Cadair Idris--NW

The Cambrian rocks of the Harlech Dome are surrounded by the younger rocks of Ordovician age, with volcanic rocks making up the Cadair Idris Range south of the Barmouth estuary (Fig.52). Cadair Idris has one of the most spectacular glacially scoured cirques in Wales; a steep slope that faces north carved into an igneous sill that intruded during Ordovician time. The sinking floor of the Iapetus Ocean produced huge volumes of molten magma that erupted to make the backbone of Lleyn, Snowdonia, and Cadair Idris.

South of Cadair Idris, the rocks continue their southerly dip, and south of the B4405, they pass up into Silurian siltstone and slate. The A487 crosses this sequence between Dolgellau and Machynlleth.

The road north of Machynlleth follows a narrow, forested, gorge through Silurian slate. Road cuttings near the bridge at Corris (275 308, Fig. 52)) show fine exposures of the slate. The road passes into older, Ordovician slate near Corris Uchaf.

Fig. 53. View southwest along the Tal-y-Llyn valley from the A487 just north of the intersection with the B4405. The rocks in the road cuttings, on the left, are Ordovician volcanic rocks. (273 311)

At the intersection with the B4405, the road enters a deep glaciated valley with steep walls (Fig. 53) that opens out to the southwest where the rocks are mudstone. This valley was eroded along the Bala fault, which extends at least to Llangollen in northeast Wales, one of the largest faults in Wales. It moved at various times since Precambrian time.

A landslide southwest of the intersection dumped a pile of debris more than 20 metres high that impounded a small lake, Tal-y-Llyn. The scar from which the landslide came lies high on the south side of the valley.

The road climbs the valley to the divide, passing a series of very large road cuttings in massive Ordovician volcanic rocks, mostly flinty rhyolite, and dark green andesite. The mountain north of the road is Cadair Idris. It is eroded into massive Ordovician volcanic rocks. North of the divide, the road crosses bleak moors eroded into less resistant slate and the terrain becomes much more subdued. Glaciers littered this surface with very large boulders, giving it an untidy look.

North of the intersection with the A470, the road passes again through green Ordovician volcanic rock. The houses in Dolgellau are built from blocks of these rocks.

Fig. 54.Cadair Idris from the Barmouth estuary. The mountains are Ordovician volcanic rocks.

The A493 west of Dolgellau follows the estuary of the Dovey River out to the coast at Fairbourne (262 313, Fig 52). The hills north of the estuary are the southern end of the Rhinog Mountains, composed of the Rhinog Grits, a sandstone laid down in Cambrian time. The hills to the south are the Ordovician volcanic rocks of Cadair Idris. The road cuttings are in slate of Cambrian age, the Ffestiniog flags. They are colour banded and at Fairbourne are uniform enough to be worth mining as roofing slate.

The coast road south of Fairbourne continues in banded slate until it crosses the Afon Dysinni, a valley carved out by glaciers flowing along the Bala Fault. The mouth of the valley is at Tywyn (258 301).

A detour up this valley along the A493/B4405, leads to exposures of flinty rhyolite in high and glacially carved cliffs. 2.3km along the B4405 from the intersection of the A493, a tributary valley hangs at its preglacial level above the glacially deepened main valley. Dolgoth Falls (265 304) drops from this hanging valley in a series of three cascades, cutting into the Ordovician slate to create races filled with potholes. The soft Ordovician slate erodes too easily to sustain the falls for very long, geologically speaking. Farther along the valley, where it becomes narrower, the steep walls of the glaciated valley rise above large scree slopes of blocks fallen from the cliffs. Eventually, those screes will bury the glaciated valley walls, restoring the valley to a V shape typical of river valleys in mountains.

Returning to the A493 at Tywyn and continuing south to Aberdyfi (261 396), the A493 crosses from the volcanic rocks into Ordovician slate. A roofing slate quarry operates about two kilometres north of Aberdyfi. Between Aberdyfi and Machynlleth the A493 follows the estuary of the Afon Dyfi, the River Dovey. Silurian slate, showing very steep cleavage, appears in many road cuttings (Fig. 55). Its uniform colour makes the bedding very hard to see. The slate was originally mud laid down in deep and quiet water undisturbed by turbidity currents. A quarry next to the station yard at Machynlleth has fine exposures of these Silurian slates.

Fig. 55. Silurian slates in road cutting on the A493, a kilometre west of Machynlleth.

THE ROADSIDE GEOLOGY OF WALES

THE CADAIR IDRIS RANGE