Kington (330 256) to Aberystwyth (258 282). 61 miles (98 km)
Fig. 15. Geology along the A44 between Kington and Llangurig.
The A44 from Worcester and Leominster crosses into Wales just west of Kington, then wends its way to Aberystwyth. Almost all the rocks along the road are sand and silt laid down in the Welsh Basin during Ordovician and Silurian time. The Caledonian collision at the end of Silurian time folded the rocks of the Welsh Basin, gently in the east and more tightly in the west.
Kington (330 256) to Rhayader (297 268)
The road west of Kington passes through a gorge cut in dolerite, one of the few igneous intrusions in the Silurian rocks. The dolerite intruded along a fault to make a nearly vertical dyke. That probably happened during the last phases of the Caledonian collision. Later movements on the fault sheared the dolerite.
Farther west, the road climbs steeply out of the valley, with the Radnor Forest rising to the north. West of the intersection with the A481 (317 258), several good road cuttings on the sharp bends show banded silts that were deposited during Silurian time. They include a few sandstone beds that give a flaggy appearance to the outcrops. Some beds contain calcite. Their weathered surfaces display an irregular ribbing, typical of weathering limestone. The rocks do not show any slaty cleavage so we are probably beyond the reach of the major effects of the Caledonian collision.
Ordovician slate comes to the surface in the core of a large fold east of Rhayader. You will not see much slate, but watch for the prominent ribs of a black rock in the hills west of the A44 north of the intersection with the A481 (317 258). This is an igneous rock, dolerite, like basalt but with larger crystals. It intruded as thin sheets of molten magma into the Ordovician mudstone. The stone is valued for road surfaces and quarried wherever it occurs.
Rhayader (297 268) to Ponterwyd (275 281)
Fig. 16. Geology along the A44 between Rhayader and Aberystwyth.
The A44 and A470 follow
the upper reaches of the River Wye between Rhayader and Llangurig (291
280). A
few ribs of sandstone, dropped as sand from turbidity currents during
Silurian
time, are exposed in the valley walls. The A44 crosses the moors
between
Llangurig and Ponterwyd. A few outcroppings of Ordovician and Silurian
slate
appear in the hills and in road cuttings.
Fig. 17. Fractured rock invaded by mineral veins at the Castell mine east of Ponterwyd.
The old Castell mine near the intersection of the A44 and the B4343, about four kilometres east of Ponterwyd, was worked in the nineteenth century to extract zinc ores deposited in fractures along a fault that breaks the slate. The main vein minerals are calcite and ankerite (a carbonate of iron and calcium) (Fig. 17). The ore is sphalerite (zinc sulphide). The veins were deposited about 355 million years ago, during the relaxation at the end of the Caledonian collision, after it fractured the rocks and opened pathways for the metal solutions.
Ponterwyd (275 281) and Devil's Bridge (274 277)
During
the last ice age, huge volumes of glacial meltwater flowing in the
River
Rheidol cut a deep narrow gorge at Ponterwyd (Fig. 18). It is worth a
side trip
on the A4120 to Devil's Bridge.
A path down from the church about three
kilometres south of Ponterwyd leads to Parson's Bridge, where the gorge
is only
a few metres wide. Its floor is full of fine potholes (Fig. 19) drilled
as
stones turning under persistent eddies ground cylindrical holes in the
slate.
Fig. 18.
Meltwater from
the Plynlimon glacier scoured the gorge cut in the plateau.
The path between Ponterwyd and Parson's Bridge passes fine outcrops of the Ordovician slate with their graptolite fossils (they look like hack saw blades). None exist in the lighter shale, perhaps because it accumulated in water that contained more oxygen, in which their remains decayed. Graptolites thrived in Ordovician time. They were floating animals that apparently drifted in the surface waters, either free or attached to floating debris. It is considered poor form to collect from the outcrops. The loose pieces fallen from the cliffs are fair game.
Fig. 19. The River Rheidol gorge with large potholes at Parson's Bridge. The rocks in the gorge are Ordovician slate.
Glacial meltwater from nearby Plynlimon deeply entrenched the gorge in the Rheidol River, but the Mynad Valley south of Devil's Bridge had no glacier, so no meltwater to cut a gorge. That explains why the Mynad River descends to the deepened valley of the Rheidol River over a cascade of waterfalls at Devil's Bridge.
