depositing them as they swept across the mudflats. As the floods reached deeper water, the sand settled on the sloping front of the pile of sand making the cross beds. Rocks higher in the sequence are dominantly red mudstone.
The firing range sign prohibits looking at the rocks higher in the sequence. They are red mudstone, overlain by the Ridgeway conglomerate, the same rocks you see west of Tenby, and on the Gower.
The cliffs at Freshwater West show a wonderful display of the effects of the last ice age. Cemented scree, collected as frost-shattered pieces at the beginning of the ice age and covers the rocks in the cliffs. Boulder clay, or till, lies on the scree, laid down by the advancing glacier. The till is in turn overlain by sand deposited from melt water shed from the glaciers as they melted at the end of the ice age. Finally, recent sand dunes advanced about two kilometres inland from the shore, covering all older rocks. The wave-cut platform at the base of the sea cliffs is also a recent feature.
A tightly folded syncline in the cove at the west end of Angle exposes the Carboniferous Limestone in the core; the older rocks of Old Red Sandstone occur in the limbs of the syncline to the north and south. The beds on the north side of the cove dip about 50 degrees to the south, and those on the south dip about 40 degrees to the north, outlining the large syncline formed during the Hercynian collision. The youngest rocks, in this case Carboniferous rubbly limestone, are in the trough of the syncline. These Carboniferous rocks have beds as much as 50 centimetres thick alternating with shale. Several small folds in the limestone complicate the larger syncline.
Fig.
33. Thin-bedded Carboniferous Limestone lies on muddy limestone at
The path to the north of the cove crosses into the Old Red Sandstone. These rocks include mudstone, sandstone and conglomerate similar to other Old Red Sandstone outcrops in the area.
Pembroke (198 202) -- Begelly (212 207). 17 miles (27 km)
The road from Pembroke to Manorbier crosses a flat grassy plateau with no outcrops.
Manorbier (206 198)
Fig. 34. Vertical sandstone and mudstone of the Old Red Sandstone at Manorbier.
The shore at Manorbier, below the castle ruins, shows fine outcrops of Devonian mudstone and thin sandstone so strongly folded that the sedimentary layers are vertical. The sandy beds, which are as much as ten centimetres thick, have rippled upper surfaces. Some of the mudstone contains layers of white nodules as large as five centimetres in diameter. These are similar to those at Llanstephan (235 210) , and are the typical calcite concretions, caliche, that precipitated in the Devonian soils during dry periods when Wales was at latitude about twenty degrees south.
South
of Tenby, folds repeat the Carboniferous Limestone and underlying Old
Red
Sandstone, both of which appear as lines of poor exposures across the
Tenby (Dinbnych-y-Pysgod) (214 201)
Tenby is a popular resort where cars are prohibited during summer business hours. It is on the southern edge of the Pembrokeshire Coalfield and lies in the keel of a large east-west trending synclinal trough. The same Carboniferous Limestone you see at Pendine on the north side of the boat-shaped fold, or syncline, reappears on the south side of the harbour at Tenby. A major fault thrusts this limestone up to the north over the younger Coal Measures. The cliffs along the shore north of Tenby contain sandstone and shale of the Coal Measures. Some sandstone beds are as much as three metres thick and show deltaic features such as cross bedding. There are excellent examples of tight folds from the Hercynian collision north of the steps to the middle part of the beach. Some of the folded layers are nearly vertical.
Fig. 35. Geologic map and cross section between Pendine and Penally. The cross section is from Tenby to north of Amroth.
Tenby to Begelly
The best examples of Hercynian folding lie along the coast road south of Begelly.
Saundersfoot (214 205)
The rocks at Saundersfoot are sandstone and mudstone of the Lower Coal Measures. The southern harbour wall has fine large folds in the sandstone.
Lady Cove, the second cove south of Saundersfoot harbour, boasts one of the most famous folds in the world. It is not particularly unusual, just photogenic, and appears in many geology texts as a classic example of an arch-like fold, or anticline.
Fig. 36. Anticlinal fold at Lady Cove, south of Saundersfoot.
The fold is in thinly bedded sandstone and shale showing many ripple marks on their surfaces. The rhythmic layering makes the rock easy to fold because the thin beds can slide over each other, like folded pages of a magazine. Scratches left on many of the bedding surfaces show where this sliding occurred.
Begelly (212 207) -- Pendine (223 208). 10 miles (15 km)
The C-class road between Begelly and Pendine and Amroth (216 207) follows a high grass-covered terrace, and passes no bedrock outcrop.
Fig. 37. Coal Measure rocks at Amroth West. The light coloured rock is a "seat earth," a leached tropical soil.
