« PreviousContinue »
The May Hill Sandstone yields species of corals, Favosites, Heliolites, and Petraia; Annelides, Trilobites, and Mollusca, the Brachiopoda being most abundant. Amongst the species are Pentamerus lens, P. oblongus, Atrypa reticularis, Strophomena compressa, Bellerophon trilobatus, Encrinurus punctatus, and Petraia subduplicata.
In South Wales these beds first appear in Marloes Bay,and, at intervals, range across Pembrokeshire; but further north and east they disappear for a space, being overlapped by the Old Red Sandstone. They re-appear south of Llandeilo, and varying from a few feet to 1,000 feet in thickness, they range north-east in a narrow strip through parts of Caermarthenshire, Breconshire, and Radnorshire, lying indifferently upon Lower Llandovery, Caradoc, or Llandeilo Beds. Near Builth, only a few feet thick, they rest quite unconformably upon the Llandeilo Flags and their associated igneous rocks. They are also found near Presteign, where they are locally called . Corton grit.' They occur at Nash Scar, and in Shropshire they lie very unconformably on the Caradoc Sandstone, between the neighbourhood of Cardington and Coalbrook Dale. In the Longmynd country they also lie quite unconformably in the form of a calcareous conglomerate on Cambrian rocks, and beyond this in Wales they are not known anywhere at the western base of the [Silurian] strata between Radnorshire and the mouth of the Conwy. (Ramsay.)
The Upper Llandovery rocks are developed in the Lower Lickey Hills in Worcestershire, and in Staffordshire; at the Lickey, low heathy hills occur chiefly composed of quartzrocks, lithologically identical with those masses on the flanks of the Caradoc and Wrekin which have been formed by the fusion of sandstone. (Murchison.)
They occur at May Hill, Shelve, and Tortworth. At Malvern the beds consist of grey and purple laminated sandstones and shales having a thickness of 500 feet, resting upon grey and purple sandstones and conglomerates about 600 feet in thickness. (Phillips.)
TARANNON SHALE. (Geol. Survey.)
These beds comprise smooth pale-blue and greenish-grey slates and shales 1,000 to 1,500 feet in thickness.
They were first mapped by Mr. Aveline. These shales form the lowest part of the Silurian rocks of North Wales, and from beneath the Denbighshire grits they are exposed in a narrow and nearly unbroken line from the mouth of the Conwy to near Builth in Radnorshire, where they are strikingly unconformable to the various underlying members of the Cambrian strata.
In South Wales they rest conformably upon the Upper Llandovery rocks. No fossils have been found in the beds.
Graptolitic Mudstones. (Harkness and Nicholson.)
The Stockdale slates (which occur in the Lake District) are of a pale colour, passing downwards into black shales with graptolites, and containing calcareous grit and conglomerate.
They are placed on the horizon of the Tarannon Shale.
Amongst the fossils are Graptolites, Diplograpsus, Rastrites, &c., also Endoceras.
Dr. Nicholson states that the beds rest in the Lake District upon the Coniston Limestone, and in the Sedbergh District upon the Trinucleus and Strophomena Shales.
LOWER WENLOCK BEDS.
DENBIGHSHIRE GRITS. (Bowman, 1841, and Sedgwick.)
This formation consists of a series of shales, flagstones, sandstones, and grits, attaining a maximum thickness of at least 3,000 feet. According to Professor Ramsay, they form but a local variety of the Wenlock formation; and apparently, where the grits thin away and disappear, instead of being overlapped by the shale, they rather pass by lithological gradations into strata of a shaly character.
Professor Ramsay observes, that in some areas fossils are absent or very scarce in the grits, a few fragments of Encrinites or of bivalve mollusca alone showing that they are fossiliferous. In many places, however, fossils are plentiful, as near Conway, at Plas Madoc, near Pentre Voelas, Craig-hir, &c. Amongst the fossils may be mentioned Phacops Downingiæ, P. caudatus, Calymene Blumenbachii, Rhynchonella, Strophomena, Leptæna, Euomphalus, Murchisonia, Bellerophon, Orthoceras, &c.
