Fig. 17. Geologic map of the Lleyn peninsula.
The 75 metre high coastal platform of the Lleyn Peninsula stretches southwest from Anglesey, along the north side of Cardigan Bay. Precambrian rocks occur in the cliffs along the north coast, southwest of Nefyn. This belt includes ocean-floor volcanic rocks like those on southern Anglesey, and some crushed gneiss similar to those near Porth Trecastle on Anglesey. These Precambrian rocks are the remains of the ancient basement on which was deposited the Palaeozoic rocks of North Wales.
The Cambrian Sea invaded this Precambrian surface, leaving a blanket of sand on the ancient lavas and gneiss. These Cambrian sandstones appear in cliff sections on the Tudwal Peninsula south of Abersoch, and along the railway track east of Criccieth (Fig. 17), but it is easier to study them near Harlech, south of Snowdon, where they are close to the road.
Most of the exposed rocks on the Lleyn Peninsula are Ordovician volcanic rocks, erupted during the closing of the Iapetus Ocean as the ocean floor plunged beneath the continent. They make up the grass-covered rounded hills west of Pwllheli and north of Criccieth. These Ordovician rocks are mostly basalt lavas and volcanic ash thrown out in explosive eruptions when the Iapetus oceanic plate was sinking beneath Avalon. Mud and sand settled on the sea floor between eruptions, but soil covers most of these softer rocks.
The Cambrian and Ordovician rocks were strongly folded and broken along faults during the Caledonian collision, when the North American continent pushed against them. This collision compressed the rocks, changed the mud to slate, folded the volcanic rocks, and gave the rocks their distinct northeast to southwest grain. A few small masses of molten granite magma that did not make it to the surface intrude the deformed rocks along the coast road northeast of Nefyn.
During the ice ages, Lleyn was a battleground between glaciers flowing off the Welsh highlands and glaciers pushing down from the Irish Sea. The Welsh ice spread across the southern part of the Lleyn, while Irish Sea ice pressed against the north coast. The Welsh ice, carrying debris from Snowdonia, contained many grey rocks. Welsh till covers the land from east of Criccieth to the Tudwal peninsula in southwest Lleyn. Irish Sea till is common along the north coast of Lleyn, is reddish in colour and is full of shells scraped from the sea floor.
Ice age glaciers plastered a cover of glacial till over most of the Lleyn Peninsula, effectively hiding the bedrock. Outcrops are rare, except in the spectacular cliffs along the coast. The broad valley between Morfa Nefyn and Pwllheli (Fig. 17) carried torrents of meltwater heavily laden with glacial sediments when the Irish Sea ice pressed against the north coast of the Lleyn Peninsula. Sand and gravel now fill the valley to a depth of about 30 metres. You can see that debris in the sea cliffs at Morfa Nefyn.
THE SOUTH COAST OF THE LLEYN PENINSULA
The A497, A499 (part), and B4413
Porthmadog (257 338) to Aberdaron (217 326). 29 miles (47 km)
The A497 follows the coast from Porthmadog to Pwllheli; then the A499 and B4413 continue to Aberdaron in the southwest. Glacial till, soil, and grass cover most of the rocks, but there are fine exposures of Ordovician volcanic rocks in the hills and in the cliffs.
Porthmadog is at the mouth of an estuary filled with sediment carried by torrential rivers emptying from the melting glaciers during the waning stages of the last ice age. The A497 west to Criccieth climbs onto Ordovician volcanic rocks. At a layby on the hill east of Criccieth you can see folded sediments laid down between the Ordovician eruptions--coarse sandstone between layers of finer sandstone and slate. There are Cambrian sandstones in the railway cutting below.
Criccieth (250 358)
Fig. 18. Criccieth Castle is built on an Ordovician dolerite sill.
During Ordovician time part of the oceanic slab sinking beneath Avalon-Wales melted, and lava erupted to build volcanoes in North Wales. The best place to see the Ordovician volcanic rocks on the Lleyn is in the cliffs below Criccieth Castle. Black basalt lava flows alternate with light-coloured rhyolite ash showing banding typical of ash fall deposits. Some of the molten rock did not erupt to form a lava flow, but solidified beneath the surface along bedding planes to form sub-horizontal sheets called sills. Many of the igneous layers show distinctive columns across the layers. These columns have four, five and six faces. These are vertical shrinkage cracks split open when the rocks cooled. All the rocks rotated to vertical during the Caledonian collision, so the columns are now nearly horizontal.
Fig. 19. Right. Columnar jointing in Ordovician lavas. The columns are rotated from their original vertical orientation. Beach below Criccieth castle.
Criccieth Castle stands on a hard slab of igneous rock, but the town perches more precariously on a cliff of glacial till deposited by the Welsh ice sheet; the till erodes easily to undermine the houses (Fig20.)
Fig. 20. The town of Criccieth is built on glacial till. The cliffs are in danger of collapsing; gabions full of rocks temporarily slow the hazard.
