The South Staffordshire Coalfield/Chapter 9

by Joseph Beete Jukes

CHAPTER IX.

Original Relation of the Formations to each other as regards their Conformability or Unconformability.

I shall assume the conformability among themselves of the several members of the Silurian formation in South Staffordshire. This is rendered probable by their conformability in the neighbouring district of Shropshire, and there is no evidence against it in Staffordshire. Starting with that assumption, we must necessarily conclude that before the Coal-measures were deposited the Silurian beds had been slightly tilted at one end, namely, on the eastern side, and made to dip towards the west, and that their surface had suffered considerable denudation. The uppermost member of the Silurian formation, the Ludlow group, is only found on the western side of the main part of the 'coal-field. If we draw a nearly north and south line, starting from Ettingshall Park farm, running between Hurst Hill and Sedgley Beacon, and continue it down through Cradley to the south, we find that to the west of that line, wherever the Silurian reels rise to the surface, namely, at Sedgley, at Turner's Hill, and the Lye Waste, they consist of the Ludlow or upper division; while all to the east of that line, wherever the Silurian rocks rise to the surface, or have been reached by shafts through the Coal-measures, they consist of the Dudley and Wenlock division of the upper Silurian series. The farther we go east, moreover, from this line, the more nearly do the Dudley limestones rise into proximity to the Coal-measures and to the surface, until at Walsall they crop broadly out, and the shales below them still rising gently to the east, there comes out at Hay Head, near Barr, a lower limestone than has been seen in any other portion of the district.

A little to the east of that Barr limestone or of the beds which represent it, the May Hill or Llandovery sandstone, which lies next below it, makes its appearance, and would be much more largely exhibited if it were not immediately cut off by the boundary fault a little to the east of Shustoke Lodge, while near Hay Head it is concealed by a thin skirt of Coal-measures lying unconformably upon it, before both are thrown down by the boundary fault.

South of the true coal-field lies the Silurian district of the Lickey, where these same sandstones rise up from beneath the shales containing the representative of the Barr limestone. These also were elevated and denuded before the Coal-measure period, as we find thin Coal-measures resting directly upon them. It appears then that previous to the deposition of the Coal-measures there was here a broad and generally level plain of Silurian rocks {whether above or under water), the beds of which had a slight dip to the west, and cropped out successively towards the east, at the surface of the plain. The boundary of the Ludlow formation must have run somewhere along the line before mentioned, that of the Wenlock nearly along the line of the eastern boundary of the coal-field, while the Llandovery rocks spread some distance farther to the east. If any of the outcrops formed escarpments, they must have been low and gently sloping.

That this gradual rise to the east was continued yet further in that direction beyond the bounds of our district, is rendered probable by the fact of rocks still older than the Upper Silurian (perhaps older than any Silurian) appearing in the Warwickshire and Leicestershire coal-fields, with the Coal-measures resting directly upon them. It is, indeed, highly probable that all this tract of country, together with much of the adjacent district from Montgomeryshire to Leicestershire, became dry land after the close of the Silurian period, rising, perhaps, very slowly, and undergoing a very gradual and long-continued process of degradation as it passed through the destructive plane of the sea level; and that it remained above the waters during great part of the period marked by the formation of the Old red sandstone and Carboniferous limestone, and that accordingly those two rocks were never deposited upon it.

However that may be, we find that when the Coal-measures of South Staffordshire came to be formed, their earliest beds were deposited on a rather rough and cliffy (see p. 80, and Fig. 11), but generally horizontal surface, and that the dip of the Silurians, though still markedly to the west, was so slight that their beds could not at any one place have been seen sensibly to differ in inclination from the horizontal or pearly horizontal beds of the Coal-measures. We have already, in examining the base of the Coal-measures, seen the way in which at particular spots the lower beds of that formation filled up hollows in the Silurian rocks and obliterated their little pre-existing cliffs, and thus formed a smooth floor for the deposition of the chief mass of the Coal— measure beds. (See p. 80.)

The Coal-measures, then, are distinctly unconformable to the Silurian rocks, but never so much so as that their respective angles of inclination are strikingly different at any one locality.