The Denbighshire grits succeed the Tarannon shale, and, interstratified with slaty shales, form the base of the Wenlock strata. They run from north to south, in a long sinuous and sometimes broad strip, from the mouth of the Conwy to Melenydd. East of Bala Lake they lie in a trough, from two to four miles wide, and the Tarannon shale and older rocks of the Berwyn bills rise from underneath 'their eastern boundary. North of the Berwyn hills, between Llangollen and Corwen, the Denbighshire grits, more shaly in character, overlie the Tarannon shales; and in the valley of the Vyrnwy, and eastward by Welshpool and the Longmountain, and round the older rocks of the Shelve and Corndon country, the sandy character of the base of the Wenlock Shale has entirely disappeared. In Radnorshire, 10 or 12
miles north of Builth, the Denbighshire grits die out, but their equivalents in a more shaly form are believed by Mr. Aveline to strike into South Wales. (Ramsay.)
UPPER CONISTON GROUP.
CONISTON GRITS AND FLAGS. (Sedgwick.) The Coniston grits and flags consist of hard siliceous sandstone or grit, flags and conglomerate, with thin bands of slate, which are placed on the horizon of the Denbighshire Grits and Flags. The upper part contains, especially, beds of grit or tough sandstone; the lower comprises flaggy beds, or sandy mudstones.
The thickness of this group is estimated by Professor Hughes at between 6,000 and 7,000 feet in the Sedbergh and Howgill districts: Dr. Nicholson considers it may be more in the Lake district proper, but there is no continuous section, and the beds are much disturbed and faulted.
This group is stated by Professor Hughes to rest unconformably upon the Coniston limestone, but Professor Harkness and Dr. Nicholson have failed to detect this, and consider the group to have no exact equivalent in the typical Silurian district in Wales, but to be intercalated as a unique deposit between the Bala Limestone and the Lower Llandovery.
The Coniston Flags yield many species of Graptolites, an Orthoceras, worm-tracks, &c.; the Coniston Grits contain fewer fossils, including Graptolites, Phacops Downingiæ, Orthoceras Ludense, Encrinites, and Rhynchonella navicula.
Prof. Harkness mentions that in the vale of Trout beck the Coniston Flags have been very extensively worked ; and at Applethwaite Common they are succeeded by higher strata belonging to the same series, to which Sedgwick gave the name “Sheerbate Flags.'
The term “Woolhope’ is derived from the occurrence of these strata at Woolhope, near Hereford.
The Woolhope Limestone and Shale are regarded as forming a subordinate part of the Wenlock formation. This formation, which rests on the Upper Llandovery rocks, as seen in Shropshire and parts of North Wales, consists of dark grey shale, agreeing, according to Murchison, with the Tarannon shale, and containing subordinate and thin nodular masses of limestone and calcareous sandstone. The limestone is more largely developed in Radnorshire. The formation is rich in Trilobites, Brachiopoda, and also in Cephalopoda: these include Homalonotus delphinocephalus, Illænus Barriensis, Phacops caudatus; Spirifer, Strophomena, Leptana, Atrypa; Orthoceras annulatum, &c.
The Woolhope limestone at Malvern is a rough, impure limestone, with occasional beds of sandstone intercalated, having altogether a thickness of about 150 feet. (Phillips.)
It may be studied north of Crumpend Hill, and near the Wych.
UPPER WENLOCK BEDS.
WENLOCK SHALE. (Murchison.) This formation consists of shales, with flags and sandstones.
It is the largest and most persistent member of the Wenlock formation, and occurs both below and above the Woolhope limestone,—the latter being absent in many tracts, and in others represented solely by a few small rounded concretions of impure earthy limestone. The shale is well exposed near Coalbrook Dale and the Iron Bridge, and may thence be followed all along the escarpment of Wenlock Edge.