Harlech Dome--NW

The Caledonian collision compressed the rocks of the Harlech region into large folds, and that folding raised them into a huge dome. Cambrian rocks are in the centre within a surround of the Ordovician volcanic rocks of Snowdon and Cadair Idris (Fig. 43).

All the rocks were deposited as muddy sediments in the Welsh Basin, a trough created in Cambrian time when the pull from the sinking Iapetus ocean slab stretched and thinned the Welsh crust. Gravel, sand, and mud poured into the basin, and submarine landslides carried the sediments to the deepest part of the basin. The Harlech region contains the thickest section of Cambrian rocks in Britain, nearly three kilometres. They consist of conglomerate, sandstone, and mudstone, the last now changed to slate.

The Harlech Dome is bleak. The coarse, hard Cambrian sandstone resists weathering, so it stands in bold erosional relief as the Rhinog Mountains. The coast road (A496), and the A470 between Ffestiniog and Dolgellau, circle these rugged mountains, following valleys cut into the softer slate that lies on the Harlech Grits.

All the rocks in the Harlech Dome are Cambrian in age. The A470 between Dolgellau and Ffestiniog crosses large areas of soft slate, though most of the road cuttings are in harder rocks. Forests cover the landscape north of Dolgellau, giving way to moors about half way to Ffestiniog.

Rocks just north of Dolgellau are rusty slate, called the Maentwrog Beds that are well exposed in a layby about 1.5 kilometres north of the intersection with the A496. The fresh surfaces are the normal grey colour of siltstone and slate, and you can see irregular layers of sandstone. The rusty color is from the weathering of an iron mineral--pyrite, iron sulphide-- in the original mudstone. Some rhyolite dykes cut across the slate. North of this layby, for almost two kilometres, the road cuttings expose an intrusion of quartz-diorite, a rock like granite but with more dark minerals. Another body of similar looking igneous rock lies north of Ganllwyd (273 324). These igneous rocks were injected during early Ordovician time somewhat earlier than the volcanoes of the Snowdon region.

Fig. 44. The Rhinog Mountains are of hard Cambrian sandstone and conglomerate. The lower foreground is in slate and softer sandstone. View from the east.

Five kilometres north of Ganllwyd, the road breaks out of the forest and onto the moors, providing fine views of the Rhinog Mountains to the west (Fig. 44), eroded into the Cambrian Rhinog Grits. The grits are well-cemented coarse sandstone and conglomerate, so they are very hard and resist erosion. No outcrops of the Rhinog Grits appear along the A470, but several road cuttings expose the somewhat older Dolwen Grits. These are thick beds of coarse light-grey sandstone that dip gently to the northeast. Geologists believe the sand was carried down the slopes of the basin into deep water as fast moving slurries called turbidity currents. These slurries are like underwater landslides.

The Coast Road. Dolgellau (273 317) -- Ffestiniog (271 342). 31 miles (50 km)

The coast road skirts the rugged Rhinog Mountains and has some of the finest exposures of the oldest sedimentary rocks of the Welsh Basin. Traveling between Dolgellau and Harlech is to take a trip through Cambrian time, 570 to 500 million years ago.

The rocks dip to the east between Dolgellau and Barmouth (261 316), so every step west is a step onto older rocks. The youngest rocks, near Dolgellau, are the rusty slates of the Maentwrog Beds. The road then drops through the Clogau shale to the older Barmouth Grit.

The Clogau Shale contains the mines that traditionally provide gold for royal wedding rings. The dark shale rich in organic matter reacted with hot fluids that rose through cracks in the grits. The gold precipitated in quartz veins that trend northeast.

A road cutting west of Dolgellau, and a little less than a kilometre west of the intersection of the A496 with the A470, is good place to see the Cambrian Maentwrog Formation. The rocks are mostly laminated mudstone with thin layers of siltstone every centimetre or so. A sandy layer about twenty centimetres thick shows convolutions near the top and laminations at the bottom, an arrangement typical of rocks deposited from turbidity currents (Fig. 45).

Fig. 45. The convolutions in the sandstone are typical of a bed laid down from a turbidity current. Cambrian Maentwrog Beds. View is one metre wide

The sandy layer contains cubes of pyrite about a centimetre across. Pyrite is iron sulphide, often called fools' gold. It means the original sediments had abundant decaying organic matter that used all the available oxygen, leaving none to oxidize the iron. Pyrite weathers readily, and spreads a rusty stain of iron oxide across the rocks.

A large layby just east of Bonndu (267 319) offers another good exposure of the Maentwrog Formation. Here the laminated mudstone and siltstone show a good slaty cleavage cutting across the layering. A small igneous intrusion at the east end of the outcrop was emplaced during latest Cambrian time, when the nearby Rhobell volcanic rocks erupted.

