•   Rhobell y Big Explosion Breccia

Geography and Geology of the area

An overview of the Geography and Geology of the Bala & Penllyn area, North Wales follows. References to further information are also provided.

The Bala & Penllyn area comprises fast-flowing rivers, lakes in deep valleys and mountains almost touching 3,000 feet. The area lies between several mountain ranges. To the east and north is the Berwyn mountain range and its sub-classification Hirnant mountain range, to the south is the Aran mountain range, while to the west and south-west is the Arenig mountain range. A major valley, running north-east to south-west, is occupied by Bala Lake and the River Dee, which flows both in and out of the lake. A further valley - which initially runs north-west from Bala is occupied by the River Tryweryn and further upstream Llyn Celyn - a reservoir, which was created in the 60s. The main communication routes follow these valleys.


The Bala Fault is a south-west to north-east trending geological fault in Wales that extends offshore into Cardigan Bay. The resulting valley is occupied by the town of Bala, Bala Lake (Llyn Tegid) and further south and east by Tal-y-Llyn and the Mawddach Estuary. The Bala Fault has an extremely long and complex history - it has been intermittently active between about 800 My (million years) and 10 My before the present! It was probably initiated as a tensional fracture during the opening of the Iapetus Ocean in late Precambrian times. The fault is very deep, perhaps extending down to the mantle lithosphere, and could be better termed a 'geofracture'.

The Bala fault has had both vertical and sideways transverse motions at various times under different crustal stress patterns.

During the Cambrian, the fault probably formed a sea bed escarpment, separating shallower water to the south east from deeper water to the north-west. It was over this sea bed scarp that turbidites were discharged - evidence comes from the very coarse proximal turbidite grits, above Barmouth.

In Ordovician times, the Bala fracture zone was a major pathway for the rise of magma, producing a series of volcanic centres from the Arans, through Rhobell Fawr to Cader Idris. The fault then continued across the present Cardigan Bay to reappear on the tip of Pembrokeshire, where it was responsible for the Strumble Head basalt pillow lava eruptions. The diagram shows the positions of the Ordovician volcanic centres in relation to the Bala Fault.

West of Dolgellau, the main fault seems to spilt into a series of parallel fractures, along the Tal y Llyn valley, the valley of the Gwernan Lake, and the Mawddach estuary. This is typical of a major deep fault, which can split upwards into a 'flower structure' of parallel fractures at the surface.

In late Lower Palaeozoic times with closure of the Iapetus Ocean, metamorphism below Wales released hydrothermal fluids which rose along the major fracture zones. This can explain the location of gold deposits along the line of the Bala fault branch through the Mawddach estuary (e.g. Clogau mine), with further deposits following the north-south Rhobell fracture to form the gold deposits in Coed y Brenin (e.g. Gwynfynydd).

The final closure of Iapetus in the Silurian seems to have been oblique, which may have caused sideways motion on the Bala fault to allow sideways adjustment of crustal blocks.

The Bala fault may well have been active again at various times up to the Tertiary, as blocks of crust in the Wales area moved up and down in response to various plate movements. Evidence for these recent movements comes from the huge movement along the Mochras fault which runs north-south along the coast and has down-faulted and preserved Triassic, Jurassic and Tertiary sediments under Cardigan Bay.

Close by, are other areas rich in geological interest, e.g. the Harlech Dome with turbidite sequences and Cader Idris with widely studied examples of volcanic action, including pillow lava, and glaciation.

The Bala Lake/River Dee valley which runs north-east to south-west was originally caused by a fault line (The Bala Fault) which extends south-west to Tal-y-Llyn with a westerly separate fork forming what in now the Mawddach estuary, extending to Cardigan Bay. The Llandderfel syncline is to the east with the Berwyn dome further east. The area is flanked by the Harlech dome, much further to the west.

The predominant lowland rock type is sedimentary principally mudstone, siltstone and grit with some tuff (a rock formed from volcanic ash) and some limestone bands. Many of the upland areas are the result of volcanic action of the Aran Volcanic Group and the Rhobell Volcanic Complex. The Berwyn mountains were also influenced by volcanic action - Pistyll Rhaeadr the highest waterfall in Wales is the result of a the river flowing over a band of harder volcanic rock.

The Aran Volcanic Group comprises several mountains which are the highest, most rugged and impressive of the area, these include the Aran ridge, Arenig Fawr (see picture), and nearby Cader Idris. The Rhobell Volcanic Complex resulted in volcanic action centred on Rhobell Fawr, the first volcanic centre in Snowdonia, where there are outstanding examples of explosion breccia at Rhobell y Big (see picture above and lower-right).

It is though that Bala Lake previously flowed west into Cardigan Bay, whereas, now the River Dee flows north-east eventually into the Irish Sea. The lake was much larger than it is now and was previously probably constrained by glacial moraine until erosion allowed the lake to reduce in water level and size. The northern end of the lake extended significantly further than now - including the area now occupied by Bala town and to Bodweni (north-east of Bala). The south end of the lake also extended much further, probably to Llanuwchllyn.

The ice-age period lasted from around 1.5 million years ago to 12,000 years ago - during this period there were extensive glaciation of Snowdonia. During the last ice age a major ice cap formed over Snowdonia. The centre was located at the Migneint area near the Arenig Mountains, where the depth of ice was 1,400m. Glaciers radiated from this ice cap, deepening valleys forming mountain passes. All the classical glacial erosion features are present in the area, including: cirques, hanging valleys, truncated spurs, U-shaped valleys and melt-water channels. There are excellent examples of glacial cirques on Arenig Fawr (see picture), Arenig Fach, the Aran ridge and Cader Idris. The Bala Lake/River Dee valley and Tryweryn valley show signs of glaciation. It is thought that the Gwyniad, a unique species of fish, was trapped in Bala Lake after the last ice age.

Mines, Quarries and Mineralisation
There were many mines and quarries in the area, including:

Building Stone: Craig-y-Fron, just outside Bala to the north-west, provided stone for many buildings in Bala, including Coleg-y-Bala and Bodiwan. The excavations have left a cavern with the roof supported by regular pillars of rock, known locally as “the caves” (see picture on right). The rock type is tuff (a rock formed from volcanic ash), sandwiched between mudstone (above) and siltstone (below). The internal roof (mudstone) has ripples - indicating sedimentary rock.
Gold at Castell Carn Dochan Mine, near Llanuwchllyn. There are also several gold mines in the Dolgellau area.
Manganese from several mines, including: Fron Feuno, Llanycil, south of Bala, Moel Llyfnant (Arenig mountain range) and Mynydd Nodol overlooking Llyn Celyn.
Stone (Quartz-Latite) for various uses associated with roads at Arenig Quarry: Quartz-Latite is a very hard igneous intrusive rock.
Limestone is found near Llandderfel at the appropriately named “Moel y Calc”.

On the east side of Bala Lake, the area was known as Bryniau Golau (The Lit-up Hills) derived from the fires of lime-kilns lighting up the area at night. The remains of several lime-kilns and a small limestone quarry are still visible over the ridge to the East.


Information compiled by Ray Hind (Mountain Guide)Additional Information provided by Dr Graham Hall 

Further Information: