A broad area, lying between the Hollybush sandstone and the unconformably overlying Silurian rocks of Buildwas, is occupied by a series of Upper Cambrian rocks, denominated by Dr. Callaway the Shineton Shales, and characterised by the forms:—Olenus Salteri, Sphæropthalmus, Asaphellus Homfrayi, &c., Bryograptus Callavei, &c., allying them with the Tremadoc Beds of North Wales.
Silurian Rocks.
The rocks of the Silurian System are fully developed within the limits of the Birmingham District, under their most typical aspect. The well known localities of Dudley and Barr have been famous in the geological world since the publication of Murchison’s great work, the Silurian System; and the abundance and beauty of the fossils of the limestone rocks of the district place it next to the typical area of Central Shropshire as the representative country of the Silurian rocks.
The Silurian strata are all of the well-known Salopian type, shewing several thick-bedded limestones, occurring on distinct horizons in a great thickness of dark blue or grey nodular shales and mudstones. They make their appearance in sharp anticlinal arches in the South Staffordshire coalfield, and along its faulted margins. Four of these exposures occur along the crest of the Lickey-Dudley anticlinal—at Rubery, Dudley Castle Hill, the Wren’s Nest, and Sedgley. The largest continuous exposure is that near the town of Walsall, on the eastern margin of the coalfield.
All the Silurian formations from the Mayhill Sandstone to the Ludlow (Aymestry) Limestone are recognisable, but the terminal Downton sandstone is lost below the unconformably overlying Carboniferous. None of the localities, however, shew the complete consecutive series, which is made out by piecing together the sections occurring in the several areas.
(1.) Lower Lickey Hills.—
(a.) Landovery or Mayhill Rocks.—The usual red and grey Pentamerus sandstone of the Mayhill formation is exposed along the north-west flank of the Lower Lickey Hills. It may be seen resting unconformably upon the Cambrian quartzite in the village of Rubery, and at Leach Heath. Casts of fossils are abundant in some of the sandstones a few feet above the base of the formation, and include the well known forms:—Pentamerus oblongus, Pentamerus lens, Strophomena expansa, Attrypa reticularis, etc.—These may be collected from the rocks at the village of Rubery, and from the fragments of sandstone scattered over the fields between the Asylum and Leach Heath.
(b.) Woolhope or Barr Limestone.—The Llandovery sandstone is followed (in a stream section below the Asylum) by pale blue shales and mudstones containing a bed of hard calcareous rock, affording examples of Illenus Barriensis, Atrypa reticularis, Encrinurus punctatus, Rhynchonella Lewisii; but the section is a poor one, and is covered up almost immediately by the overlying Carboniferous.
(2.) Walsall and Barr.—This is the typical area for the well known Barr Trilobite Illenus Barriensis. The quarries of the Woolhope Limestone which afford it are now disused, but a good section of the fossiliferous shales above is displayed in the railway cutting between Aldridge and Walsall. The overlying Wenlock or Dudley Limestones are mined at the town of Walsall itself, but good fossils are now comparatively rare.
(3.) Dudley and the Wren’s Nest.—By far the most notable and interesting of the Silurian exposures are those of the neighbourhood of Dudley. In the three exposures of Dudley Castle, the Wren’s Nest, and Sedgley Hill, the Silurian limestone rises up in steep dome-like forms. This limestone, which is that of the Wenlock of Siluria, is here composed of two calcareous bands—the higher about 28 feet in thickness, and the lower about 42 feet—separated from each other by an intermediate zone of about 90 feet of gray shales. The limestone has been worked for centuries as a flux for the ironstones of the coalfield. The hills have been mined to a great depth, and all the best limestone rock extracted. The intervening and enveloping shales have been allowed to remain, and the present structure of the hills is that of a central dome surrounded by two enveloping shells, separated from each other by two more or less empty spaces. Where the dip of the rock is high, and these excavated parts are exposed, they form deep moat-like hollows, bounded by walls of shale. Where the dip is low, and the overhanging rocks are supported by the vast pillars left by the workmen, these excavations form magnificent caverns of peculiar weirdness and beauty. In the heart of the hill at greater depths they form damp gloomy chasms of enormous extent, which can only be seen to perfection when lit up by artificial light.