Ponterwyd is in a formerly prosperous mining district, with ores of silver, lead, and zinc. Hot fluids, perhaps heated in proximity to granites deep in the crust or emanating from them, flowed through fractures in the slate, leaving calcite and quartz veins that contain metallic minerals. The old Llywernog lead and silver mine, three kilometres west of Ponterwyd, was discovered in the late 18th century and worked for a hundred years. It is now a private museum. You can visit the old mine buildings, but not the underground workings. Watch for old mine tips near Goginan, where the road descends into the Rheidol Valley.
The A44 follows the Rheidol Valley for about ten kilometres to Aberystwyth. The few road cuttings reveal slate with some turbidite sandstone. Torrents of glacial meltwater pouring down from glaciers that covered Plynlimon to the east deposited sediment in the Rheidol Valley forming the flat river valley. The main valley reaches to Devil's Bridge, where meltwater cut a gorge into the plateau.
Devil's Bridge (274 277) to Rhayader (297 268). 23 miles (37 km)
The unclassified mountain road between Devil's Bridge and Rhayader, provides a scenic alternative to the A44-A470 between Aberystwyth and Rhayader. The rocks are mostly folded sandstone and mudstone deposited from turbidity currents during Silurian time. The finer mudstone and siltstone show good slaty cleavage.
The B4574 east of Devil's Bridge to Cwm Ystwyth passes the Jubilee Arch, a memento of George III. The Cwm Ystwyth Mine, which dates from the Bronze Age, is one of more than a hundred old mines in the region. Huge piles of waste rock on the hillside below the mine give a sense of the size of this old mine, which produced zinc, lead, and silver. Good specimens of galena (lead sulphide) and sphalerite (zinc sulphide) still exist in the tips by the road. The ore lies in grits of early Silurian age in veins in rock broken along a fault zone. The main fault trends slightly north of east. It is visible on satellite pictures. The mineralization dates from between 390 and 330 million years ago, during the Hercynian collision.
The
unclassified road east of Cwm Ystwyth crosses the moors, with fine
views of the
high Welsh plateau (Fig. 20). This plain is more than 500 
metres
above
sea
level in this area, but varies from about 600 metres near Devil's
Bridge to 450
metres to the south. The peak on Plynlimon (Fig. 21) is the highest
point, 752
metres high. Before the plain rose to the present high plateau,
Plynlimon was a
small hill on the erosional plain that sloped gently down to near sea
level. It
rose to its present height between thirty and forty million years ago.
During
the ice ages, the plateau was the center of one of the main continental
ice
sheets that covered Wales.
Fig. 21.
Plynlimon, the
highest mountain (752 metres) in central Wales, was at the
centre of the central Wales glacier.
Builth Wells (304 251) to Oswestry (329 330). 58 miles (93 km)
Fig. 22. Geology along the A483
Rocks between Builth Wells and Welshpool (323 307) are poorly exposed siltstone and sandstone. The original sediments were deposited during Silurian time as layers of silt and mud on the floor of the Welsh Basin. There are good road cuttings in the sandy units about five kilometres south of Llanbadarn Fynydd (310 278). These are sandstones of the Castle Vale Formation. They are the same age as the thick Silurian Denbigh Moors turbidites.
The finer silt and mud
are now slate with a weak cleavage crossing the bedding. A tendency to
split
along both the cleavage and the sedimentary bedding produces fragments
shaped
like pencils (Fig. 23). Rocks with a stronger cleavage would break into
slabs.
Good outcrops of this pencil slate occur about three kilometres north
of
Llanddewi Ystradenni (311 269),
and at about nine kilometres south of Newtown (312 292). These beds are of
the Llanbadarn Formation, and
lie above the Castle Vale Formation sandstone. These pencil slates have
no
commercial value, except as rough building stone.
Fig. 23.
Pencil slate in
Silurian silt and mudstone south of Newtown on the A483.
The view is about 2 metres across.
