The South Staffordshire Coalfield/Chapter 13
CHAPTER XIII.
On Faults.
The previous Chapters have been confined almost entirely to the description of facts actually observed. In the present Chapter it is intended to make some remarks on the proper use of the term "fault," and on some facts tending to throw light on the origin of faults and their mode of production.
The word "fault" is one of several that have been selected by geologists from the language of practical miners, and adopted as scientific terms. Various synonyms for it are used in different districts. "Dyke" is the word most commonly used in the north of England, and "trouble," "slip," and other similar terms, often occur. In South Staffordshire the word "thing" is very commonly used instead of "fault." They speak of an "upcast" or "downcast thing."
I am not exactly aware how far "fault" and its synonyms may be used accurately in other mining districts, but in South Staffordshire both "fault" and "thing" are terms which are used by colliers, ground bailiffs, agents, and all concerned with them, in the most vague and puzzling way. These terms are applied to matters of the most incongruous natures, so incongruous that it is impossible to include them under any one general term without introducing great confusion of ideas, and consequently falling into all sorts of blunders in practice.
For instance, we have already seen that a cake of sandstone included in a mass of coal is called a "fault," and though in the preceding pages it has always been spoken of as a "rock-fault," yet that limitation or specification of its nature is not one commonly used in the district. Again, the "rolls" or "swells" in the floor of the coal are called "faults." Mere partial and irregular thickenings or thinnings of a bed are spoken of as "faults." I have even heard the natural outcrop of a bed of coal at the surface, especially if it rose to the crop a little more suddenly than usual, spoken of as "the fault" by many old miners of the district.
One of the most eminent and long experienced coal and iron masters of South Staffordshire gave, as his definition of a fault, on a legal examination, "anything which interrupted and deteriorated the coal." Under this definition dykes and intrusive masses of "green rock" (or trap) would be called faults.
A very intelligent ground bailiff once described to me a "sand fault," which I could not understand till I found it meant the outcropping of the New mine coal into a mass of drift at a depth of 90 feet below the surface.
Mr. Keir, in his account of the district, evidently applies the term "fault" not to the mere fracture and displacement, but to the substance, whatever it be, that comes in between the walls of the fissure, or the substance that is met with in place of the coal. This idea of a substance of a different kind from that expected seems to be the prevailing one attached to the word fault, for not only do they call "faults" things such as those above, but I have on several occasions had true faults, such as the Brockmoor or Corbyn's Hall fault, described to me as "not really a fault, but only a slip."
Examples of the great pecuniary loss and practical blunders likely to arise from this confusion of ideas might be easily accumulated. I will select two. In the case of the gap in the Thick coal at the Gower colliery, described in page 45, it is clear that whatever cause it was which destroyed the coal, that cause had ceased to act before the Cat heath, or the bed immediately above the Thick coal, was formed; yet the ground bailiff who described it to me was surprised that the fault did not affect the Brooch coal 150 feet above the Cat heath. He, knowing that true faults when found in one bed must necessarily affect all the beds above it up to the surface, and looking at this gap in the Thick coal as a fault, expected to meet a similar gap in the Brooch coal above. He of course, would never have recommended, therefore, any operations in search of this Brooch coal; and much of it might have been left behind if it had not been worked in the adjoining ground, and gradually followed by him over this space against his preconceived Opinion as to the possibility of its occurrence.
Another case was a dispute between two gentlemen arising from the uncertainty as to whether "rolls" or "swells" could be considered faults or parts of faults, which involved an expenditure of several hundred pounds, and left the question still undecided.
A. let to B. the Thick coal under a certain tract of ground. This ground was known to be traversed somewhere by a large fault, although its exact place in the ground was not known. The Thick coal was to be got, at a rent of so much per acre, up to the fault wherever it might be, allowance being made for so much of the coal as was injured or diminished by the fault, or by any branches or offsetts of the fault. It was found, that in addition to the fault, the Thick coal was traversed by two "swells," "rolls," or "horse backs," (see ante, p. 52,) which ran side by side across a part of the ground, diminishing the thickness of the coal by cutting out a certain portion of the lower beds of it. These "rolls," or long ridges, were traversed obliquely by the fault, and it was contended by B., that they were branches or offsets of the fault, and he claimed compensation accordingly.