The practical importance of these apparently quite theoretical discussions will admit of a very apt illustration in this district. If the reader will look in the map of the district, at Walsall and its neighbourhood, he will see the Silurian limestones peeping out from under the Coal-measures here and there, and again becoming concealed by them, the Coal-measure boundary not at all strictly following the line of the limestones. Let us suppose the Coal-measure boundary to represent the outcrop of the Blue flats ironstone, which it really does very nearly, and let a sinking be made, say at the Butts, near Rushall, in which sinking, after passing through the Blue flats, the limestone was found at a certain distance, say 30 yards, below it; a person not understanding the fact of the unconformability of the two formations, might, after getting the Blue flats further east near Caudy-fields, sink down again for the limestone, and feel absolutely certain of reaching it at some depth not varying greatly from 30 yards. Instead of finding it, the pit would be sunk beyond the outcrop of the limestone altogether, as in the following diagram.

Fig. 17.

A. The first shaft.
1. Coal-measures.
2 Blue flats.
3. Lowest Coal-measures.

B. The second ditto.
4. Silurian shale.
5. Silurian limestone.

Now this very case absolutely occurred, although not exactly in the method above stated, and was described to me by Mr. Roberts, the mine agent at that very locality. It had always been a rather puzzling and incomprehensible occurrence to him, till I explained the way in which it had taken place.^[1]

Having established the fact of the general unconformability between the Coal-measures and Silurian rocks, let us now examine the relations between the Coal-measures and the rocks above them, namely, the Permian and the New red sandstone.

1. As to the Permian.—We have already seen that in the southern part of the district we had nearly or quite a thousand feet of Coal-measures above the Thick coal, without including any Permian, or any other rock than the true Coal-measures. Now, in the sinkings at West Bromwich that took place a few years ago, several shafts passed down through a considerable thickness of Permian sandstone, and through a part of the Coal-measures into the Thick coal. So far, however, from there being a thousand feet of Coal-measures between the bottom of the Permian and the Thick coal, there are not more than 520 feet at the Lewisham pit, 350 feet at the Lyng colliery, 330 feet at Messrs. Davis's pits at Spon-lane, and only 30 feet or 40 feet at the Heath pits of Lord Dartmouth. We have in these facts a clear case of decided unconformability between the Permian beds and the Coal-measures. We see that after the Coal-measures had been deposited they had suffered largely and very irregularly from denudation, several hundred feet of strata having been removed at one place which were left untouched at another, before the Permian beds were begun to be deposited on them. The belief of this denudation having taken place is confirmed by the appearance of angular and rounded fragments of Coal-measure rocks and pebbles of coal being found in the bottom beds of the Permian rocks at Quarry Hill near Halesowen, and at the Heath pits, West Bromwich (see Vertical Sections. Sheet 17, No. 17). We can hardly suppose this denudation to have taken place without a previous elevation and disturbance of the beds, although, as in the other case of the Silurian rocks, this elevation may have been so steady and equable that it did not cause the Coal-measures very sensibly to incline from a horizontal position.^[2]

It is perhaps rash to generalise from the very scanty data we possess as to the precise relations between the Permian and Coal measures. On so important a point, however, it is. I believe, a duty to state every opinion that may be fairly arrived at. I will therefore state, as my belief, that not only near West Bromwich, but generally in South Staffordshire and the adjoining counties, the Coal-measures suffered very greatly from denudation before the deposition of the Permian, and that the red sandstones of that formation were largely deposited in hollows and excavations worn in the Coal-measures by this denudation; and, moreover, that this excavation and denudation had in places proceeded to the length of being continued right through the Coal-measures down to the rocks below.

It may be useful, in order to arrive at a right understanding of this subject, if we discuss in detail the operations that were carried on at the celebrated Heath pits of Lord Dartmouth at West Bromwich. For this purpose we have the data given in Sir R. Murchison's Silurian System, and others partly collected by myself, but principally received from Mr. H. Johnson, Mining Surveyor, of Dudley. In the section published as No. 17, Sheet 17, of the Vertical sections, the actual shaft is drawn to scale, with the measures passed through, as given in the Silurian System, p. 476. On the left or west side of the shaft the section is copied from one lent by Mr. Johnson, which he compiled from the accounts of the men engaged in the sinking of the shaft, and corrected from his own measurements; and on the east side of the shaft are delineated to scale the headings or gateways that were driven in that direction, with the jackey pits and borings that were put up and down, together with a small ground plan both of the surface and the underground operations.