The A483 between Newtown and Welshpool (Trallwng) (323 307) follows the Severn River through its broad valley. Silurian slates are in the hills on either side. The road northeast of Welshpool follows the Severn Valley to the English border. Its valley floor is flat. Silurian slate makes Long Mountain to the southeast. About five kilometres northeast of Welshpool, the road passes west of Breidden Hill, which is eroded in black dolerite.
North of Breidden Hill, the road enters a broad plain with the Severn Valley turning to the east and the Dee Valley stretching to the north. Red Triassic mudstones of the New Red Sandstone lie beneath the plain. They were deposited in a valley that dropped between faults as the Atlantic Ocean began to open.
The road between Welshpool and Oswestry skirts the eastern edge of the Berwyn Mountains. The Carboniferous Limestone dips gently down to the east. Its turned up edges make scarps that face west. You see the first of these scarps near the border with Shropshire.
Brecon (305 228) to Dolgellau (273 318). 104 miles (168 km)
Fig. 24. The geology of the A470 between Brecon and Newtown.
The A470 connects Cardiff in the south and Conwy on the north coast. The area between Brecon and Dolgellau was the deepest part of the Welsh Basin. Most of the rocks were deposited as sand and mud during Ordovician and Silurian time.
The A470 between Cardiff and Brecon is described in the Southeast Wales chapter.
Brecon (305 228) lies in the Usk Valley. The river eroded it in the softer red mudstone, deposited in shallow lakes and floodplains, that lies beneath the resistant sandstone of the Brecon Beacons. North of Brecon, red soil developed on the Old Red Sandstone continues to the intersection with the A438 to Hay-on-Wye (322 242). Road cuttings in horizontal beds of red sandstone show layers about ten to fifteen centimetres thick.
The
beds north of the intersection of the A470 and the A438 dip gently down
to the
south and the road passes into much older brownish grey sandstone that
was
deposited during Silurian time. The Welsh Basin had finally filled by that
time, so the
rocks are sandstone and mudstone laid down on floodplains. The Silurian
sandstone continues to Builth Wells
(304 251). Nearly horizontal layers
of
sandstone are exposed at a bridge (309 243) (Fig. 25) across the Wye River, about ten kilometres
south of Builth Wells.
Fig. 25. The River Wye south of Builth flows through a broad valley underlain by horizontal Silurian sandstone that makes low ledges in the river.
Builth
Wells is in the centre of a broad arch, an anticline. The oldest rocks
in the
region come to the surface at its crest. They
are siltstone and sandstone that
were deposited during Ordovician time, but they look almost like the
younger
Silurian rocks. Rocks in the bed of the Wye River at Builth are thin sandstone
of
Ordovician age. Black volcanic and intrusive igneous rocks of
Ordovician age
appear in quarries northeast of town, and in the surrounding hills.
Fig. 26. The bridge over the River Wye at Builth Wells. The quarry in the background is in a dolerite that dates from Ordovician time.
The A470 between Builth (304 251) and Rhayader (297 268) passes through several large folds that have Ordovician slate in their cores. Iron oxide derived from weathering pyrite (iron sulphide) gives many of these slates their rusty colour. The rocks were mainly mudstone laid down in the deep part of the Welsh Basin. A few layers of sandstone make shelves in the riverbed south of Rhayader.
The A470 follows the River Wye to Llangurig (291 280). Layers of resistant sandstone make ledges on the hills. The River Wye crosses them in picturesque little rapids.
The road between Llangurig and Newtown (310 292) cuts across Silurian slate before passing into the Severn drainage system. The side road, B4518, from Llanidloes (296 284) to the Llyn Clywedog dam (291 287) passes a lead mine that was active during the 1700s.
Fig. 27. The geology between Newtown and Dolgellau.
The A470 northwest of Newtown crosses the Cambrian Mountains to Cemmaes (282 304). The rocks are Silurian slate and outcrops are sparse. Barbed wire fences and hedgerows substitute for the dry stone walls of the landscape on volcanic rocks to the north. Silurian rocks in this part of Wales are a series of mildly compressed mudstone. They make very poor slate, and erode rather easily to make a subdued landscape. Road cuttings appear near Cemmaes.