The Carboniferous rocks west of Amroth are tightly folded, and much broken along faults. Some are smashed beyond recognition. The first rocks in the cliff section are typical of the Coal Measures--bleached mudstone full of brown ironstone concretions, overlain by black, organic-rich mudstone deposited in a swamp. There are many repetitions of the delta to swamp cycle in the rocks west of Amroth. Unfortunately, the folds and faults have so disrupted the layers of rock that it is very difficult to keep track of the individual sequences.
In the short drive between west and east Amroth the road crosses the old Hercynian mountain front and the rocks to the east are relatively undeformed.
Sandstone of the Coal Measures is exposed in the cliffs about 150 metres east of the promenade at Amroth East (218 207). The rocks at the base of the cliffs are brownish sandstone that shows cross beds and ripples on the bedding surfaces. These sedimentary structures are typical of deltas. The upper parts of the cliffs are grey mudstone, and are white towards the top. This white band of bleached rock is called a "seat earth" as a thin band of coal sits on this layer. Above the coal is more cross-bedded sandstone.
Fig. 38. Coal Measures sequence in the cliffs east of Amroth East.
This sequence shows the typical history of the Coal Measures in one place. The environment begins as shallow water that the delta fills with the sand. Then the river shifts its course to another channel leaving behind sheltered backwaters that fill with mud. Rainwater soaks into and through the mud, leaching it of soluble material and turning it white. Swamp vegetation and trees then flourish, die, and turn into peat. Sea level rises, and the whole process starts again, burying the peat beneath a new delta.
The change from the deltaic deposits of the Coal Measures to the underlying Carboniferous Limestone, is not visible from the road. The contact is at Ragwen Point, about 1.5 kilometres along the beach southwest of Pendine. However, the cliffs at Pendine provide fine examples of the limestone.
The
back roads between Pendine
and Carmarthen
Fig. 39. Map
of area
between
Pendine (223 208) -- St.
Clears (228 216) 9
miles (14 km)
Pendine (223 208)
Waves created the fine sandy
beach at Pendine from sand brought to the region by ice-age glaciers.
The
Pendine sands were once famous for land speed record attempts before
the
Bonneville Flats in
A quarry (225 208) east of
Pendine works red mudstone of the Old Red Sandstone. The contact with
the
overlying Carboniferous Limestone is not visible, but the limestone
makes the
magnificent set of cliffs west of the village. These cliffs offer one
of the best
sections of Carboniferous Limestone in
Fig. 40.
Carboniferous Limestone in the Pendine cliffs.
The limestone layers are typically
ten to twenty metres thick (Fig.
40). The
Some
of the limestone layers are masses of rubble. Water dissolved the
limestone to
create sinkholes and caves when sea level dropped; eventually the roofs
collapsed filling the caves with rubble (Fig. 41).
.
Fig. 41.
Rubble breccia
in collapsed cave in Carboniferous Limestone. West end of Pendine beach.
The A4066 east of Pendine
crosses Devonian and Carboniferous rocks laid down near the coastline
of the
Avalon/North American continent. The road between Laugharne (230 211)
(the
setting for Dylan Thomas' "Under
Milk Wood") and St. Clears follows the Afon Taf. Bedrock outcrops
are
rare in this region, but there are a few exposures of red mudstone of
the Old
Red Sandstone near Thomas' boathouse at Laugharne.
Dunes south of Laugharne get
most of their sand from ice-age sediment carried to the coast in rivers
swollen
with water from melting glaciers of the last ice age. Waves swept the
sediment
down the coast and onto the upper beach, where the wind whipped it into
the
dunes.
The low
peninsula southwest of
Rocks of the Devonian Old Red
Sandstone that lie on the folded Ordovician rocks make up the cliffs of
the
peninsula; the best exposures are along the shore south of Llanstephan.
However, the first outcrops in the cliffs south of the Llanstephan car
park are
of red
The
higher part of the Old Red Sandstone formation is mostly grey to green
mudstone, the red mudstone becoming dominant in the older rocks farther
south.
The original sediments were probably laid down on mud flats. Look
carefully in
the mudstone for blotches of limestone that look like white pebbles.
These are
blobs of calcium carbonate-- or caliche-- that crystallized in the soil
during
Devonian time. Caliche forms in semi-desert soils today, when the
occasional
rain soaks into the ground, then evaporates, and leaves nodules of
calcium
carbonate behind.
These limy concentrations are
more common where the coastline turns west; the rocks look like
limestone
gravel. It seems reasonable to suppose that water collected in
temporary
shallow lakes on the mud flats as the Old Red Sandstone was laid down,
since
similar limestone layers, often called tufa, collect in shallow
dry-climate
lakes today. In Devonian time as the lakes evaporated, dissolved
calcium
carbonate precipitated to become uneven beds of limestone.
Fig.
42. Red and green Old Red Sandstone rocks, to the right of the steps,
and a
rubbly caliche beneath the steps. Cliffs south of