THE ROADSIDE GEOLOGY OF WALES
Chapter 3 CLWYD--NORTHEAST WALES
Fig. 1. Geologic map of northeastern Wales.
The Denbighshire Moors and the Clwydian Ranges are the backbone of northeast Wales. Three major valleys mark the edges of the ranges: the broad Dee Valley on the east, the Vale of Clwyd between the two ranges, and the Vale of Conwy separates the Denbigh Moors from Snowdonia to the west (Fig. 1).
Fig. 2.
Geologic cross
section of northeast Wales.
The rocks beneath the ranges are mostly mudstone and sandstone collected in the Welsh Basin during Silurian time, and folded during the collision between Eastern Avalon and North America during late Silurian and Devonian time. After the resulting Caledonian hills eroded, a tropical sea laid gravels followed by limestone in Carboniferous time. The Llandudno cliffs expose excellent examples of these battleship-grey limestones.
In early Permian time, about 200 million years ago, the Atlantic Ocean began to open and cracks appeared in the Welsh crust and opened the Vale of Clwyd and the Dee Estuary in Northeast Wales. These valleys filled with Permian and Triassic New Red Sandstone—muds and wind-blown sand dunes. Rivers and ice eroded the New Red Sandstone but left some of the red and yellow sand on the popular beaches of Rhyl and Prestatyn.
Shrewsbury (349 312)-- Betws-y-Coed (279 357). 61 miles (98 km)
Fig. 3. Llangollen to Betws-y-Coed
The A5 north of Shrewsbury crosses the western part of one of the valleys that opened during Permian and Triassic time when the crust stretched, split, and foundered. The rich farmland developed on Permian and Triassic mudstone that lie beneath the soil. About twelve kilometres north of Oswestry, the A5 leaves that valley, and enters the narrow Vale of Llangollen cut into the Berwyn Mountains.
The Vale of Llangollen cuts through sandstone deposited during Carboniferous time as deltas, now hardened into the Millstone Grit. Near Llangollen (322 342), the road drops down into older rocks, the familiar grey Carboniferous Limestone exposed in the scarp on the skyline. A fault that runs north along the scarp face raised Silurian slate to the surface on its west side. The slate was originally mud in the Welsh Basin that was compressed during the Caledonian collision. Castle Dinas Bran above Llangollen stands on this Silurian slate. Large, ugly, quarries exploit them near the Horseshoe Pass, on the A542 north of Llangollen.
Stop at the sharp bend in the A5 about four kilometres west of Llangollen to view the slate quarries at Horseshoe Pass, and the Eglwyseg escarpment of Carboniferous Limestone above Llangollen. Road cuttings near the layby expose slate with thin sandy layers. These are older than the colourfully banded slate at Horseshoe Pass.
The rocks between Llangollen and Betws-y-Coed are Silurian slate and siltstone. Slate from the quarries that dot the hills between Llangollen and Corwen was used to build the local houses and churches. It is from the same layer of rock as that in the quarries at Horseshoe Pass. At Corwen (308 343), the A5 crosses the River Dee, (Afon Dyfrdwy). The road crosses grazing land to Betws-y-Coed, passing a few slate outcrops that poke through the grassy landscape.
A side trip of a kilometre or so south along the B4406, just to the south of Betws-y-Coed, takes you to Conwy Falls (281 352). The water tumbles over the end of a hanging glacial valley, more than a hundred metres above the main valley. The weak flow of ice from the moors did not cut as deeply as the much bigger glacier that came down the Afon Lledr from Snowdonia. So the River Conwy drops over the falls into the deeper Lledr Valley. The rocks in the river bed are all dark grey slate. Watch for the splendid potholes, cylindrical holes as much as a metre wide drilled in the rock as pebbles turn in the grip of persistent eddies.
The A5 passes many road cuttings in slate as it descends into the broader valley below the falls.