A., however, contended that they had nothing to do with the fault, that they were mere ordinary accidents to be expected now and then in coal mining; and if compensated for at all, were to be so on totally different grounds from those put forward by B.
There can be no doubt that if we construe the agreement technically, according to the only possible accurate definition of the terms. A. was right. It is clear that the "rolls" or "swells" existed before the deposition of the upper beds of the Thick coal, while the fault was produced not only after the deposition of those beds, but subsequently to the formation of the whole of the Coalmeasures above them. The "rolls" were traversed by the fault just in the same way as all the other parts and portions of the whole formation were traversed by it, and as they existed long prior to the fault, clearly could not have been produced by it, or be anything like "branches or offsets of it." Owing, however, to the vague and uncertain use of the term fault, which is the "custom of the country," and the confusion of ideas in men's minds as to the real nature and origin whether of beds or faults, as much evidence, and from as respectable parties, was adduced on B.'s side as on A.'s, and the matter was ultimately compromised.
This matter is mentioned here thus prominently because it is clearly a practical point well worthy the attention of those engaged in all coal mining operations. Unless such common terms as that of "fault" can be assigned a definite technical meaning, it is plain that their insertion into legal documents, agreements, &c., can only be the fruitful cause of error and dispute, leading to much useless expense, and possibly to much necdless ill feeling.
The remedy for the correction of error both in practical operations and in legal agreements, depending on the proper understanding of the nature of faults, is obvious. It is that the ground bailiffs, mine agents, and surveyors, the men on whose word and authority these things mainly depend, should have the opportunity of acquiring larger and more accurate knowledge, and more correct ideas, as to the rea] nature of the matters they are engaged with. A comparatively slight acquaintance with the rudiments of practical and theoretical geology, such as might be acquired by any intelligent person from a few months' instruction from a competent teacher, would be sufficient to produce a perfect uniformity in all such men's Opinions as to what was and what was not "a fault." Such instruction would enable every mine agent in future life to observe accurately, and to arrange and record his observations truly and methodically, so that all his subsequent experience would be applied in the right direction, instead of, as is too often the case, in the wrong one, and tend to increase his real knowledge, not to add to his misconceptions.
The South Staffordshire coal-field has been so thoroughly worked and explored, that a study of its faults and dislocations will add something of precision and completeness even to the knowledge of a professed geologist.
It is clear that a single fault, that is a fissure running along one straight or slightly curved line, can only produce a "throw" or dislocation of the beds along a part of its course. There must be a point in the fissure where the dislocation begins, from which it increases to a maximum, and another point where it gradually ends, and it is scarcely possible that the mere fissure will terminate at either of those points. Such a fault may be likened to a crack in a deal board which ends in the board each way before it reaches its extremity. It is evident that we can only elevate or depress a portion of the board on one side of the crack above or below the corresponding portion on the other side by pushing it in or near the centre of the crack, and causing that portion to bulge. The protuberance of the bulge will be greatest at this part, and gradually diminish each way to the points, where, though the crack may continue, the parts on opposite sides of it retain their relative position. Such "single-lined faults" are those that extend east and west across the part of the coal-field between Dudley and Bilston. Although single in the centre of their course, they often split into one or two small branches near their extremities. The dislocation may be increased by the measures on one side of the fault being bent up into a bulge along its course, while those on the other are bent down. We may, moreover, conceive it possible for the beds ranging along the fault to have more than one "bend" in them, and thus the amount of the throw to diminish and again increase; and if the beds on opposite sides of the fault bend in opposite directions, the flexures may cross each other, and thus the fault apparently disappear, and again set on again along the same line.
