Owing to the soft nature of the strata, valleys are usually hollowed out in the Lower and Upper Mottled Sandstones, while the Keuper marls form an undulating plain. On the other hand the harder nature of the Bunter pebble bed, and the Lower Keuper Sandstone, causes these two rocks to form escarpments or lines of hills, parallel to each other; the abrupt face generally looking west or north-west, while the gentle slope is to the east or south-east, agreeing with the average dip.
The Lower Red and Mottled Sandstone.—Round Bridgenorth this division rests unconformably on Permian strata, and is about 650 feet thick. It is a homogeneous sandstone, of reddish-brown, yellow, and bright red colours, entirely devoid of pebbles. As we follow this stratum to the east it decreases in thickness, being only 200 feet near Stourbridge. East of the South Staffordshire field, the Lower Mottled Sandstone is entirely absent, and the Bunter pebble beds repose directly upon the Permian rocks. Geologists desirous of examining the “Lower Mottled” should visit Kinver Edge, west of Stourbridge, where this rock is well seen underlying the Bunter conglomerate. Its upper portion has here been hardened by calcareous matter, and projects beyond the pebble beds above. Caves, or “rock-houses,” have been excavated in the Edge, and in a detached mass of sandstone called the Holy Austen Rock. The Lower Mottled Sandstone is again visible on Whittington Common, between Kinver and Stourbridge. It is everywhere quite unfossiliferous. “False bedding” is especially characteristic of this division, but it is common in all the Midland Triassic sandstones.
The Bunter Conglomerate or Pebble-bed occupies the surface of the Birmingham area, along a line running from south-west to north-east. It extends from Worcester, by Bridgnorth, Stourbridge, Cannock Chase, and Sutton Park to Lichfield. At all these places it is seen as a remarkable mass of rounded pebbles—mostly yellow, brown, or liver-coloured quartzites—and attains a thickness of 300 feet at Cannock Chase. West of Stourbridge the Conglomerate forms the “Ridge,” and caps Kinver Edge, dipping east or south-east at from five to eight degrees. Thence it is traceable northward by Upper Penn and Bushbury to Cannock Chase, where it forms a wide undulating heathy moorland, six miles in breadth from Bednall on the west to Rugeley on the east, and is exposed in many gravel pits and other excavations. The Staffordshire Coalfield lies like a great wedge between the Trias on its western and eastern sides. Crossing over from Stourbridge, we again find the Bunter Conglomerate or Pebble-beds extending between Harborne and Smethwick, and thence it runs northward in a broad band across the western suburbs of Birmingham, by Winson Green, Handsworth, Perry Barr, and Perry, to Sutton Park (where its outcrop is 3½ miles wide), and on to Aldridge, Wreford, Hopwas Wood, and Lichfield Racecourse. All along this line the Pebble-beds rest on Permian marls, and their thickness at Barr Beacon is about 400 feet. Good sections of the pebble-beds are rare on the south-west or west of Birmingham, but in the north-west they are well seen in a quarry south of Great Barr Station, and in one or two sections near the Beacon. At Sutton, exposures along the new railway lines to Lichfield and to Walsall have been numerous and good, and the Quarry in the Park, close to Blackroot Pool, gives a vertical section of thirty feet. Each section shows a mass of well-rounded hard pebbles, which have been so pressed together during the earth-movements that have taken place since their deposition that many are cracked, while all bear white indentations. The Bunter Conglomerate contains no fossils of contemporaneous age, but many species of shells have been obtained from the hard, rounded lumps of rock of which it is composed. These fossils being of necessity of the age of the rock-fragments in which they are included, they furnish a clue to the sources from which the pebble-beds were derived. The following list of these derivative fossils will give some idea of the results which have already been obtained:—
Ordovician (in quartzite pebbles).—(Arenig Beds.) Lingula Lesueurii. (This interesting brachiopod shell has not yet been found in its parent rock in England, though it is not uncommon in the Gres Armoricain of Brittany, a quartzite on the same horizon as the Stiper stones); various lamellibranchs such as Modiolopsis, Palaearca, and Lyrodesma occur.
(Caradoc and Bala Beds.)—Seven or eight species of brachiopods, of which the commonest is Orthis Budleighensis; a crinoid (Glyptocrinus basalis), &c.
(May Hill Sandstone.)—Lumps of coarse sandstone, identical lithologically with the rock which flanks the Lickey Hills, occur commonly; they contain numerous casts of Stricklandinia lirata, &c.
Devonian.—Nine or ten species of brachiopods (especially Spirifera Verneuilii). Remains of trilobites, such as Phacops and Homalonotus are not unfrequent.
Mountain Limestone.—Mr. Molyneux enumerates twenty-two species of mountain limestone fossils—brachiopods, corals, crinoids, &c.—which he obtained from the Bunter pebble-beds of Trentham. Near Birmingham, fragments of partly decomposed chert, in which the stems of crinoids are beautifully shown, are common in the same strata.
The Upper Red and Mottled Sandstone.—Stourbridge stands on the bright red sands of this division, which extend northward through Kingswinford to Trysall and Tettenhall. South of Birmingham we find the same strata at Harborne Heath and Mill—there is a good exposure underneath the drift in Flavel’s brick pit at “California”—from which point we can trace the “Upper Mottled” across the western part of Birmingham, by Rotton Park Reservoir and the Botanical Gardens; the beautiful soft red sandstone forming a strip about a mile in width between Spring Hill, Hockley Brook, Aston Villa, and Birchfields on the west, to the foot of Snow Hill, and Aston Park on the east. In the cemetery adjoining the Great Western Station at Hockley, there is a grand section, forty feet in height, where the incoherent sand is largely worked for moulding and foundry purposes. It is also exposed in and round Aston Park.
The Lower Keuper Sandstone.—The lower member of the Keuper is the most consolidated part of the Triassic formation, being best known as a tolerably hard sandstone, white or pink in colour, which often yields good building stone.