At Betws-y-Coed the A5 turns west into the rugged mountains of Snowdonia (see NW Wales itinerary) while the A470 continues north along the Vale of Conwy, a broad, flat, river terrace between steep valley walls. A large glacier flowing down from Snowdonia deepened and straightened the valley, and steepened its walls. Later, as the glacier melted, vast torrents of meltwater filled the valley floor with sand and gravel. Ordovician volcanic rocks hold up the steep west side of the valley. Much less resistant Silurian sandstone and slate make the eastern wall with its gentler slope.
The picturesque houses of Llanrwst (280 361) are built of blocks of volcanic rocks and slate quarried from the nearby hills. North of Llanrwst are many cliffs, quarries, and a few road cuttings in the banded sandstone and slate of Silurian age. Excellent exposures of thinly banded Silurian siltstones appear in the road cuttings about twelve kilometres north of Llanrwst.
Chester (341 366) to Conwy (278 377). 38 miles (61 km)
Fig. 4. Chester to Conwy.
Chester is on the western edge of a valley filled with New Red Sandstone. Blocks quarried from it give many of Chester's buildings and bridges their distinctive red color.
Very few rocks are exposed along the A55 northwest of Chester. The road climbs out of the Dee Valley onto the rolling grasslands of the Clwydian Range. The Dee Valley is one of the valleys in the continent created when the Atlantic Ocean began to open in Triassic time. A block of the crust dropped as the crust stretched (Fig. 2).
A few low road cuttings expose grey Carboniferous Limestone, and piles of grey limestone blocks scatter across the fields near Holywell (318 376). On the western edge of the range, the road cuts through thick layers of this limestone as it drops into the Clwyd Valley, another of those that opened as the crust stretched when Pangaea began to break up during Triassic time. Rich farmland completely covers the soft sandstone and mudstone of Triassic age that lie beneath the valley floor, and contrasts strongly with the grassy grazing land of the surrounding hills.
The
road west of the Clwyd Valley skirts the northern edge of
the
Denbighshire moors. Carboniferous Limestone makes the rounded scarp
that faces
east near Abergele (295 378).
West of Abergele, the road follows a
narrow
wave-cut terrace bounded by ancient limestone sea cliffs south of the
road.
Waves cut the terrace and the cliffs during a time between ice ages
when sea
level was higher than now. Quarries in the limestone supplied the
blocks that
masons used to build Conwy Castle for Edward I.
Fig. 5. The Carboniferous Limestone in the cliffs at Llandudno consists largely of microscopic fossils, though corals and bivalves abound in some layers.
The wave-cut terrace widens behind Colwyn Bay (286 378), and expands into a peninsula that stretches northwest to Llandudno (282 382). Grey Carboniferous Limestone is exposed in the impressive cliffs of Great Ormes Head at Llandudno (Fig. 5). Footpaths and the marine drive that encircle the headland cut through the massive grey limestone. The headland stands above the wave-cut terrace carved during a period between two ice ages when melting ice raised the sea above its present level. Great Ormes Head was then an island, a stack similar to Puffin Island between Llandudno and Anglesey.
A542/A525
Llangollen (322 342)
to Rhyl (301
381) and the Vale of Clwyd. 35 miles
(56 km)
The A542 connects Llangollen on the A5 with the A525 to the north. Between Llangollen and the Horseshoe Pass (319 347), the A542 crosses folded Silurian slate with the gently tilted Carboniferous Limestone lying above them.
Watch for the many road cuttings in the dark-grey Silurian slate near the pass. Near Crucis Abbey (321 344), the slate contains layers of sandstone as thin as a sheet of paper about every five centimetres. Look for the ripples on the upper surfaces of the sandstone layers. Their closely spaced parting, or cleavage, makes these slates excellent roofing material. The Carboniferous Limestone was deposited on an erosion surface planed across the already folded and cleaved slate. The light grey cliffs east of the road are made of this limestone that tilts gently down to the east.
Several large slate quarries near the Horseshoe Pass exploit rocks that are mostly free of sandy beds. The quarry at the Horseshoe Pass (Fig. 7) shows fine examples of slate with folded colour banding, the only trace of the original sedimentary layers. The cleavage surfaces are closely spaced. Slates from these quarries are on the roof of nearly every house in the region.
Fig. 6 Llangollen to Rhyll and the Vale of Clwyd.