If now, recurring to the deal plank, we were to make a cross cut to the crack before mentioned, it is obvious that by applying the requisite force near the junction of the two, we might permanently elevate or depress the included corner of wood. We should then have a representation of two faults, each increasing towards their point of intersection. Or if we continue the crack to the edge of the plank, which we may look on as a great boundary fault, we may bend one side so as to make the dislocation gradually increase from its commencement up to its termination. Of the latter case we have examples in the Corbyn's Hall and Brockmoor faults, that gradually increase their "throw" as they approach the western boundary fault.
It follows from the above considerations, that if there be only one or only two lines of fracture they must gradually diminish and end somewhere, and that the dislocated ground must have at least one place of maximum disturbance towards which it bends down either in one or several directions.
To have any piece of ground altogether elevated or depressed with regard to the whole of that surrounding it, it is necessary that it be bounded by at least three rectilinear faults, or by such an arrangement of curved fractures as amount in effect to three or more rectilinear ones.
These statements will, of course, be to the geometrician exceedingly simple, and scarcely worth making. The considerations involved in them, however, are sometimes hardly sufficiently attended to in geological surveying. One often sees a mere straight line representing a fault on a geological map, and sometimes is tempted to wonder what becomes of it, or how any mass of beds can be elevated or depressed in consequence of a single crack running through them. That such single-lined fractures do occur, however, the beds on one side bending down, while those on the other have remained stationary, or are perhaps bulged upwards, is proved by the occurrence of the east and west faults, before described, traversing the South Staffordshire coal-field.
The above considerations have to do with the longitudinal extension only of faults, but their vertical extent is equally worthy of study. This, however, is so wide a field that we shall only venture on a small portion of it, the subject of "Trough faults," and refer the reader for other parts of it to the papers of Mr. Hopkins in the "Cambridge Philosophical Transactions," vol. 6[1], and in the "Philosophical Transactions of the Royal Society of London," vols. 133 and 134, 1842-3, as also to a paper by Mr. Darwin, in the "Transactions of the Geological Society," vol. 5, p. 601.[2]
Of "Trough faults" we have several excellent and well-explored examples in the South Staffordshire coal-field, both on a large and small scale. We will take for examination one example of each, namely, the Dudley Port Trough faults for the large scale, and a small pair of faults in the Victoria colliery. West Bromwich, for the other.
The Dudley Port Trough faults are shown in the section given in this volume (a reduction from No. 7, sheet 25, of the Horizontal sections), and their general form is reproduced in Fig. 28. They have previously been described (p. 164), to which the reader is requested to refer.
Mr. H. Johnson of Dudley, in the year 1849, showed me in his office a carefully-executed drawing of a singular appearance he had observed in the Thick coal in one of the gate-roads of the Victoria colliery at West Bromwich. At this spot the lower beds of the Thick coal were apparently unbroken and regular, but in the three upper beds there was a trough-shaped gap, eight feet wide at bottom and fifteen feet wide at top, in which reposed a corresponding mass of the beds that on either side lay on the topmost bed of the Thick coal. This gap in the coal gradually, descended in one direction till it reached the bottom of the Thick coal, and assumed the form represented in the following figure:—
Fig. 26.
| A. Trough-shaped gap in the Thick coal. | |||
| B. and C. The Thick coal. | |||
| 1. | Shale and clunch above the Thick coal, containing some bands of ironstone nodules. | ||
| 2. | A black batt (bituminous shale). | ||
| 3. | Roof's. | Thick coal. | |
| 4. | Top slipper. | ||
| 5. | White coal. | ||
| 6. | Lambs. | ||
| 7. | Tow coal. | ||
| 8. | Brazil. | ||
| 9. | Foot coal. | ||
| 10. | Hard stone parting. | ||
| 11. | Stone coal. | ||
| 12. | Sawyer and slipper. | ||
| 13. | Bench batt. | ||
| 14. | Benches coal. | ||
In the portion of the Thick coal marked C, the lamination and stratification of the coal is uninjured, but in the part marked B, while the stratification is for the most part probably unaltered, as is shown by the "partings 10 and 13 continuing through it almost uninterruptedly, the "lamination of the coal is totally destroyed, and the coal has the appearance of a paste made up of coal dust and very small coal. It appears to have attained its present consistency from compression and not from heat."[3]
