Wales (Cymru) - Northwest Wales
The geography of this area is dominated by the mountains of Snowdonia and the Rhinogs, where the landscape has been deeply eroded by glaciers. This area extends from the Menai Strait in the north to the major break in the geology – the Bala Fault which runs in a north-east to south-west line from Llangollen to Aberdovey. The eastern boundary of this area is taken where younger bedrock layers are found at the surface. The area is predominantly rural — the main settlements include Conwy, Llandudno, Betws-y-Coed, Dolgellau, Harlech and Ffestiniog.
Sedimentary bedrock
The youngest rocks in the area comprise a succession of soft silt and clay (Palaeogene), underlain by harder mudstone and siltstone with limestone layers (Lias), which were deposited up to 250 million years ago. These are preserved in a small area on the coastal plain between Barmouth and Harlech, and proven in a very deep borehole (Mochras) near Llanbedr. Elsewhere layers of Carboniferous Limestone form the prominent hills around Llandudno and in a ridge extending from Colwyn Bay to Ruthin; these are host to historic mineral workings. In the east of the area, the Vale of Clwyd contains a block of red sandstones deposited between around 320 to 200 million years ago, which has moved downwards due to geological faults or breaks in the rock. This layer is a major aquifer due to the spaces between the sand grains in these sandstones and provides a significant public water supply.
Basement rocks
Youngest basement rocks (Silurian), exposed in the east of the area, generally comprise thick units of sandstone and mudstone and extend south of the Bala Fault into mid Wales. These rocks are underlain by dark grey slates and sandstones deposited in an ancient sea between 475 to 445 million years ago and are thought to extend to depths of greater than 1 km. They alternate with thick layers of volcanic rocks with small intrusions mainly of granite. Here the volcanic tuffs are generally resistant and form the main mountain peaks of Snowdonia (Plate P006965). Copper mineralisation, such as that at Coed y Brenin, is present within the sediments and has been mined in the past. These are underlain by a sequence of sedimentary rocks that were deposited between around 550 to 480 million years ago and form the central part of the Rhinogs. The lower part of this sequence is dominated by strong thick sandstone layers that are overlain by slatey mudstones with thin sandstones. These rocks are host to the gold and manganese deposits that have been mined in this area in the past. The oldest rocks in the area comprise volcanic tuffs, erupted from ancient volcanoes, proven in a borehole but not exposed at surface.
The basement rocks are tightly folded and are cut by a dense network of faults and fractures. Throughout the area they form local aquifers in which the water flows along fractures and from which abstraction of small private water supplies is widespread.