Sedimentary layers in the mudstone dip to the southeast, so the rocks at the surface become older to the northwest. The road passes into slates of Ordovician age at Mallwyd (286 312). To the northwest the road climbs onto rugged moors covered with peat and underlain by volcanic rocks that erupted during Ordovician time. Massive outcrops of these pale rocks armour the mountains. These are the northeastern extension of the volcanic rocks of Cadair Idris (271 313). The parent magma formed in the crust above the sinking floor of the Iapetus Ocean as it descended beneath the Eastern Avalon continent. The road follows a glaciated valley with characteristic steep walls.
Road cuttings east of Dolgellau expose sandstone and slate. The landscape is wooded, and soil covers the rocks.
Ffestiniog (271 342) to Bala (292 336). 26 miles (42 km)
Fig. 28. Ffestiniog to Bala.
The A4212 follows valleys between Ffestiniog and Bala. The west part of the route crosses Cambrian rocks, largely silt and slate with few outcrops. A few small masses of intrusive igneous rocks, black dolerite, make steep bluffs in the hills. About eight kilometres east of the intersection with the A470, the road crosses a hummocky moraine (279 338) dropped from one of the valley glaciers when it melted. Road cuttings in the glacial till reveal a mess of unsorted clay, sand, and boulders, the typical stuff of moraines.
Fig. 29. The Arenig Hills northwest of Llyn Celyn are a range of Ordovician volcanic rocks, some lava but mostly ash from volcanic explosions. Llyn Celyn is an artificial lake.
Almost two kilometres west of Llyn Celyn (286 340), the road passes between the Arenig peaks. They were eroded into volcanic rocks that erupted during Ordovician time. The lava flows are nearly horizontal, and difficult to distinguish. You can see them exposed in a large quarry about half a kilometre down a side road opposite the intersection with the B4391.
At first glance the volcanic rocks look a little like black flint, but a closer look reveals white feldspar crystals and clear quartz grains as much as three millimetres across. The abundance of quartz and feldspar labels this rock as rhyolite. It resembles the rhyolite of the Snowdonia passes. Regular joints in the quarry face break the rock into the vertical columns commonly seen in lava flows. They are a cruder version of the more famous lava columns of the Giant's Causeway in Ireland, or Fingal's Cave on the Scottish Island of Staffa.
White rhyolite is exposed in road cuttings along Llyn Celyn, just west of the car park that overlooks the dam. These erupted during Ordovician time as red hot ash suspended in a cloud of steam. The ash flow poured across the countryside, then settled while the shreds of ash were still at least partly molten. They welded together into the very solid rock you see exposed in the road cuttings. Geologists call it welded tuff. Look for the crude flow banding in the rock, and for the large fragments of other rocks the ash flow picked up as it surged across the ground surface.
The landscape between Llyn Celyn and Bala is gently rolling grazing land, in striking contrast to the more rugged terrain west of Llyn Celyn. The difference reflects the existence of siltstone and mudstone east of Llyn Celyn, and volcanic bedrock farther west.
Bala (292 336) -- Llanfyllin (313 320). 28 miles (45 km)
Fig. 30. The B4391 from Bala to the intersection with the A495.
The B4391 across the Berwyn Mountains crosses sandstone and siltstone deposited during Ordovician and Silurian time. Their layering is typical of that you see in sedimentary rocks deposited from turbidity currents.
The road climbs onto the moors south of Bala. Glacial debris covers most of the bedrock, but a few cuttings reveal siltstone and fine sandstone, with a few thick layers of sandstone. Some beds contain fragments which passing turbidity flows ripped up from the surface beneath, and incorporated into their own deposits.
Where the road descends through a glaciated valley into Llangynog (305 326), you see outcrops of thick layers of sandstone and siltstone. A few Ordovician volcanic rocks make prominent ribs in the slopes. Road cuttings show examples of the sandstone and mudstone layers, and a weak slaty cleavage that dips steeply down to the south.
The glaciated valley broadens south of Llangynog. Watch for a small tributary valley high up the west side of the main valley. The glacier deepened the main valley, and left its tributary hanging at the elevation of the valley floor before glaciation.
Southeast
of Llanfyllin (313 320),
towards Welshpool, the road passes road cuttings in
turbidite
sandstone and siltstone. They make very few outcrops and are so subdued
that it
is easy to miss them.