Fig. 27
A. The Trough.B. The compressed Thick coal.C. The unaltered Thick coal.
Fig. 27 is a more extended representation of a side of the same gate-road on a smaller scale, that of 60 feet to the inch, its total length being about 150 yards. It is reduced from a much larger drawing of Mr. Johnson's, which was drawn to scale from very careful measurement, every fissure and flexure being as nearly as possible exactly represented.
The two drawings given in Figs. 26 and 27 were taken, I believe, in consequence of the explanation I proposed to Mr. Johnson, to account for the appearance mentioned first (p. 194), that in which the trough did not end in a point downwards, but was still eight feet wide at its base. The explanation then given was fully confirmed by these facts since observed and recorded by Mr. Johnson; and as it is applicable to all "Trough faults," and bears on the question of faults in general, it is now given here.
The simplest form of a Trough fault is that represented in Fig. 28, in which a portion of the bed A is dropped down between two
Fig. 28.
equal faults, C and D, which incline in such a manner that they must meet before reaching the lower bed B. It is clear that these faults, having an equal "throw" in opposite directions, must, when they meet, neutralize each other, and that no portion of the bed B can be "thrown" by them either upwards or downwards, or in any direction whatever. We may conceive it possible, however, that C may be continued as a fissure towards c or D similarly continued to d, though it seems much more likely that they should coalesce and continue in one intermediate fissure E.
The most obvious explanation of the cause of such a condition of things seems to be the following. Let a set of beds of rock of indefinite extent, among which are two marked ones. A and B, be acted on by an elevating force from below, causing them to bulge upwards as in Fig. 29. Mr. Hopkins has shown us that if this expansion be continued far enough, the result will be the production of one or more longitudinal fissures, commencing at some point below the surface. Let such a fissure be produced running from F to E, and at E let it split into two, E D and E C.
Fig. 29.
Fig. 30.
that the lower single fissure F E widens into a triangular gap, and consequently E C spreads to E C', and E D to E D'. In this case the included triangular mass of beds between D and C must fall down into the open fissure F E. Moreover, as for every inch that the parts A B rise and that E F widens, the included mass D C will settle down into it, the corners at E will be worn and ground down by the friction, so that they will very shortly be ground off to the lines h g, which will have the effect of still further widening the fissure, and consequently of admitting the wedge-shaped piece D C to sink further down into it.
Suppose now that the expansive and elevatory force ceases to act, and the upraised beds begin to settle down again towards a horizontal or nearly horizontal position, it is quite possible for the lower part of the fissure E F to close again, and to close in some cases so accurately and perfectly, that if the beds are at all soft and cohesive, scarcely a trace of it may be left after some period of compression. That part of the fissure, however, into which the wedge has sunk will, of course, not be able to close again, but it will strive to do so, and great force of compression will consequently be exerted on the sides of the wedge, and on the beds which come against them. The effect of this compression is shown in Fig. 26 in the destruction of the lamination in the coal marked B B. As, however, the beds sink into a very low arch, the downward pressure on the summit of the arch is made more and more of a horizontal thrust on its sides or abutments, the parts A and B on the sides of Figs. 29 and 30. When the arch becomes very low, the lateral pressure hereabouts must be enormous; and it is important to remark,[4] that we have here a real cause for bonâ fide lateral pressure, and consequent lateral shifting, and even of the sliding of one bed over the other, so that during the subsidence greater dislocations and more crumplings and contortions may take place at the outer ends of the arched beds, or at some distance outside of them, than took place in any part of the beds during the action of the elevating force.
