Bala Series
Encyclopedia
In geology
, the Bala Series is a group of dark slate
s and sandstone
s with beds
of limestone
which occurs in the neighborhood of Bala, Gwynedd
, in north Wales
.
It was first described by Adam Sedgwick
, who considered it to be the upper part of his Cambrian
System. The series is now placed at the top of the Ordovician
System, above the Llandeilo beds. The Bala limestone is from 20 to 40 ft (12.2 m). thick, and is recognizable over most of North Wales; it is regarded as the equivalent of the Coniston Limestone
of the Lake District. The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by 1400 ft (426.7 m). of slates from the BaJa limestone, below this are more slates and volcanic rocks. The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick.
In South Wales the Bala Series contains the following beds in descending order: the Trinucleus seticornis beds (Slade beds, Redhill shales and Sholeshook limestone), the Robeston Wathen beds, and the Dicranograplus shales. The typical graptolites are, in the upper part, Dicellograptus anceps and D. complanatus; in the lower part, Pleurograptus linearis and Dicranograptus Clingani. In Shropshire this series is represented by the Caradoc and Chirbury Series; in southern Scotland by the Hartfell and, Ardmilian Series, and by similar rocks in Ireland.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, the Bala Series is a group of dark slate
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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
s and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s with beds
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
which occurs in the neighborhood of Bala, Gwynedd
Bala, Gwynedd
Bala is a market town and community in Gwynedd, Wales, and formerly an urban district of the historic county of Merionethshire. It lies at the north end of Bala Lake , 17 miles north-east of Dolgellau, with a population of 1,980...
, in north Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
.
It was first described by Adam Sedgwick
Adam Sedgwick
Adam Sedgwick was one of the founders of modern geology. He proposed the Devonian period of the geological timescale...
, who considered it to be the upper part of his Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
System. The series is now placed at the top of the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
System, above the Llandeilo beds. The Bala limestone is from 20 to 40 ft (12.2 m). thick, and is recognizable over most of North Wales; it is regarded as the equivalent of the Coniston Limestone
Coniston Limestone
Coniston Limestone is the sedimentary rock formation around Coniston in the English Lake District, and forms part of the Windermere Supergroup. It is late Ordovician or possibly early Silurian in age and rests unconformably upon the Borrowdale Volcanic Group of rocks, which subsided beneath the...
of the Lake District. The series in the type area consists of the Hirnant limestone, a thin inconstant bed, which is separated by 1400 ft (426.7 m). of slates from the BaJa limestone, below this are more slates and volcanic rocks. The latter are represented by large contemporaneous deposits of tuff and felsitic lava which in the Snowdon District are several thousand feet thick.
In South Wales the Bala Series contains the following beds in descending order: the Trinucleus seticornis beds (Slade beds, Redhill shales and Sholeshook limestone), the Robeston Wathen beds, and the Dicranograplus shales. The typical graptolites are, in the upper part, Dicellograptus anceps and D. complanatus; in the lower part, Pleurograptus linearis and Dicranograptus Clingani. In Shropshire this series is represented by the Caradoc and Chirbury Series; in southern Scotland by the Hartfell and, Ardmilian Series, and by similar rocks in Ireland.