The following is a succinct account of these operations. In sinking the shaft they went down for 800 feet, through red standstones and other rocks certainly belonging to the Permian series. Below these, or at 804 feet from the surface of the ground, they entered the Coal-measures, and the following beds were passed through:—

		FT.	IN.
1.	Grey clunch, with streak of coal	2	0
2.	Ditto, with batt at the bottom	4	6
3.	Grey clunch	18	6
4.	Grey fire-clay and ironstone	3	9
5.	Light greenish grey and red rock, containing a thin streak of coal	21	0
6.	Dark grey clunch and batt	4	6
7.	Dark-coloured fire-clay	8	6
8.	Coal	0	6
9.	Grey fire-clay	2	0
10.	Coal	2	6
11.	Grey fire-clay rock	6	0
12.	Coal	9	0
13.	Batt, gubbinstone, and table batt	2	0
14.	Grey clunch and lambstone	11	0
15.	Coal, afterwards traced to Sulphur ? coal	2	0
16.	White ironstone ground	4	0
17.	Grey fire-clay and ironstone	4	0
18.	Dark grey clunch and flattened concretions of ironstone	14	0

No. 12, the nine-foot coal, was recognised ns the bottom part (Slipper, and Sawyer, and Benches) of the Thick coal, with the Gubbinstone and Table batt thinner than usual below it. No. 15 was said to have been afterwards traced into the Sulphur coal; but if No. 16 was rightly identified as the "White ironstone ground," this could not have been the case, as the Sulphur coal lies below the White (or New Mine) ironstone, and therefore No. 15 must have been the Heathen coal.

In examining the engraved Vertical Section. Sheet 17, No. 17, the reader must bear in mind, that while the measurements and position of the beds, as found in the pits, headings, and actual explorations are certainly accurate, the lines connecting these parts may not be exact representations of nature. They are probably only approximations to the truth, especially the shapes and positions of the coals to the west of the shaft, and the line marked "general boundary of the rock fault" on the east.

A "heading" was then driven in No. 12 for about 50 yards to the south-south-east, when the coal was found to thin out by the descent of its roof and come to nothing. A second heading was then driven 140 yards to the east-south-east, through Coal-measures lying in a very irregular condition, and at the end of that heading a bore hole was driven upwards, which at 12 or 15 feet above the heading passed through a three-foot coal, and was continued through dark clunch, for a total height of 100 feet, up into the "red rock." A third heading was then driven nearly parallel to this for about 35 yards, and at the end of it a "jackey pit"^[3] was sunk for 81 feet, through fireclay and sandstone, and from the bottom of that a boring was made for 180 feet lower. This bore hole was said to pass almost entirely through "Bavin measures," (by which Silurian shale is meant,) and these were said to be "very strong towards the bottom, in fact, defied all attempts to bore lower."^[4] A fourth heading was then driven to the east by north for about 170 yards. This, after passing through the nine-foot coal before described, "went level away for 70 or 80 yards through an intermixture of black ground and binds. We then faced the Bavin measures, which seemed to pitch a little for 20 yards, and then ran level away to the back of the head." At the back of the head a two-inch bore hole was put up for 128 feet, which passed through about 17 feet of Bavin, and then dark clod for the remainder of the distance, above which they struck the "red rock," when so much water followed that the hole was obliged to be plugged up immediately. A two-inch bore hole was also put down here for 180 feet, passing through "Bavin" measures the whole way, and finally ended in what we considered to be (as in the case of the hole in the jackey pit) the limestone. Shells were found in great abundance in the Bavin measures of this head."^[5] After making these fruitless explorations in search of the Thick coal to the east of the shaft, they came back to it, and went up to a higher level, and followed a little two-inch coal in No. 5 towards the west, and that shortly led them into the Thick coal.

Examining these facts by the light of the information gained in exploring the Baremoor colliery (see page—) we shall. I think, have little doubt that the Thick coal is cut off at West Bromwich on its eastern side by a large "rock fault;" that beds of sandstone with fireclay and clunch come in, suddenly splitting up the Thick coal, and almost entirely occupying its place, and that the coal in No. 5, together with those numbered 8, 10, and 12, in the section (page __) are all ends or detached parts of some of the beds of the Thick coal.

