The Triassic Rocks.
BY W. JEROME HARRISON, F.G.S.
A considerable portion of the Midland Counties of England is composed of red sandstones and marls. The town of Birmingham stands upon, and is surrounded by rocks of this character. They form the Triassic System of geologists, the first of the four grand members of the Mesozoic series.
Strictly speaking, the title Trias is a misnomer as applied to the English development of the rocks of this system. The central member of the typical German succession, the Muschelkalk, is wanting in Britain; and only the upper and lower divisions, the so-called Keuper and Bunter, are represented. The Bunter or lower Trias, consists in the Midland areas of a mass of pebble beds or conglomerate, usually underlain and overlain by variegated sandstones. The Keuper is formed of a great thickness of red marly strata, with a thick sandstone (Waterstones) at the base.
The following table shews the subdivisions of the Trias which have been recognised in England, together with (a) their maximum thickness, and (b) their thickness in the neighbourhood of Birmingham.
Classification of the Triassic Strata—
| Thickness in Cheshire. Feet. | Thickness near Birmingham. Feet. | |
|---|---|---|
| Keuper Red Marls (with the Upper Keuper Sandstone) f. 6. | 3,000 | 700 |
| Lower Keuper Sandstone, f. 5. | 450 | 200 |
| Muschelkalk | (wanting in England). | |
| Upper Mottled Sandstone, f. 3. | 500 | 200 |
| Pebble Beds, or Bunter Conglomerate, f. 2. | 600 | 400 |
| Lower Mottled Sandstone, f. 1. | 400 | (wanting.) |
The Trias enters England on the south coast, between Torbay and Exmouth. At the little watering-place of Budleigh Salterton there is a bed of quartzite pebbles in the Trias 100 feet thick which is worthy of study in connection with the great numbers of similar pebbles that occur in the same strata round Birmingham. In West Somerset and Devon, the Triassic strata are 3,500 feet in thickness. Their subdivisions cannot be correlated with those of the Midlands, for they appear to have been deposited in a separate basin, of which the Mendips, &c., still mark the northern boundary.
Crossing into Gloucestershire, we find the vale of the Severn composed of Triassic marls, and thence northwards the “red rocks” broaden till they form the plains of Cheshire and South Lancashire on the west, and extend eastward to Warwick, Leicester, and Derby. From this great central plain of our island a long strip of Triassic sandstones and marls runs northwards, forming the Vale of Trent and the Vale of York, until finally it reaches the coast between Redcar and Hartlepool. Along the main line of outcrop—from the Malvern Hills to the mouth of the Tees—the Triassic strata incline gently, or dip, to the south-east, at from two to five degrees.
Of the two great divisions of the Trias, the lower (Bunter) is mainly sandy; while the upper (Keuper) is chiefly a stiff marl or clay. One result of this is that while the outcrop of the former is usually barren, forming much heath or waste land, as Sherwood Forest, the Keuper marls produce a rich soil, well fitted for the plough.