Fig. 27 is an admirable instance of the arched condition of the beds, of the compressed central parts B B, and of the cracks and fissures that are likely to be caused on the sides of the arch as the beds sink down again and are endeavouring to adjust themselves to their altered condition, consequent on the intrusion of the wedge A.
As all these actions may take place in nature simultaneously and very irregularly, according to a multitude of varying conditions, we may have a great number of modifications of the simple action here described. Among other things we may easily imagine the apex of the wedge to suffer considerably in some places, and to be ground off till it may have a blunt base, not only of 8 feet, but of many yards in width.
I have spoken before of the benefit Practical Mining might receive from the study of the science of Geology. This case is an instance of the great reciprocal advantage the science of Geology would receive from the observations of practical miners, if they knew what to observe, and how and where to record their observations. In the matter of faults and dislocations in the Coal-measures especially, there are many curious observations yet to be made, and many cases occur that would at first puzzle a geologist, and might be deemed by him impossibilities till he saw them before bis eyes. Some of these came partially and imperfectly under my own observation when surveying this coal-field, but they could only be properly described and recorded by a person who was-frequently engaged in measuring and drawing them during the progress of works extending, perhaps, over weeks or months.
We may, perhaps, here advantageously say a word or two as to the age of the principal faults and dislocations of the South Staffordshire district.
It is clear that, however the Silurian rocks may have been partially disturbed and denuded before the deposition of the Coal-measures, none of the prominent dislocations and disturbances which give the marked features to the structure of the district, took place till after the formation of the whole of the Coal-measure series. All the great faults, then, are of a later date than the deposition of the newest Coal-measures of the district.
How far the pre-existing rocks were fractured and disturbed at the commencement of the Permian period, or during the interval between the deposition of any Coal-measures and that of any Permians, is very difficult to be determined.
From considerations partly derived from this district, partly from other portions of the Midland Counties, it would appear probable that great disturbances, producing both large dislocations and an immense amount of denudation, took place some time during the Permian period, or some time between the period of the Coal-measures and that of the true New red sandstone. I should incline to look on it as possible that the disturbances along the line of the Lickey, and the Dudley and Sedgley anticlinals are of this date, and possibly also some of the principal faults of the coal-field. It is, however, plain, from the description given in the last Chapter, that the great boundary faults of the coal-field were, if not produced, at least largely acted on, subsequently to the deposition of the whole of the New red sandstone, and even to that of the Lias. It is, of course, quite possible that the first impulse towards these faults, the strain that produced the cracks, was communicated to them at the period when the great disturbances in the Carboniferous rocks took place generally through Britain; and that period seems certainly to have been previously to the existence of the New red sandstone. A further subsidence or elevation, or in other words a relative displacement, may have at some subsequent period, or even at several periods, taken place among these cracks, extending them into the more recently deposited beds of the Red marls and the Lias, and "throwing" those rocks up or down from their original level, as we see to be the case with the Red marls on both sides of the coal-field, and with the Lias south of Bromsgrove.
No one, I think, will now be able to look at a geological map of the centre of England without connecting in his mind's eye the Lias of Worcestershire and Warwickshire with that of Staffordshire and Cheshire, and being convinced that these outlying patches once formed one broad connected sheet, a level plain of Lias spreading over all the intervening districts, and sweeping up into the borders of Wales. Wherever a sufficient thickness of the upper Red marls has been left undenuded, to render it possible for beds of Lias to come in, there we find them; but for the denudation, therefore, we should have found Lias wherever we now find the Red marls. The same reasoning will apply to the several members of the New red sandstone, down to the base of that formation. Wherever, therefore, the New red sandstone spreads (in the Midland Counties, at all events), it was in all probability once covered by Lias.
At all events, it is clear from our previous descriptions, and from an inspection of the maps and sections, that before the production of the dislocations of the great boundary faults, the New red sandstone spread over the coal-field and whole district of South Staffordshire, and we can see no reason why it should not have had the whole of its beds everywhere, or why these beds should not have been everywhere covered by the Lias.