The general course of the boundary of this rock fault is nearly north and south, as it was found again, with Silurian shale beyond in the same heading, at the Lewisham pits near Virgin's End, one mile north of the Heath pits, likewise cutting out the Thick coal towards the east, and overhanging with a general inclination of about 22°.^[6] Similar facts with regard to the ending of the Thick coal at a depth of 200 yards against some "rock and rig," were observed in some pits east of Lyndon near Hall End colliery, where there are no "red rocks" or Permian beds over the Coal-measures at all.

The most curious fact, however, is, that there is a sudden rise of the Silurian rocks both at the Heath and the Lewisham pits, through the Coal-measures; and this I am inclined to believe is not in consequence of any fault, but is due to their having formed an old Silurian bank of rising ground during the Coal-measure period, and that the Coal-measures were deposited against that bank, its existence being favour able to the formation of sandstone and the accumulation of clay, but unfavourable to the formation of coal. The extension of this "rock fault" and old Silurian bank is at present uncertain; it appears, however, either that they extend continuously for three miles to the south of West Bromwich, or that they recur at that distance near Langley Green, as we shall see hereafter when describing the boundary faults.

We have in these facts, then, an instance of the unconformity of the Coal-measures both to the Silurian below and the Permian above; and it is probable that a little further east of the Heath pits the Coal-measures are entirely wanting, and the "red rocks" of the Permian formation rest directly on the shale or "bavin" of the Silurian formation. This would then be one of those cases where the denudation of the Coal-measures had proceeded the length of totally removing that entire series of rocks previously to the deposition of the Permian beds. The whole history of the Heath pits gives us a good example of the value of geological knowledge to the practical miner; Ist, assuring him of the general fact of the existence of coal beneath the "red rock" of the Permian and New red sandstone formations; 2nd, putting him on his guard as to the possibility of coming down to a spot where the coal had been removed by denudation, showing him what he might expect as possible as well as what was probable, and teaching him what to do in any case.

We have now to examine the original relations between the New red sandstone and the Permian. - These two formations seem likewise to be unconformable to each other, but we cannot yet exactly ascertain either the amount or the precise method of this unconformability. The possibilities of the case are,—

1st. After the partial denudation of the Coal-measures the Permian rocks may have been deposited, not uniformly over the whole district, but in large patches here and there, filling up hollows, but leaving bare the higher spaces of older rock. On this surface the New red sandstone might be deposited, resting sometimes on the Permian, sometimes directly on the Coal-measures, or perhaps on still lower rocks.

2nd. The Permian, after having filled up and levelled the old hollows in the Coal-measures, was continued, as to its upper beds, over the whole district. In that case those upper beds must have been again more or less denuded, as we now find the New red sandstone resting sometimes directly on the Coal-measures without the intervention of any Permian rock whatever.

This is the case at Brereton near Rugeley, where they have sunk in several pits through the quartzose gravel or conglomerates of the New red down into the Coal-measures, the two lying distinctly in an unconformable position, the coals cropping gently up into the base of the New red.

After attaining the New red sandstone there appears no further occurrence of unconformability, the beds appearing to lie with perfect parallelism and regularity through the red sandstones and gypsiferous marls up to the Lias of Needwood Forest.

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1. This is one of the numerous minor difficulties in the way of intelligent practical men that I heard of while surveying this coal-field, that would have been no difficulties, had the knowledge of a little "practical geology" been attainable by them at an early period of life.
2. A dip of even 3° is often hardly perceptible either to the eye or to the clinometer, although it produces very large results if continued over a wide space. A bed dipping at 3° for the space of 3 miles will be 839 feet lower on the "deep" than it is on the "rise" side.
3. By a jackey pit is meant a small occasional shaft sunk in any part of the underground workings for a particular and temporary purpose.
4. I believe this hard rock was the one met with while the "Silurian System" was passing through the press. It was, from the description, supposed by Sir R. I. Murchison to be trap. It probably, however, was the upper part of the Dudley limestone, or possibly the Barr limestone.
5. MS. notes of the evidence given by the men who were engaged in these operations, lent to me by Mr. H. Johnson.
6. The general inclination of the sides of the rock fault at Baremoor colliery was much the same, varying from 18° to 25°.