Slate appears in many road cuttings near the harbour east of Barmouth. The siltstone and mudstone along the shore below Parkington Terrace (2619 3159, Fig. 46), at the east end of Barmouth, contain thin sandy layers with current ripples on their upper surfaces. These are the Gamlen Beds, which are part of the Barmouth Grits. Some beds contain rounded pebbles as much as four centimetres across. The rocks have a good slaty cleavage.

Road cuttings at the west end of the terraced houses show the contact with the underlying Barmouth Sandstone. Turbidity currents pouring into the Welsh Basin deposited these thick sand beds interlayered with silts. The sandstones are green and coarse; some are gritstones. They dip steeply to the east, and the finer beds have a vertical slaty cleavage.

Fig. 46. Cambrian slate with vertical cleavage. Below Parkington Terrace, Barmouth. View is one metre wide.

At Barmouth, the road crosses into the older Dolgellau Beds, rich in manganese, and then into the Rhinog Grits.

You can see the Dolgellau Beds near St. John's Church, high above Barmouth. The church stands on a terrace carved into green turbidite sandstone. These turbidites are muddy sandstone in layers as much as sixty centimetres thick, coarser at their base and grading up to finer silts. This fining upward is typical of turbidity currents; the fines drop after the main current passes. Small mines formerly worked some of the layers for manganese, deposited on the sea floor from volcanic fluids.

The steep road north of the church passes down into the next oldest beds, the Rhinog Grits (Fig. 47). These grits are pale coloured and rich in quartz with grains up to five millimetres across, and the bedding so thick that it is difficult to see in most outcrops. These hard gritstones form the backbone of the Rhinog Mountains.

Fig. 47. Coarse sands of the Rhinog Grits. On path above St. John's church, Barmouth.

More outcrops of the Rhinog Grits appear in large road cuttings about a kilometre north of Barmouth (Fig. 48). These grits are greenish, and rich in quartz. The layers are hard to see. They are about two metres thick and dip moderately to the east.

Fig. 48. Right: Thick bedded sandstone of the Rhinog Grits. Road cut just north of Barmouth. View is five metres across.

Farther north, the road hugs the edge of the Rhinog Mountains, above the plain of the Mochras flats (Fig. 43). This abrupt escarpment follows a large fault that separates the Cambrian rocks of the hills from two thousand metres of Jurassic to Tertiary sediments lying beneath the Mochras flats, unsuspected until a borehole was sunk into them. This basin of sediments is bounded by one of the cracks in the continent that opened up during the opening of the Atlantic Ocean, beginning in Triassic time. Similar basins developed on the other side of the Atlantic Ocean.

Beneath the Rhinog Grits lie the siltstone and mudstone of the Llanbedr Slate. These softer rocks make a subdued landscape in which outcrops are rare. A good place to see these slates is in an old quarry less than a kilometre east of Llanbedr (259 326). A thin dolerite dyke that crosses the quarry was in place before the cleavage developed in the compressed mud, so must have intruded into wet mud. Those who want to see how slate was mined can visit the Llanfair Slate caverns (258 329) south of Harlech.

The road north of Llanbedr crosses the northwest end of the Harlech Dome and begins to climb the geologic succession. The castle at Harlech stands on the Rhinog Grits, coarse turbidite sandstone with beds between 50 centimetres and one metre thick. The rocks in the car park below the castle are finer grained laminated green slate alternating with thin layers of sandstone every 15 centimetres or so. The sandstone layers grade from coarse at the bottom to fine at the top, typical of the way sediments settle from a slowing turbidity current.

Edward I built Harlech Castle in 1286 on a seacliff that looked directly down on the water. Supply ships docked against the Water Gate. The waves have since washed enough glacial debris north to build a spit two kilometres wide that now separates the castle from the sea.

Fig. 50. A sandy turbidite layer, the lighter layer in the middle, in slate of the Cambrian Rhinog Grits. In the car park below Harlech Castle.

North of Harlech, the road follows the flats of Morfa Harlech across the recent sediments of the estuary of Traeth Bach. The road cuttings along the estuary are in the familiar rusty Maentwrog Beds, which here are sandy flagstone. Near Maentwrog (267 341) the road climbs into the higher rusty slate of this sequence. The road crosses the northern end of the Harlech dome, where the rocks dip to the north.

Fig. 51. A fault separates the Cambrian rocks on the left, from the low plain, Morfa Harlech, on the right. Tertiary gravel underlies the plain. A bore hole penetrated 2000 metres of Tertiary and Jurassic rocks.

THE ROADSIDE GEOLOGY OF WALES

THE HARLECH DOME