THE COAL FIELD
OF
EAST SOMERSET
BY G. C. GREENWELL.
Any detailed account of this coal field, my short residence in the district necessarily precludes my entering upon at present; some of its features are, however, so prominent, and, as applicable to certain portions of the Northumberland and Durham coal field, so interesting, that I trust no apology will be necessary for presenting the paper which I have now the honour of submitting to the Institute. It is of the utmost importance to a society like the present, that its sources of information should be widely disseminated, and that, by its embracing the consequent facilities for centralization and comparison of facts, it should be prevented from falling into the too frequent error of arriving at conclusions founded upon insufficient data.; and, I trust that other distant members of this influential body, who, by their position, are so well fitted to add to its store of knowledge, will, ere long, show that they agree with me in this opinion.
It will be seen by reference to the Geological Map (Plate 1), accompanying these notes, that the coal- measures of East Somerset may be shortly defined as resting, as in the North of England and elsewhere, upon millstone grit, which is again based upon the mountain limestone, &c; and, as these same substrata rise from beneath the coal measures in the Northern, Western, Southern, and in all likelihood (as seen near the Golden Valley and Chipping Sodbury) Eastern direction, the whole, though most probably uncomformable in detailed structure, may be stated to possess a certain conformability in general position.
By this expression, I would be understood to mean that there is not here, any abutting up of these inferior formations against an overlay of coal measures, such as we shall observe when treating of the position of the coal measures, with regard to the rocks of which they in turn form the substrata.
The probable thickness of the coal measures, at the deepest part of the coal field in the neighbourhood of Radstock, may be 6,000 feet, in which there are three distinct series of seams of coal, as shewn in the general section (Plate 2).
The aggregate thickness of the various seams of coal (or veins, as they are locally named), 33 in number, is 81 feet 4 inches, as shewn in the detailed section following
The proportion of coal contained in the entire thickness of coal measures amounts to only 1-5 per cent.
The coal of the first series is the best in quality; it is of a bituminous character forming a cheerful and very durable fire, but produces more ash than the better classes of coal found in the Northumberland and Durham coal field. The long wall method of working is universally adopted, the undermining or benching being performed in the under clay, by which means the coal is brought to the surface and sent to market in very large blocks.
The strata interposed between the seams of coal, consist of shales and grits of different kinds; the shales are similar in character to those of the Newcastle district, although perhaps abounding more in bright and glassy partings ; they consist of sagre clay (called " pan,") capable of being formed into firebricks of fair quality, and blue metal (ìcliftî) of various thickness, and containing organic remains in, I think, greater abundance and better defined than those found in the North of England.
The character of the siliceous rocks (ìgrayî) is however different ; they are much harder and frequently contain a curious intermixture of coal, the entry of which into the composition of these rocks is a some‚what difficult problem to solve : so far as I have yet been able to ascertain there is no white sandstone (or "white post") in the Somersetshire coal field.
A very remarkable stratum occurs in the first series; it consists of 'a contorted mass of coal pebbles, cemented firmly together by a mixture of grit and shale.
The coal field is traversed by several dislocations, some of which are of considerable magnitude; one of these passes in a North and South direction, through Radstock, in the position shewn on the section (Plate 3), and depresses the strata to the West about 100 fathoms. On the dip side of the dyke, the strata decline towards it at the angle of 20ƒ.
The geological age of the faults of this district appears to vary; some of them (as at Timsbury,) affecting both the older formation of the coal measures, and those newer ones of the red sandstone and lias, by which it is covered, whereas others, of which the great fault at Radstock abovementioned is a notable example, produce no effect whatever upon the formations overlying the coal measures, their continuity being unaltered in the slightest degree.
For some further observations on this subject see Messrs. Conybeare and Phillip's "Geology of England and Wales," part I, p. 257.
There are also some other faults of a singular description, termed overlap faults and rises, which constitute some of those peculiarities which it is the object of this paper to describe.
