The South Staffordshire Coalfield/Chapter 10
CHAPTER X.
Position[1] and Lie or the Rocks.
General Description.
The South Staffordshire coal-field seems like an island of Palæzoic rocks rising up through the secondary New red sandstone plain of the centre of England. These rocks appear at the surface partly in consequence of their being elevated to a higher level than the Palæzoic rocks immediately adjacent to them, which are still covered by the New red sandstone, and partly in consequence of the New red sandstone, which once covered them also, having been removed by denudation.
The general form of the Palæzoic district is that of a rather rude and irregular spindle-shaped band slightly bent so as to have both its sides convex to the west, and terminating in a point both to the north and to the south.
These points are at Barnt Green, which forms the southern termination of the Lower Lickey range, and at "Brereton Coal Pits," near Rugeley. They are very nearly 26 miles apart, and bear about N. 7° E, and S. 7° W.[2] from each other. Neither this line, however, nor its bearing, is of any importance, or requires further notice in the description of the district.
There are, however, two lines which can be drawn within the Palæzoic area which are worthy of notice in themselves, and indicate bearings that are very important, giving us, indeed, the key to the structure of the district.
The first of these lines runs from Barnt Green to the termination of the Sedgley Silurian ground near Parkfield school, south of Wolverhampton. This line is 14½ miles long, and bears as nearly as possible N. 22° W., and S. 22° E.
The second line starts from the south-west angle of the coal-field near Pedmore, and terminates at the Brereton coal-pits before mentioned. It is 22 miles long, and bears N. 23° or 24° E. and S. 23° or 24° W.
These two lines intersect m the town of Dudley, where they cross each other at angles of about 45° and 135°. A line bisecting the angle of 45° would run as nearly as possible north and south. A line bisecting the angle of 135° would be as nearly as possible east and west.
The first line runs along the ridge of the Lower Lickey, and is parallel to the general direction of the boundaries of the southern part of the coal-field, to the Russell's Hall fault, to the greatest surface extension of the Rowley basalt, and to the general direction of the Dudley and Sedgley anticlinal.
The second line forms the axis of the Netherton anticlinal, runs nearly parallel to the Dudley Port Trough faults, to the outcrop of all the beds (both Coal-measures and Silurian), along the eastern margin of the district from West Bromwich to the Brown Hills, to the outcrop of the beds about Wyrley and Essington, to the western boundary fault north of Wolverhampton, and to the general direction of the northern part of the coal-field from Dudley northwards.
These two lines then point out to us the two directions alo which the disturbing forces acted that have produced the principal inclinations of the rocks, and the principal fractures that traverse them. It may at once be seen by inspection of the maps that all the lines of outcrop and all the faults of any importance either run parallel to one of these lines of bearing or parallel to the lines which are their resultants, or those which lie half-way between them. That is to say, all the features mentioned above strike—
| either | N.N.W. and S.S.E., |
| N.N.E. and S.S.W., | |
| N. and S., | |
| or | E. and W. |
All faults and all strikes that do not coincide very closely with one of those four lines of bearing are either small and unimportant, or occupy spaces that may be described as merely the connecting links where the one line of bearing is passing into the other.
Having thus sketched the outline of the framework of the district, let us commence our description of the lie of the beds at its southern portion.
The Permian rocks stretch in nearly horizontal and continuous sheets through the Clent Hills and the high ground of Hunnington and Romsley, Frankley Hill and Kitwell to Bartley Green. These beds dip with a very gentle inclination to the south. They are cut off both to the west and to the east by faults which afterwards form the boundary faults of the coal-field.
Towards the south these rocks have been largely denuded, especially in the middle portion south of Frankley Hill, where the surface of the ground dips more rapidly than the dip of the beds, and the rocks below the Permian are consequently exposed by the denudation. We find these rocks to consist of altered Llandovery sandstone and lower Wenlock shale and limestone, forming the broken anticlinal of the Lower Lickey, with a little unconformable deposit of Coal-measures on each side of it.
The Permian rocks and the beds below them soon, however, become concealed, both to the south-east, south, and south-west, under the New red sandstone, either in consequence of their continued dip, or from their being suddenly thrown down by downcast faults in those directions.
If now we return to the high ground stretching through the Clent Hills and Frankley Hill, and descend its northern slopes towards Halesowen, we shall see that the beds of Permian rise gently to the north, and that at the foot of the slope the Coal-measures appear from underneath them. These Coal-measures consist, first of thin grey shales and clays, then of the Halesowen sandstones, and then of red clays, each successively rising out to the surface towards the north as we descend into the valley of the Stour.
We are here fairly within the coal-field which thence runs off continuously to the northward, or a little cast of north, between its boundary faults, till we arrive at Hednesford and Brereton. The boundary faults are sometimes single large downthrows bringing the New red sandstone into direct contact with the Coal-measures, and are sometimes more complex, allowing of the appearance of more or less of the Permian rocks between the Coal-measures and New red sandstone.
At Hednesford and Brereton the two boundary faults seemingly run off into the New red sandstone, and over the space between them the Coal-measures are apparently covered by an overlap of the New red sandstone resting unconformably upon them. It is probable, however, that the Coal-measures are here, either let down by faults, or largely eroded under the New red, which formation probably thickens much more rapidly to the north than would be supposed by the mere inclination of the beds.
The coal-field then is a narrow plateau of Coal-measures bounded both on the E, and the W. by downcast faults, covered on the north by unconformable New red sandstone, and dipping on the south under apparently conformable Permian rocks.
The coal-field itself may be divided into two parts, each of which may be again subdivided into two.
