Down the hill towards Talyllyn lake on A487 - geograph

The Bala Fault is a SW-NE trending

geological fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecto ...

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

that extends offshore into

Cardigan Bay Cardigan Bay ( cy, Bae Ceredigion) is a large inlet of the Irish Sea, indenting the west coast of Wales between Bardsey Island, Gwynedd in the north, and Strumble Head, Pembrokeshire at its southern end. It is the largest bay in Wales. Geo ...

. In the offshore area it is a major normal fault and forms the bounding structure to the Cardigan Bay Basin, with a fill including about of

Lias Group The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, s ...

. Onshore it is responsible for the

lineament ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-aligne ...

which runs through

Bala Bala may refer to: Places India *Bala, India, a village in Allahabad, India * Bala, Ahor, a village in the Jalore district of Rajasthan * Bala, Raebareli, a village in Uttar Pradesh, India Romania * Bala, Mehedinți, a commune in Mehedinţi ...

and south of

Cadair Idris Cadair Idris or Cader Idris is a mountain in the Meirionnydd area of Gwynedd, Wales. It lies at the southern end of the Snowdonia National Park near the town of Dolgellau. The peak, which is one of the most popular in Wales for walkers and hi ...

to the coast at

Tywyn Tywyn (Welsh language, Welsh: ; in English language, English often ), formerly spelled Towyn, is a town, community (Wales), community, and seaside resort on the Cardigan Bay coast of southern Gwynedd, Wales. It was previously in the histo ...

. At its northeastern end it links to the similarly orientated Llanelidan Fault. The fault is believed to have had two separate stages of movement. The horizons between the upper Carboniferous sequence and the underlying

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

sequence are parallel, so little rotational movement occurred. The second stage of movement happened in the middle-to-late Jurassic period, when strong rotation happened, up to 24 degrees.

References

Geology of Wales Cardigan Bay {{Wales-geo-stub