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CARADOC, OR BALA BEDS.
UPPER CARADOC. (J. Phillips, 1842.)
The Caradoc Sandstone was first noticed by Murchison in 1833, and in 1834 he described the strata under the name of Horderley and May Hill Sandstone. Subsequently, in the Silurian System (1839), they were called Caradoc Sandstone from the circumstance of their being typically developed in the neighbourhood of Caer Caradoc. (Ramsay.)
The term Bala Beds was given by Sedgwick.
The typical Bala or Caradoc beds lie in the Bala district, between Dinas-Mowddwy, Bettws-gwerful-goch, and Bettwsy-Coed. They consist of black and blue slates, and grey and brown arenaceous beds; the Bala limestone, generally very impure, lying about the middle, and averaging from 20 to 30 feet in thickness. Between the limestone and the lower traps of the Arenigs and Llyn Conwy, two, and sometimes three thin and imperfect beds of volcanic ashes represent the whole of the vast volcanic accumulations of Moel Hebog, Snowdon, and Carnedd Llewelyn. The middle part of the Bala beds, including the limestone, is most fossiliferous, the black slates below, and the slaty and sandy interstratifications above, being comparatively barren. (Ramsay.)
The thickness of the series is estimated at from 10,000 to 12,000 feet. Murchison states that the limestones and sandstones in North Wales do not present a thickness of more than 1,050 feet, while those of Shropshire attain about 4,000 feet.
The beds are found in Anglesea, but the best localities are Caradoc, Horderley (shelly sandstones), Norbury, Bala, and Snowdon. The Meifod beds may also be included. Northwest of Bala, the Rhiwlas limestone-a grey limestone, 30 to 40 feet thick, contains many fossils.
Dolwyddelan, for they contain a much greater number of interbedded felstones and volcanic ashes, which range northward to Conway, and from thence south-west along the higher
e. Columnar Felspathic Rock.
c. Felspathic Porphyry.
North of Moel-Siabod (according to Professor Ramsay) the Bala beds assume a markedly different character from that which they possess between Dinas-Mowddwy and
d. Volcan c Ashes, sometimes calcareous and fossiliferous = Bala Limestone.
b. Greenstone (intrusive).
a Fossiliferous Grits overlying slaty beds.
Caernarvonshire mountains. Carnedd-Llewelyn, CarneddDafydd, Y-Glyder-fawr, Snowdon, and Moel-Hebog are the chief mountains in this, the wildest and grandest part of North Wales. And these, like the ranges of Cader Idris, the Arans, and Moelwyn, consist in a great degree of volcanic products. These volcanic rocks belong to two sections of the [Cambrian] period, for the felstone porphyries and felspathic ashes, and perhaps even the intrusive greenstones of Merionethshire, were formed during the deposition of the Llandeilo strata, while the same species of thickbedded traps and ashes on Snowdon and the surrounding mountains are high in the Bala or Caradoc series. In both cases they form the highest mountain ranges in Wales, not from upheavals caused by the intrusion of igneous masses in special areas, but simply from the circumstance that long after their formation, while lying deep below thousands of feet of newer strata, the whole of the rocks of the area have been disturbed; and the hard igneous masses now rise so high because they have better withstood degradation than the slaty rocks with which they are interbedded. The ranges formed of the lower porphyries, &c., of Cader Idris, Aran Mowddwy, Arenig, and Moelwyn, lie midway up in the strata of the great Merionethshire anticlinal, while the peaks of the still higher range of Moel-Hebog, Snowdon, and Carnedd-Llewelyn actually lie in the middle of a basin. The whole form but minor parts of an old mountain system, of which Wales is only a fragment. (Ramsay.)
In Shropshire the 'Caradoc sandstone admits of the following divisions, according to Messrs. Salter and Aveline:
5. Thin-bedded sandy shales full of Trinucleus concentricus. (Trinucleus shales.)
4. Very fossiliferous brown and yellow sandstones.
3. Thick beds of freestone (Horderley flags).
2. Coarse yellow sandstone, in places a conglomerate, and containing calcareous bands. (Hoar Edge grits.)
1. Sandy and argillaceous shales, sometimes fossiliferous.
The fauna of these beds is particularly rich in Trilobites and Brachiopods, amongst which may be mentioned Trinucleus concentricus, Orthis (Strophomena) grandis, Orthis vespertilio. Two genera of starfish, Protaster and Palaaster, are found at Bala.
In the Caradoc sandstone, remains of fossils are often so abundant as to render some of the beds sufficiently calcareous to be burnt for lime: these beds are known to the workmen as Jacob's Stones.' (Murchison.)
Some valuable deposits of phosphate of lime (phosphorite) have been discovered on the top of the Bala Limestone in North Wales. Beds of jasper occur at DinasMowddwy, and other places.'
Welsh oilstone is obtained from the vicinity of Llyn Idwal, Snowdon, and sometimes called Idwal stone. From Snowdon the Cutler's green stone' is also obtained.
The Gore stone, which occurs near Kington in Radnorshire, is considered to be an altered rock of Caradoc age.
Hirnant Limestone. (Sedgwick.)
This limestone is locally developed in the valley of Hirnant, south-east of Bala, and in a tortuous tract between Bala and Dinas-Mowddwy. It is described as a black pisolitic and fossiliferous limestone, containing species of Orthis (0. Hirnantensis, O. sagittifera, &c.), Arca and Modiolopsis.
It occurs locally near the junction of the Bala Beds and the Upper Llandovery, and was considered of Upper Bala age by Sedgwick.
1 The Dinas sand is used for lining copper-furnaces.
Mevagissey. The grey quartzites of Veryan Bay, first described by Murchison, have been considered by Sedgwick and M'Coy to be of Upper Bala age, the fossils from Carn Goran (Gorran Haven) being Orthis calligramma, O. alternata, Calymene brevicapitata, C. parvifrons, Homalonotus bisulcatus, &c. They were first made known by the collections of Mr. Peach. The rocks at Bolt Head, South Devon, are metamorphosed Devonian slates.
LOWER CONISTON GROUP.
CONISTON LIMESTONE. (Sedgwick.)
This group consists in its lower division of bluish-grey limestone and calcareous slate, and above of flagstone and slate (Ash Gill or Lower Coniston Flags), generally calcareous, having a total average thickness of about 300 feet.
It is placed on the horizon of the Bala beds, but does not contain so great a development of limestone nor so many fossils as do those rocks. The beds rest conformably upon the Green Slates and Porphyries.
The Coniston Limestone stretches from the estuary of the Duddon to near Ambleside and Wastdale Crag. It occurs also in the Furness district, where the formation was called Ireleth limestone by Sedgwick; and it may be studied on the western side of Trout beck.
The Coniston Limestone is rich in fossil Corals, Brachiopods (Leptana, Orthis, Strophomena), species of Orthoceras, and Crustacea (Agnostus, Calymene, Cheirurus, Illanus).
The upper portion of the series, consisting of slaty beds, is termed Trinucleus and Strophomena shales by Professor Hughes, from the occurrence in them of these fossils.