Fig. 7. View northeast from a quarry near Horseshoe Pass (319 347). Silurian slate was quarried for roofs and billiard tables.
A detour along an unclassified road from Pentredwr to Minera passes through World's End (323 348) with a Roman lead mine that worked in an east-west fault zone that runs parallel to a deep valley eroded into the Carboniferous Limestone escarpment. Small pits in the limestone exploited similar ores near Minera (327 351) and in the hills southwest of Flint. The lead minerals deposited in veins fill the spaces between blocks of limestone broken during the Hercynian mountain building. The lead mines of the Flint district produced about one eighth of Britain's lead between 1845 and 1913. The Minera Mine was the largest. It produced four fifths of the district's lead and silver, and nearly all its zinc. The A525 just north of Minera passes onto moors underlain by Coal Measures about a half kilometre west of Bwlchgwyn (326 353), then crosses the A5104 that follows the trace of the Bala fault to the southwest. The slate quarries at Horseshoe Pass lie on the ridge to the south.
South of Ruthin (312 358), at the southern end of the Vale of Clwyd, the A494 and A525 follow the foot of the limestone scarp to the west. Several quarries work the limestone for road metal.
The Vale of Clwyd is one of those blocks of the Earth's crust that dropped between faults as the Atlantic Ocean began to open during Permian and Triassic time. Sediments eroded from the flanking hills filled the depression as the block sank. The New Red Sandstone that filled the valley was deposited in a desert environment. It is mostly red mudstone and sandstone. Sand that modern rivers eroded from these sediments is now on the beaches at Rhyl (301 381) and Prestatyn (306 383).
Ice flowing from Scotland filled the Irish Sea during the great ice ages. It reached into the Vale of Clwyd as far south as Denbigh, and deposited its load of sediment across the valley to make a terminal moraine. Though it is not obvious from the road, the A541 crosses this moraine between Bodfari (309 370) and Denbigh. The southern margin of the ice impounded a large lake. You can see horizontal layers of the fine sediments that settled on its floor in some of the riverbanks.
The A525 continues north to St Asaph and Rhyl on the west side of the Vale of Clwyd with the scarp of Carboniferous Limestone to the west. An alternate route is the B5429 running along the eastern edge of the Vale of Clwyd, following the trace of the fault active during Permian and Triassic time. The Clwydian Range to the east contains much older Silurian rocks. Like many large faults, this one broke the rocks into many more or less parallel slivers, one of which brought a narrow band of Carboniferous Limestone to the surface near Bodfari (309 370). Some of the limestone is stained red as a result of leaching from the overlying red mudstone and is a favoured building stone for the houses of Bodfari. Red limestone also appears close to the road about three kilometres north of Bodfari. Follow the road beyond the A55 to Prestatyn (306 383) to the top of Craig Fawr for a good view of the Vale of Clwyd and the Denbigh Moors to the west.
Fig. 8. View from Craig Fawr. The Clwyd Valley filled with desert sand and mud during Permian and Triassic time.
The large hill, Craig Fawr, above Prestatyn has good outcrops of Carboniferous Limestone. It also provides a fine view across the Clwyd Valley to the Cambrian Mountains, Snowdon, and the limestone cliffs of Great Ormes Head. A limestone quarry to the south has prominent red stained streaks in the limestone (Fig. 8.). The red pigment is iron oxide, which leached down from the New Red Sandstone before it was eroded from the land surface above.
The Clwydian Range (Figs. 6 & 9) lies between the Dee and
Clwyd valleys
and rise to 600 metres. The western part of the range south of St.
Asaph
(Llanelwy) (304 374) consists of folded
Silurian siltstone;
Carboniferous
Limestone laps across the folded and eroded Silurian rocks. The lowest
Carboniferous rocks are massive limestones that lie beneath the
prominent
ridges. Edward I used this limestone to build Rhuddlan Castle (303 378), as did the
builders of the
older houses of St. Asaph and the bridges of the A5. Carboniferous
Millstone
Grit and Coal Measures lie above the limestone.
