The “basement beds” of the Keuper are certain coarse sandstones and chocolate coloured marls seen in a pit at “California,” near Harborne. Above these come massively-bedded sandstones, of which there is a good exposure in the now disused quarry at Weoley Castle.
Commencing at Edgbaston, we can trace the Lower Keuper Sandstone by the Five Ways, and across the central highest part of Birmingham to Nechells, Gravelly Hill, and Erdington. Its lower boundary line, where it reposes on the Bunter, may be indicated by a line drawn from the junction of Monument Road with Hagley Road to the bottom of Snow Hill, and thence to Aston Station. From this point it extends eastward, for from one mile (Edgbaston) to half-a-mile (Aston). The ridge on which stands the Town Hall, St. Philip’s Church, &c., (475 feet above sea-level) is formed by the Lower Keuper Sandstone, and deep excavations for foundations in the centre of the town, frequently disclose the thin-bedded, dull-coloured, pinkish sandstones (known to workmen as “skerry”) which constitute the upper portion of this rock. Its thickness under Birmingham may be estimated at 200 feet. The average dip is from three to five degrees south-east. Similar beds are exposed in the cutting at Bromsgrove Railway Station, and it was from a quarry here (now closed) that the remarkable fossil fish was obtained, which was described by Sir Philip Egerton as Dipteronotus cyphus. From quarries at Coton End, Guy’s Cliff, Cubbington and Blakedown Hill, near Warwick, bones and teeth of four species of the Labyrinthodon have been obtained, and foot-prints of the same creature have been found in the Lower Keuper Sandstone in many localities.
The Keuper Red Marl is the uppermost member of the Trias. Near Birmingham it is abruptly separated from the Lower Keuper Sandstone by a line of fault, which can be traced from Selly Oak northwards to the junction of the Rea with the Tame. East of this line of fault, the red marls extend for ten or twelve miles forming an undulating fertile plain, on which stand Moseley, Smallheath, and Castle Bromwich, Coleshill and Whitacre. The thickness of the Keuper Marls is considerable. A boring in Smallheath Park was made to a depth of 440 feet entirely in such strata; but quite lately another boring at King’s Heath has been continued to a depth of 700 feet. Gypsum is plentiful in the red marls, occurring in white fibrous layers, but not of sufficient thickness to be of any value in this district. At Droitwich (eighteen miles south-west of Birmingham) the Keuper Marls contain a thick bed of rock salt, which yields an inexhaustible supply of brine.
The Upper Keuper Sandstone is a thin band of sandstone, not exceeding thirty feet in thickness, which occurs irregularly in the upper part of the Keuper Marls. It is well exposed at the entrance to the canal tunnel at Shrewley Common, and in a small quarry at Rowington (thirteen miles south-east of Birmingham), and also crops out on the hill sides at many points in South Warwickshire. From this thin stratum, the Rev. P. B. Brodie, F.G.S., has obtained a fossil fish (Palæoniscus superstes), and the crustacean (Estheria minuta). Specimens of these may be seen in the Warwick Museum, which contains the finest collection of Triassic fossils possessed by any provincial museum.
How the Triassic Rocks were formed.—According to the writer’s views, the area now occupied by central England, alternated in condition during the Carboniferous epoch, between a low plain and a shallow sea. In the Permian period, land conditions prevailed, except in the North and North Midland Counties, where a brackish sea somewhat like the Baltic, it may be—occupied a shallow depression. In Triassic times this central sea appears to have been completely cut off from the open ocean, and to have formed a large inland lake, comparable to the Caspian or the Dead Sea of our own day. The southern boundary of this inland sea was formed by a ridge of old rocks which extended from Charnwood by Hartshill and the Lickey to the Wrekin and Malvern Hills. In the basin north of this axial ridge, all the subdivisions of the Bunter and the Keuper were in turn deposited; and the cliffs and reefs of the Palæozoic rocks of which this coastline was composed, yielded large contributions to the pebble-beds, sands and marls, which constitute the Trias. According to a theory originally advanced by Professor Hull, and ably supported by Professor Bonney, the pebbles of the Bunter were mainly derived from the Paleozoic Rocks of the N.W. and N.E., some being possibly furnished by the ancient strata of N.W. Scotland.
The waters of the Triassic sea were so overcharged with salts of iron, that every grain of sand was encrusted, before its deposition, with a pellicle of peroxide of iron: of chloride of sodium (common salt) and sulphate of sodium (gypsum), there was also an excess, so that much was deposited on the sea-floor, producing beds of rock-salt and of gypsum, of considerable thickness. The presence of these mineral substances in the water was prejudicial to life, so that—as in the Dead Sea, and in Lake Utah to-day—few living creatures could inhabit the Triassic sea, and fossils are consequently of extreme rarity in strata of this age.
The Trias as a source of Water Supply.—The Triassic strata are so porous, that they absorb a large proportion of the rain which falls upon them, and they consequently form an underground reservoir which, when tapped by wells or boreholes, is capable of yielding an almost inexhaustible supply of good, though somewhat hard water. In this way Birmingham receives three-fourths of its water from three deep wells—two on the north-east of the town, at Aston and at Perry respectively, and one on the south-west, near Selly Oak. These wells extend to depths of 400 feet, passing through the Upper Mottled Sandstone, and piercing the pebble beds, and the average supply of water from each is three million gallons per day. The hardness varies from nine to fifteen degrees. There are many other deep wells in and round the West of Birmingham, and at Stourbridge, Wolverhampton, etc., which derive their water from the same source.
Liassic and Rhætic.
BY REV. P. B. BRODIE, M.A., F.G.S.
The Lias occupies a large area in the south and east of the Birmingham District, and consists for the most part of the middle and lower divisions. The highest position of the Lias is seen on the south and south-eastern division of Warwickshire, the middle Lias forms the hills projecting in spurs to the north-west, and the lower division extending in the same direction, at a lower level, up to the southern edge of the Trias.