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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 Downingia, P. caudatus, Calymene Blumenbachii, Rhynchonella, Strophomena, Leptana, 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 hills 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 Downingia, 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, Illanus 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.
At Malvern, the Wenlock Shale, consisting of dark blue or grey sandy shale with thin bands of limestone-nodules, is estimated by Professor Phillips to have a thickness of 640 feet.
The Wenlock Shale contains several species of Orthis, Leptana, and Rhynchonella, also Euomphalus, Bellerophon, Theca, and Orthoceras. Encrinurus, Calymene, Sphærexochus, and Phacops. Crinoids also are not uncommon.
WENLOCK LIMESTONE. (Murchison.)
HAY HEAD OF BARR LIMESTONE. (Walsall.)
The Wenlock Limestone consists of thick beds of concretionary or nodular limestone of a light grey colour, containing numerous fossils and separated by beds of shale. It rests conformably upon the Wenlock Shale.
In parts of Wenlock Edge the rock is more crystalline; and where varied colours prevail, the matrix being charged with Encrinites and corals, it forms a pretty marble, though the slabs are of no great dimensions. (Murchison.)
Some of the beds consisting of impure earthy limestone and shale, contain large concretionary masses of good limestone, called 'wool-packs' or 'ballstones:' some of them near Wenlock have, according to Murchison, a diameter of 80 feet, and they are quarried out, leaving large cavities. The same geologist observes that, though very thick near Wenlock, the limestone thins out so rapidly in its range to the south-west, that even in the interior of the Ludlow promontory it is represented by thin courses made up of small concretions only, and near Aymestry it is merely represented by a few concretions, varying in size from 2 inches to 2 feet, but still full of beautiful and characteristic corals. It thins out entirely in Radnorshire, and is very feebly represented in Brecon, Caermarthen, and Pembroke; for, according to Murchison,
its place is only marked in the cliffs of Marloes Bay, west of Milford Haven, by some fossils and a small quantity of impure limestone in grey and sandy shale.
The Wenlock Limestone is well-developed at Malvern (280 feet), Woolhope, May Hill, and Usk. Among the most noted localities, however, are the Castle Hill and Wren's Nest at Dudley, where the limestone has been largely quarried; also Hurst Hill near Sedgley.
The northern end of Wenlock Edge, Benthall Edge, and Gliddon Hill are mentioned by Murchison as the best localities for fossils.
The fossils comprise Corals, Encrinites, Trilobites, Mollusca, and Annelides, including species of Halysites, Favosites, Heliolites, Omphyma, Cyathophyllum; Periechocrinus moniliformis; Illanus Barriensis (Barr trilobite), Encrinurus, Phacops Downingiæ, P. caudatus, Calymene Blumenbachii (the Dudley Locust'), Pterygotus; Orthoceras annulatum, Bellerophon, Conularia, Orthonota, Avicula, Strophomena, Pentamerus, Spirifer, Orthis, Atrypa, Rhynchonella borealis; and species of Cornulites and Tentaculites.
The limestone is largely quarried for smelting-purposes and lime-burning at May Hill, Ledbury, Woolhope, and Abberley. The Ledbury Marble is an oolitic limestone.
LOWER LUDLOW BEDS. (Murchison.)
The term is derived from the town of Ludlow in Shropshire, and the formation consists of grey sandy shales, locally called mudstones; some of the upper beds are calcareous.
The Ludlow formation is a natural continuation of the Wenlock beds, and the inferior strata contain calcareous nodules, which differ from those of the Wenlock deposit only in being usually of a blacker colour, and which have often been formed round an Orthoceras, a Trilobite, or other fossil as a