The Nod Glas Formation (also known as the Nod Glas Black Shale, the Cadnant Shale, the Penarwel Mudstones or just Nod Glas) is an

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geology, geological science associated with the study of stratum, strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are ...

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

(a sequence of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

) in

Mid Wales Mid Wales ( cy, Canolbarth Cymru or simply ''Y Canolbarth'', meaning "the midlands") or Central Wales refers to a region of Wales, encompassing its midlands, in-between North Wales and South Wales. The Mid Wales Regional Committee of the Senedd ...

. The rock of the formation is made up of pyritous, graptolitic mudstone that is generally black in colour. It weathers to a soft, very well cleaved and coal-like material. The formation runs from

Conwy Conwy (, ), previously known in English as Conway, is a walled market town, community and the administrative centre of Conwy County Borough in North Wales. The walled town and castle stand on the west bank of the River Conwy, facing Deganwy on ...

in the north, down to

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. Geograp ...

in the area around

Aberdyfi Aberdyfi (), also known as Aberdovey ( ), is a village and community in Gwynedd, Wales, located on the northern side of the estuary of the River Dyfi. The population of the community was 878 at the 2011 census. The electoral ward had a larger ...

and

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

, though it is not a continuous over this area. The formation is the topmost of the

Caradoc Series In geology, Caradoc Series is the name introduced by Roderick Murchison in 1839 for the sandstone series of Caer Caradoc in Shropshire, England. It is the fifth of the six subdivisions (in ascending order) of the Ordovician System, comprising all t ...

in North Wales, and the name refers to all the black shale beds in the area. In South Wales, the equivalent beds of shale are called the Dicranograptus Shales.

North Wales

Between Tywyn and

Aberllefenni Aberllefenni is a village in the south of Gwynedd, Wales. It lies in the historic county of Merionethshire/ Sir Feirionnydd, in the valley of the Afon Dulas, and in the Community of Corris. Government The village has a community council. The ...

the Nod Glas Formation is about thick. The section between Aberllefenni and

Aberangell Aberangell () is a village in Gwynedd, Wales. Geography Aberangell stands at the confluence of the Afon Angell and the Afon Dyfi, and lies within Dinas Mawddwy community. On the north side of the village, the long ridge of Pen y Clipau runs d ...

is the type locality for the formation. North of Corris, towards

Bwlch y Groes Bwlch y Groes (translates from Welsh as ''pass of the cross'') is the second highest public road mountain pass in Wales, with a summit altitude of . Gospel Pass in south Wales is slightly higher. It lies on minor roads linking Dinas Mawddwy (via ...

, the formation gradually thins and ceases entirely at the pass. Just north of

Dinas Mawddwy Dinas Mawddwy () is a village in the community of Mawddwy in south-east Gwynedd, north Wales. It lies within the Snowdonia National Park, but just to the east of the main A470, and consequently many visitors pass the village by. Its population is ...

, at Aber Cywarch the exposed shale beds contain thin layers of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

. The formation appears again to the east of Bwlch y Groes and can be found in thicknesses up to as far east as

Welshpool Welshpool ( cy, Y Trallwng) is a market town and community in Powys, Wales, historically in the county of Montgomeryshire. The town is from the Wales–England border and low-lying on the River Severn; its Welsh language name ''Y Trallwng'' m ...

. There is a further outcrop approximately long west of

Glyn Ceiriog Glyn Ceiriog is the principal settlement of the Ceiriog Valley and a community in Wrexham County Borough, north-east Wales. Glyn Ceiriog translates simply as Ceiriog Valley, though there are other villages in the valley. The village and communit ...

. There is a further significant layer of the Nod Glas running along the

and Lledr vallies. Within the

Dolwyddelan Dolwyddelan ( ; ; ) – in Victorian times, often spelled Dolyddelen – is a village and community in Conwy county borough, Wales, on the main A470 road between Blaenau Ffestiniog and Betws-y-Coed. As a community, the population of Do ...

syncline the shales have been compressed and are quarried in

Chwarel Ddu Chwarel Ddu quarry was the earliest slate quarry in the Lledr Valley. It was working before 1810, and continued in sporadic operation until the late 1920s. It is about west of Dolwyddelan, just beneath Dolwyddelan Castle. History The quar ...

slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. ...

. There are further outcroppings around

Betws-y-coed Betws-y-coed (; '' en, prayer house in the wood'') is a village and community in the Conwy valley in Conwy County Borough, Wales, located in the historic county of Caernarfonshire, right on the boundary with Denbighshire, in the Gwydir Forest. ...

where again there was some quarrying of the formation as slate. At

Dolgarrog Dolgarrog is a village and community (Wales), community in Conwy County Borough, in Wales, situated between Llanrwst and Conwy, very close to the Conwy River. The village is well known for its industrial history since the 18th century a ...

the Nod Glas is between and thick. There is one further outcropping of the formation, on the

Llŷn Peninsula The Llŷn Peninsula ( cy, Penrhyn Llŷn or , ) extends into the Irish Sea from North West Wales, south west of the Isle of Anglesey. It is part of the historic county of Caernarfonshire, and historic region and local authority area of Gwynedd. Mu ...

which is about thick, and is known locally as the Penarwel Mudstones.

Fossils

Fossils of

graptolites Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian (Miaolingian, Wuliuan) through the L ...

conodonts Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

and

trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

have been found in the Nod Glas Formation near

Other uses

The term ''Nod Glas'' was also used in Wales to denote a blue-black sheep mark.

References

{{Reflist Ordovician System of Europe Upper Ordovician Series Rock formations of Wales