In the event of our finding any stratum cut off by a fault, it is the generally received opinion that when the line of hade forms with the thill of the stratum an acute angle, we may conclude that the stratum in question will be depressed on the other side of the fault, and vice versa. The overlap faults above alluded to, however form a-remarkable instance at variance with the above, the dislocation of the strata producing the effect L shewn in the diagram, occasioning thereby a repetition of the seams of coal and other strata, so that a pit sunk
at the point A, would pass through the same seams a b twice.
The principal fault of this description is also shown in the section (Plate 3); by it the seams of coal are doubled for a breadth of about 150 yards, and the alteration in level occasioned amounts to 22 fathoms.
Several minor faults, similar to the above, occur in the district, some o them producing a difference of level of a few fathoms or feet, and others a mere forcing over of the seam of coal upon -itself, as shown in Plate 4. In all these cases there is no deterioration of the foal in quality, and rarely any in its hardness, notwithstanding that the lines of stratification are sometimes contorted in a remarkable degree these lines in one part of the mass being parallel with the ordinary inclination of the seam of coal, and in other parts (separated from the first by a mere parting), lying at a very considerable angle to it.
A repetition of the coal of this kind is usually in the immediate vicinity of an entire nip out of the coal, or what is termed "dead ground."
The nature of the roof and thill is in these cases little altered, with the exception that the roof possesses more frequently than ordinary, glassy and slightly coherent partings, rendering increased caution in working beneath it, and care in propping, requisite in order to prevent it from dropping down.
Many of the faults found in this coal field consist of sudden rises of the coal, not unfrequently at an angle of from 30 to 40 degrees; the difference of level thereby occasioned, amounting in some instances to 8 or 10 yards. In these cases there is no alteration either in the thickness or quality of the coal, and the roof and floor of the seams remain parallel as in other parts of the district.
The chief distinctive feature, however, between the East Somerset and Newcastle coal fields, as at present defined, consists in the super-position over the coal measures of the former district, of formations of a much more recent date than any of those beneath which coal has been worked in the North of England, and one of the principal objects of this paper will have been attained, if these notes should draw the more particular attention of the Institute to what is undoubtedly a matter of no slight importance. I allude to the existence, or otherwise, of the coal measures beneath the new red sandstone and liassic formations of the North-East of Yorkshire.
When treating of the presence of the coal measures beneath the upper. formations in the Somersetshire district, the justly celebrated authors of the "Outlines of the Geology of England and Wales," observe, (Part I, page 254):
ìNo inference can be drawn from this district in favour of similar trials elsewhere, since two peculiar circumstances here concur: first, the manner in which these more recent beds here overly the coal field, resting unconformably in horizontal planes upon the truncated ends of the highly inclined strata belonging to the coal measures; and secondly, the thinning out of many of the beds in this direction, in consequence of which the sands of the inferior oolites and the clay of the lias are greatly reduced, and have almost vanished in many places, leaving the freestone beds of the inferior oolite almost in contact with the lower stony beds of the lias; the new red sandstone also being greatly diminished in thickness, so that a geological interval, equal in many instances to probably 2,000 feet, is here reduced to less than a quarter of that depth."
This passage, it need not be remarked, has reference, not to the absolute presence or absence of the coal measures beneath the oolitic, lias, or red sandstone formations, but to the thickness of these strata, which, under the circumstances of ordinary districts, would have to be passed through before the coal measures would be attained.
It is not my intention to enter into any speculations of this nature, but rather to leave them to form the subject of future investigation, and to confine myself to a mere description of the manner in which the "Overlay," as it is locally and most expressively termed, forms a component of the stratification of the coal region of East Somerset.
The number of formations of which the overlay at any point consists, depends,
(1) upon the elevated or depressed situation of the surface where the section is taken; for, excepting under circumstances of dislocation by faults, they are found continuous and in a nearly horizontal position and
(2) upon the local thickness of each formation; the surface elevation only introducing a superior, when the thickness of the inferior formation is so diminished as to allow of such a result.
The variation in thickness of these upper formations, within short dis‚tances, is considerable, as will be seen by referring to the following sections :
From this, it follows that an elevation which, at Radstock, would be surmounted by the lower oolite, would, at Paulton, only display more elevated beds of the lias formation.