Fig. 9. The central Clwydian Range
Mold (324 364) to Ruthin (313 358). 10 miles (16 km)
The A494 between Ruthin and Mold passes large road cuttings in banded Silurian siltstone and slate as it climbs the steep east scarp of the Clwyd valley. These rocks are the same as those on the Denbigh Moors to the west. Carboniferous Limestone that lies unconformably on the folded Silurian rocks forms a low bluff east of the crest of the main scarp. About two kilometres north of Llanferres (319 361), the A494 cuts through massive cliffs of this east-dipping limestone. No road cuttings expose the Millstone Grit that lies above the limestone, but the light-brown houses of Mold (324 364) are built of its sandstone. The Millstone Grit was deposited in deltas that built into the Carboniferous sea when the first effects of the Hercynian collision raised the land to the north.
Mold (324 364) to Denbigh (305 366). 16 miles (26km)
The A541 between Mold and Denbigh passes good outcrops of Carboniferous Limestone at Hendre (319 367). Watch for the splendid exposures of well-bedded grey limestone and limy mudstone at the entrance to a large quarry.
THE DENBIGH MOORS
The Denbigh Moors (Fig. 10) are part of the Cambrian Mountains, one of the high Welsh plateaux that began as a Permian erosional surface then rose to its present height in the past 50 million years. The surface of the plateau rises gently from about 300 metres in the north to more than 400 metres in the south. The rocks beneath the grazing land of the plateau are Silurian ocean floor sediments -- 3000 metres of gritstone, sandstone, siltstone, and mudstone deposited from periodic submarine slides, or turbidity currents. The rock layers generally dip gently down to the east, but some large folds interrupt this regional slope.
Gently dipping Carboniferous Limestone lies on the folded Silurian sediments. The limestone outlines a broad arc from Llandudno to the west side of the Vale of Clwyd (Fig. 10).
Fig. 10. Folded Silurian sandstone and siltstone underlie the Denbigh Moors. Carboniferous Limestone that lies on the Silurian rocks makes the northern and eastern scarps.
Abergele (295 377) to Llanrwst (280 362). 17 miles (27 km)
The 548 crosses the moors, with only harder sandstones making the rare outcrops in the fields. Softer siltstone and slate appear in a few road cuttings, for example along the A548 as it drops into the Vale of Conwy east of Llanrwst
Several glacial melt-water channels, broad valleys with flat bottoms and steep sides lie near the eastern edge of the Denbigh Moors. They carried torrents of water as the ice melted near the end of the last ice age. Tiny streams now flow through them. The A548 crosses the largest of them, the Elwy Valley, south of Abergele. The hill of Moelfre Isaf (295 373) and the hill fort of Pentre Isaf (397 372) provide good views of its broad trough. Raging torrents of water flowing beneath the ice cut a deep trench, a hundred or so metres wide, at the bottom of the valley. The present small river could not possibly have eroded it.
A543
Denbigh (305 366) to Pentrefoelas (287 351). 17 miles (27 km)
The A453 is the main route across the southern part of the Denbigh Moors. Prominent Carboniferous Limestone exposures occur at Henllan (302 368), west of Denbigh. The road then crosses into folded Silurian sand and silts, with several small roadside quarries providing good outcrops (Fig. 11). The largest are about four kilometres and ten kilometres southwest of the intersection with the A4554. These quarries show regular layers of sandstone a millimeter or so thick alternating with layers of mudstone some three to five millimeters thick. These thin beds settled from turbidity currents that billowed far out across the deep floor of the Welsh Basin after dropping most of their load to the south. The rocks have a weak cleavage of closely spaced fractures.
Fig. 11. Banded Silurian siltstone southwest of Denbigh. Similar rocks underlie much of central Wales.
B5113
Colwyn Bay (284 378) to Llanrwst (280 362). 19 miles (36 km)
Although rivers have deeply dissected the plateau, you can still see its original surface preserved along the B5113 between Colwyn Bay and Llanrwst. The B5113 also provides splendid views of the heavily glaciated peaks and valleys of Snowdonia to the west. These mountains are eroded in hard volcanic rocks that erupted during Ordovician time, and stand above the dissected plateau of the Cambrian Mountains.