Handbook of Birmingham / Prepared for the Members of the British Association, 1886

by British Association for the Advancement of Science

Introduction.

The Birmingham district, or that area of which Birmingham constitutes the natural capital and commercial centre, extends from Stafford on the north to Tewkesbury on the south, and from Wellington on the west to Leicester on the east, forming an area about 60 miles in length by 50 in breadth.

In no single district in Britain is the relation of the physiography of the country seen to be more strikingly dependent upon its geological structure than within the limits of the Birmingham district. Every hill, ridge, plain, and valley of any importance is a mere reflex of the underground geology. The local distribution and physical peculiarities of its rock formations afford a natural and complete explanation of all its special scenic peculiarities.

The dominant geological formation of this Midland area is the great Mesozoic formation of the Triassic or New Red Sandstone, which stretches through Southern Britain in a continuous band from Hartlepool to Exeter, and divides the broken and contorted Paleozoic rocks of the west from the flat-lying Neozoics of the east. This great band of New Red Sandstone attains its widest extension in the north of the Birmingham district, between Eccleshall and Charnwood Forest, where its transverse diameter is about 50 miles. Within the limits of the Birmingham district, however, its diameter rapidly decreases, until in the extreme south, in the neighbourhood of Worcester and the Malverns, its breadth is reduced to about 10 miles.

The red rocks of this Triassic formation not only form by far the most predominant and most conspicuous geological feature of the district at the present time, but not long since, geologically speaking, they must have extended over the entire area in one unbroken expanse. They now constitute a sheet of red sandstones and marls, through which protrude, in numerous bands and patches, the older Paleozoic rocks. Although nowhere very steeply inclined, these red beds of the Triassic have been bent into several long low arches, or broad domes, whose longer axes range approximately north and south. The summits of many of these arches have been denuded, and the underlying older rocks have again been bared to-day. Four of these arches are especially conspicuous, those of the Wrekin, Malvern, Dudley, and Nuneaton. In each of these the underlying Coal Measures are laid bare, forming the four coalfields of Coalbrookdale, Forest of Wyre, South Stafford, and Eastern Warwick, all of which shew round their outer margins a narrow band of the intermediate formation of the Permian. In each of these anticlinals, too, the denudation of the core of the arch has been sufficient to wear away the Carboniferous from its centre, and to expose to view yet older formations—the Old Red Sandstone in the Forest of Wyre, the Silurian in South Staffordshire, the Malverns, and Coalbrookdale, and even the Upper Cambrian and its underlying igneous rocks in the Wrekin, the Lickey, and near Nuneaton. With the exception of the Silurian of Abberley and Dudley, and the recently discovered Cambrian of Nuneaton, however, these pre-Carboniferous rocks are comparatively inconspicuous, rising up merely in narrow bands in the cores of long wedge-shaped hills.

From the economical as well as from the structural point of view, by far the most important of these geological arches is that of South Staffordshire, which is the southward continuation of the Pennine chain, and part of the true backbone of Southern Britain. The central axis of this arch runs, as we have seen, through the Dudley Hills, and dies away in the complex of faults to the south of the Lickeys. On the natural consequences of the rise of this arch, all the physical, scenic, and economic peculiarities of the central parts of the district are essentially dependent. The hills and plains around Birmingham are all more or less related to this grand anticlinal—the hills marking the uplifted edges of the harder rocks, the limestones, sandstones, and pebble beds; and the plains, the position of the gently inclined soft shales and marls. It has brought within workable distance of the surface the coals and ironstones of South Staffordshire, and the valuable limestones of the Dudley Hills, and it has had its final effect in bringing together the overflowing population of the town and district.

The original simplicity of the geological structure of the floor of the Birmingham District is much complicated by fractures, faults, and unconformities. The Wrekin and the Malverns are both affected by profound dislocations. The South Staffordshire Coldfield is bounded both on the eastward and the westward by faults of more than ordinary magnitude, while a long straight fault more than 20 miles in length runs through the south-eastern part of Birmingham itself, and flings down the Keuper marls of the Warwick plain against the older sandstones of the Birmingham plateau.

In the Birmingham District as everywhere in Britain, the Triassic formation rests unconformably upon everything below. At the same time, its members overlap each other more or less irregularly, and shew a rapid diminution in thickness when followed from west to east. Thus it happens that not only do the pre-Triassic rocks make their appearance in their expected positions along the main anticlinal lines, but many of the old ridges of Palæozoic rocks, which rose out of the Triassic waters, have had their enveloping pall of red sandstone removed from their flanks and summits, which have thus been bared once more to the light of day. Such are, in part, the ridges of the Wrekin, the Malverns, the Lickeys, and the Forest of Charnwood.

In the valley of the Severn, to the west of Birmingham, the strata run in narrow bands. The central portion of this valley lying between Coalbrookdale and Bridgnorth, to the west of the main anticlinal of the Birmingham district, owes all its striking beauty and variety to the rapid alternations of hard and soft strata which occur within its limits.

In the great Midland plain to the east of Birmingham, the strata are spread out in broad sheets. The plain is underlain in great part by the comparatively homogeneous flat lying Keuper marls, with their intercalated bands of harder sandstones. Its scenery is consequently less varied than that of the Severn valley, but it is rich in that sweet sylvan beauty which is almost peculiar to the English landscape, and it forms one broad expanse of gently rolling farmland and woodland, whose green crested waves sweep onward to the east and south, mile beyond mile, till they break against the long shore-like scarp of the harder Jurassics.