At some subsequent period great dislocations took place, and either the present coal-field was lifted up above the surrounding district, or it was left standing while the surrounding districts were depressed, and thus rising as a great protuberance, was of course subject to the more marked influence, and more complete action, of the denuding agencies which have worn and pared down all the country to its present surface. In this way the lower rocks have been stripped of their former covering in the district forming the present coal-field, while more or less of that covering, according to circumstances, has been left untouched in the surrounding country.
The practical bearing of these remarks is this, that wherever we find any of the upper parts of the New red sandstone, we shall there, in all probability, find all the inferior ones in their proper position below it. 'To commence sinking for coal, therefore, in any of the parts coloured as Red marl in the geological maps of the Midland Counties, would be only to throw away money; still more absurd would it be to attempt to find it by sinking in the Lias, as was formerly tried on Needwood forest.[5] It would even be a very imprudent speculation to attempt to sink for coal within any part of the New red sandstone district, unless it were first clearly ascertained that no great thickness of Permian rocks was likely to occur between the bottom of the New red sandstone and the top of the Coal-measures.
Supposing any one to be desirous of sinking for coal, either in the district between the South Staffordshire and Shropshire, or between the South Staffordshire and Warwickshire coal-fields, and assuming that Coal-measures stretch without interruption beneath, an assumption which the exposed area of the district described would not entirely warrant, he will have to calculate—
1st. The probable thickness of the beds of the New red sandstone he will have to pass through.—This will, in many places, be several hundred feet, let us say 500.
2nd. The possible or probable thickness of the Permian formation.—We have seen reason to suspect that this is in some places, perhaps in many, at least 500 yards, or 1,500 feet in thickness. He might be lucky enough to hit upon a spot where there was none of this; but there would be a great chance against such 4 piece of good fortune, and he would only act wisely to set this rock down as 1,500 feet thick.
3rd. The probable thickness of the upper and unproductive Coal-measures.— We have seen that in some part of the South Staffordshire coal-field there are 900 or 1,000 feet of Coal-measures above any of the workable beds of coal or ironstone. Our supposed speculator then would, in any place, have a great chance of coming first upon these upper measures, and would do well to calculate on the possible occurrence of 1,000 feet of them before he reached the more profitable beds.
We thus get altogether a total of 3,000 feet, or 1,000 yards, for the probable depth of good workable Coal-measures over the greater part of the space between the South Staffordshire coalfield and those of Shropshire on the one hand, or that of Warwickshire on the other, under the supposition above mentioned.
He must also take into account that after sinking for perhaps 1,000 or 2,000 feet (say 600 or 700 yards) through red rocks, he might find that he had come down upon a place where the Coal-measures are altogether absent, and might find himself penetrating Silurian shale or some other formation that lies below the coal.
- ↑ William Hopkins, Esq., M.A., F.G.S., &c. 1837. Researches in Physical Geology, "Transactions of the Cambridge Philosophical Society", vol. 6, part 1, pp. 1-84 (Wikisource contributor note)
- ↑ Darwin, C. R. 1840. On the connexion of certain volcanic phenomena in South America; and on the formation of mountain chains and volcanos, as the effect of the same powers by which continents are elevated. [Read 7 March] Transactions of the Geological Society of London (Ser. 2) 5 (3): 601-631, pl. 49, 3 figs. (Wikisource contributor note)
- ↑ Mr. Johnson's notes.
- ↑ This was remarked to me by Mr. Hopkins, in a conversation I once had with him on this point in the Museum of Practical Geology.
- ↑ I may, perhaps, be pardoned for saying here, that even within my own recollection, since the year 1830 for instance, more money has been foolishly expended in the abortive search for coal alone, in places where a geologist would have at once declared against the possibility of finding it, than would have paid the entire cost of the Geological Survey from its commencement to the present time, or its future cost for many years.
Any search for coal through the lias or through the red mar! could only be advisable in localities where those formations could be shown by geologists to have, probably or certainly, overlapped the Bunter and Permian beds, and to rest directly on the Coal-measures.