The principal line of division in the coal-field is that which runs from Parkfield a little south of Wolverhampton, through Sedgley. Dudley, and Rowley, to the Leasowes. This line forms a broad anticlinal ridge from Parkfield to Dudley, round which ridge the coals all crop out, and on which the Silurian rocks rise to the surface at four places. Three of these, namely. Dudley Castle Hill, the Wren's Nest, and Hurst Hill form long oval elevations of the Dudley limestone, the axes of which run north and south, with the beds curving round at very high angles, but more broken by transverse dislocations than they seem to be at first sight. The fourth, namely, the Sedgley ridge, is a broader and more irregular synclinal flexure, the axis of which is likewise north and south, and which is also broken by faults.
South of Dudley this anticlinal form of the beds ceases, but the line of division is continued partly by the surface feature of the Basaltic hills and a steep western slope that runs from them to the south, but chiefly by a line of fault below the surface, having a great downthrow to the west-south-west, which we may distinguish as the Russell's Hall fault. To the south of the Basaltic range this line of fault appears as if again about to take an anticlinal form, since the Coal-measures rise sharply up to it from the west-south-west, if not from both sides. The line of division is then lost for a space, but re-appears again in almost exactly the same line of bearing in the narrow and broken anticlinal ridge of the Lower Lickey Hills (Rubury Hill and the Bilberry Hills).
We may speak of the part of the coal-field lying west of the line above indicated as the South-Western portion of the coal-field. It is subdivided into two by the Netherton anticlinal, a line 3 miles long running north-north-east and south-south-west, with the Silurian showing itself in its southern portion, and the coals cropping out on both its flanks and round both terminations.
The basin between the Netherton anticlinal and the Russell's Hall fault we may call the Cradley Basin. This basin seems to be comparatively free from faults, and except those that bound it has certainly no dislocations of great magnitude. .
The basin lying between the Netherton anticlinal. Kingswinford, and Gornal we may call the Pensnett basin. This is traversed by a number of large and important faults that will be described more in detail presently.
Where the Netherton anticlinal ceases towards the north, there is a trough or hollow in the beds, nearly a mile wide, between it, and Dudley, forming a channel connecting the Cradley and Pensnett basins. We will speak of this as the Old Buffery Trough, from the name of some ironworks situated in it.
The other principal part of the coal-field lies to the east and north of the main line of division before described. It is subdivided into two by the Great Bentley fault, which a little north of the latitude of Walsall rune across the coal-field from east to west, with a downcast of 360 feet (120 yards) to the north.
South of the Great Bentley fault lies the central and south-eastern part of the coal-field. The central is the part between the Dudley and Sedgley anticlinal and the Walsall Silurian district, both on the east and west side of which the Coal-measures successively crop out, and allow the Silurian rocks to appear from underneath them. They also mse, but much more gently, towards the north, the Thick coal. Heathen coal, and New Mine coal cropping out one after another in that direction, and the beds below them being at a very slight depth when they range up to the Great Bentley fault. This central district is traversed by many east and west faults, all having their greatest throw in the middle of their range, and all being downcasts to the south till we come to the space south of Tipton occupied by the Dudley Port Trough. The pair of faults forming the a Port Trough run parallel to, eat almost in the same line with, the Netherton anticlinal. They tend, however, to produce a synclinal rather than an anticlinal effect, and the faults on each side of them aid that effect, since their downthrow is always towards the Trough. The faults running parallel to the Trough which strike from the northern flank of the Rowley Hills are mostly upthrows to the south-east till we approach the Brades, when they again have a southerly downthrow.
Beyond these the Coal-measures stretch into what we may call the south-eastern end of the central district, in which the beds lie nearly flat, as about Titford and Causeway Green, between the Rowley Hills and the eastern Boundary fault.
North of the Great Bentley fault lies the northern portion of the coal-field, in which the dip of the measures is at first steadily from east to west, at a mean angle of about 3°, gradually curving round till it dips west-north-west, and then apparently north-west at the same low angle, over Cannock Chase up towards Brereton. In the Old Park of Beaudesert, however, they appear to become first quite horizontal, and farther on at Brereton they are said to dip gently to the south-east, and to rise towards the north-west and crop up into the New red sandstone that lies upon them.
Many faults running chiefly east and west traverse the northern art of the coal-field, especially the part of it between the Great Bentley fault and Cannock Chase.
Trough Faults.—Besides the faults which have already been mentioned by name, there exists many others, some of which have been alluded to, in all parts of the coal-field. Their description must be left till we speak of them in detail, but it will be well first of all to call attention to those among them which dominate over the rest, and seem to bear to them a relation analogous to that which the keystone bears to the arch.
These are the Trough faults, of which we may enumerate six.
Two of these are in the Pensnett Basin, namely, the well-known Brierley Hill Trough, which runs nearly due east and west, and the less marked Tansy Green Trough, which, likewise, has a general east and west extension.
The central district has only one, but that is the largest and best marked of all, namely, the Dudley Port Trough, which runs nearly north-north-east for three miles, and then gradually curving round seems to merge into the system of east and west faults that traverse that portion of the coal-field.
The Great Bentley fault is itself the southern side of a trough, though of an unequal and imperfect character, since the amount of the upthrow of the corresponding fault on the northern side is only about one-fifth or one-sixth of the downthrow of the Great Bentley fault itself.
Besides these four, there are two very well marked, although only small troughs, which are known in the northern part of the field. The one is that of the High Bridge in the Pelsall district, and the other that which we may call the Rising Sun Trough, down the centre of which Watling Street runs from the old inn called the Rising Sun to the eastern boundary fault.