The bottom beds of the inferior oolite consist of calcareous sandstones, (called, locally, freestone) of different degrees of hardness, some being very soft and easily cut or sawn into blocks of any, shape, and others harder and more crystalline in texture.Ý These beds contain numerous fossils.Ý Beneath the inferior oolite is the liar; the upper part of which consists chiefly of soft blue marles of very variable thickness, according to the extent of denudation prior to the deposition of the oolitic sands. This is followed by blue and grey lias limestone beds interstratified with shale, the thickness of the whole being, also, very variable.Ý It abounds in organic remains, of which Ammonites, Gryphw, and Pectines, are most frequent; it also contains, more particularly in the neighbourhood of Glastonbury, at Street, remains of Saurian reptiles, in considerable abundance. This limestone affords a lime which possesses the property of hardening in water.Ý It is found to produce the best hydraulic mortar when fresh slaked and mixed with riddled ashes, in the proportion of two of ashes to one of lime. Immediately beneath the blue lias a bed of yellowish brown lias limestone, called the sun bed, about two feet thick, which is used as a building stone and road material, and, beneath this latter bed is the white Has limestone, which is generally about ten feet thick, and lies in pannels of from six inches to a foot in thickness.
The white lias is much used for building, and affords a durable stone for this purpose, it is easily dressed, and from the facility with which it is carved and its fine texture, it is well adopted for interior decorative work, in which respect indeed it is difficult to surpass it in beauty. Beneath the white lias is a bed of softish limestone with thin clay partings, about three feet thick called " rumples," to which the wells of the district are sunk; this bed contains muscles in abundance. Immediately beneath this bed is a thin stratum of grey marble, about 4 inches in thickness, which, when polished, presents the appearance of a landscape, and is often cut into inkstands and other ornaments.
The lowest member of the lias is a black marl, about 8 feet thick, which is possessed of fertilising properties, and is frequently, at its outcrop, dug up and laid on the land as manure. I have been informed, however, by an intelligent farmer in the neighbourhood, that the advan‚tage derivable from it is to be attributed rather to its property of retain‚ing the moisture in the soils where it is laid, and preventing it from settling into the dry stony sub-soil which is generally found where the lias lies near to the surface.Ý None of the strata above-mentioned contain much water, but further down when we enter the new red sandstone, we sometimes find it in abundance.
The new red sandstone is a peculiar formation; towards the top it consists of a very soft argillaceous sandstone, which, when dry, appears a fine red close-grained rock, but which, when wet, falls to a clayey earth. It becomes gradually harder in some places as we descend, but in others retains the character above described, to the bottom. It contains a few beds of white sandy stone of a harder nature, but these are irregular.
From the crumbly nature of this rock, and the large feeders of water met with in it, it is somewhat difficult to sink through, the water washing the soft stoneout from behind the timbering put in for the temporary support of the shaft, until a foundation for tubbing can be obtained. The water pumped during the progress of sinking through this formation resembles red paint.
At the bottom of the soft sandstone is about 20 feet of an exceedingly hard concretion of pebbles with red sandstone grit; this stratum is locally called "the millstone," it has also received the name of ìmagnesian conglomerate,î and is supposed by some to be the representative of the magnesian limestone of the Northern Counties.
This rock is very difficult to sink through; the pebbles consisting chiefly of hard flinty chert, and the cement by which they are united being of almost equal obduracy.Ý This bed contains little water.
Beneath the red conglomerate last named is a stratum of variegated soapy marl, containing one or two thin beds of dark, ferruginous sandstone; the thickness of this, which may be called the lower new red sandstone formation, may be stated as being at Radstock 101 fathoms, but the exact line of demarcation between these red strata and the coal measures is difficult to determine, the red colours gradually blending with and becoming lost in the blue of the coal measure shales, which have been already described.
In former years the existence of good coal beneath the "red ground" as it is termed, was considered quite as chimerical by the authorities on the matter in East Somerset and the adjoining coal districts, as beneath the magnesian limestone in the North of England; and it was only as other mines became exhausted that the march of adventure was extended into more doubtful districts.
An equally powerful impetus is now given to coal speculation in the North-East of Yorkshire, by the discovery of its enormous wealth in ironstone, as an addition to which, coal would be almost invaluable.