The road west of Criccieth crosses grass-covered glacial till, and passes the birthplace of Lloyd George, the last Liberal Prime Minister, at Llanystumdwy, a mile or so west of Criccieth. Some ice-age sea cliffs eroded in welded rhyolite ash rise just east of Abererch, and on the outskirts of Pwllheli (237 335). A massive outcrop of dolerite lies at the end of the spit at Pwllheli. This outcrop was a sea-stack left stranded when the waves cut the sea cliffs behind the town.
The next exposures of solid rock are near Llanbedrog (233 332), where the village school stands on fine outcrops of Ordovician rhyolite ash. The headland has granite that intruded Ordovician volcanic rocks; good exposures of the volcanic rocks appear again around Mynytho (231 331) on the B4413 and along the road north of the school.
The National Trust windmill (231 332) north of Mynytho provides a fine view of the volcanic terrain. In Ordovician time, a chain of volcanic islands stretched from here to the Lake District in England. Ordovician volcanic rocks and granites stretch in a broad arc from Snowdon along the north coast of Lleyn, then swing round to this point in a broad down-fold, or syncline, and back along the south coast to Criccieth. The rocks near the windmill are part of a small intrusion, and the low land to the west is eroded into the soft Llanvirn slate.
The St. Tudwal's Peninsula south of Abersoch (232 328) has early Cambrian gritstone in the cliffs west of the headland. The rocks are coarse sands of the Hell's Mouth Grits in beds up to four metres thick, alternating with finer silt and mud. Turbidity currents carried these sediments from the continent into one of the small fault-bound basins that opened on the edge of Eastern Avalon in Cambrian times. At the south end of the peninsula, the rocks are finer grained; the source of the sediments lay further north. Then in early Ordovician time, the sea shallowed and gravel and sand, now conglomerate and sandstone, collected on beaches. The bay at Porth Ceiriad (230 324) has Late Cambrian and Ordovician sandstone and mudstone in its cliffs.
The B4413 crosses glacial till between Llanbedrog and Aberdaron. Aberdaron (217 326) stands on low cliffs of glacial till between two headlands of the extremely resistant Precambrian rocks.
Fig. 21. The headland at Uwchmynydd, at the west end of Lleyn. Late Precambrian Gwna submarine landslides are in the cliffs.
Fig.
22. The
headland at Uwchmynydd.
Large blocks of
white quartzite in a mixture of schists and volcanic blocks, a
landslide that roared along the sea floor down a submarine slope near
the end of Precambrian
time.
A trip to the southwest tip of the Lleyn Peninsula is worthwhile to see some extraordinary Precambrian rocks. The best exposures are at Uwchmynydd on the southwest tip of the peninsula. The track from the National Trust car park to the cliffs keeps to the north side of the valley. From the end of the road, it is about one kilometre to the coast.
The rocks along the coastal path look as though they had been ground in a giant mixer; this is the Gwna Melange. Large blocks of white quartzite, dark bluish-grey limestone, and brown schist seem to float in much finer-grained rock (Fig. 22.). Geologists interpret these rocks as the result of an undersea slide that mixed rocks from the shallow continental shelf with ocean floor rocks from a sinking plate about 600 million years ago.
THE NORTH COAST OF THE LLEYN PENINSULA
Aberdaron (217 326) to Caernarfon (248 363). 32 miles (52 km)
The B4117 follows a seventy-five metre high platform along the north coast. The bedrock is Precambrian volcanic rock, but is exposed only in the cliffs.
Morfa Nefyn (229 340)
Fig. 23. Pillows in Precambrian basalt (Gwna group). Morfa Nefyn lighthouse. View is four metres across.
The cliffs at Morfa Nefyn are of loose sand deposited as a delta in front of the melting Irish Sea glaciers at the end of the last ice age. Hard Precambrian lavas beneath the soft sands protect the cliffs from rapid erosion.
Good exposures of Precambrian volcanic rocks of the Gwna group occur below the lifeboat station north of Morfa Nefyn. The basalt lavas show the distinctive pillow structures typical of flows that erupted underwater. The Caledonian collision rotated this basalt more than ninety degrees so that they now dip steeply northwest, with the original tops of the bed now pointing to the southeast.
The other distinctive Precambrian rocks, gneiss, crushed and deformed during the Caledonian collision, can be seen only along the headland between Morfa Nefyn and Nefyn.
Northeast of Nefyn, the
road climbs onto a 75 metre high platform cut into Ordovician rocks
covered
with glacial till. Then the road climbs past several granite intrusions
that
are the same age as the Ordovician volcanic rocks and date from the
time when
the floor of the Iapetus Ocean was sinking beneath the
continental
crust of Wales, then part of Avalon. The
largest mass
makes the mountain Yr Eifl
(236 345) (Fig. 24) with its large granite
quarries
on its north side visible from Trevor
(237 346). The granite is a light
grey
color with small crystals of felspar in a finer matrix of quartz and
felspar.
Fig. 24. Granite quarry in Ordovician granite. View from Trevor. The mountain is Yr Eifl.
North of Trevor, the road follows the flat surface of a raised marine terrace to Caernarfon. Hills of Ordovician volcanic rocks rise to the south.