From The Technologist – A Monthly Record of Science Applied to Art, Manufacture, and Culture. Volume V
October 1, 1864
By J. Randall
The paper which I have the honour to lay bofore this section of the British Association has reference to two subjects, both of equal local interest, the one in an economical and commercial point of view, and the other bearing upon the scientific conditions, both as regards the mode of working and geological positions of those beds in the great or Bath oolite, which may be called the “quarry stone,” and which are so extensively worked in the Bath district. I purpose, therefore, to divide my paper into two sections, or arrange the materials into short, and yet I hope sufficiently detailed a manner under two heads : first, to determine the true horizon or geological position of the workable beds of this valuable freestone in the series termed the great oolite ; secondly, to enter upon the mode of “working and getting” this extensively used and valuable building stone.
Geological Position.— Nowhere, I believe, in Great Britain (indeed, in Europe) are the lower members of the Jurassic group of rocks so extensively developed as in the Bath district, where each group seems to have attained its fullest recognised development; nowhere can the whole Jurassic series be so readily studied, nowhere so easily understood; and this applies to the lias itself in its three divisions — the fuller’s earth (here extensively employed); the member of the lower oolite under consideration ; and the Bath or great oolite, distinguished here for its economical value, and at Minchinhampton and other places for its fine and typical organic remains. Above this series, but intimately associated with it, the forest marble and cornbrash are highly and typically developed, succeeded by the Oxfordian and Kimmeridge groups, not omitting even the Portlandian at Swindon and the Purbecks of the Vale at Wardour. To each of these may be appended important notes bearing upon their high importance, economically considered, and which are extensively developed in the district ; but I purpose drawing the attention of the members of this section to the Bath oolite only, determining the position of that zone from which the freestone is extracted, and on which the wealth and comfort of the population of this neighbourhood, engaged in quarrying operations, so much depend. I have also endeavoured to fix, by detailed and measured sections, the workable beds of the district, and to correlate them over a considerable area, useful, it is hoped, both to the man of business and the geologist. These sections, which I may here refer to, are all coloured the same in their respective zones, and show the importance of carefully determining the place or position of the workable beds, prior to any outlay of capital ; and however difficult, indeed impossible, it may be to diagnose the quality of the freestone beds in depth, there can be no doubt as to their position and probable condition ; and when it is known that uniformity of condition over any large area is of extreme uncertainty, and knowing as we do that the thinning out of the marketable beds of freestone in this district, like the great colite en masse on the line of deposition and dip, is a fact now well understood, it becomes a matter of high importance to the capitalist to be assured and confirmed as to the chances of success in opening out or developing a new district. The natural grouping of the beds constituting the great oolite series in this district fall under three well-marked divisions, all well exhibited in the sections exposed at Murhill, Westwood, and Farleydown, Combe and Hampton Downs, Box and Corsham workings, &c., &c. Indeed, generally where conditions have exposed them, and reading downwards from the surface, we meet with over the Bath area, immediately below the forest of marble (where present), the following groupings : — 1. The Upper Ragstones. 2. The Fine Freestone, or Building Beds. 3. The Lower Ragstone. These constitute a series from 60 to 120 feet in thickness, depending upon local circumstances and conditions during deposition and perhaps subsequent denudation.
The Upper Ragstones. — This series consists of (in the upper part) coarse, shelly, and irregularly bedded limestones, with usually a few underlying beds of white fine-grained limestones, possessing a distinctly and well-defined oolitic structure and finely comminuted shells; these are again succeeded by tough argillaceous beds of limestone, usually pale brown in colour and smooth in texture, the whole ranging in thickness from twenty-five feet to about fifty feet. No beds of workable value occur in this upper series.
The Fine Freestone, or Building Beds, in the Bath Stone Series. — Succeeding the upper ragstone are the Bath freestone, or fine-grained building beds, which vary in the number and thickness of the various beds comprising the series, and also economically distinguished from each other by their structural condition, the size and structure of the oolitic grains, the presence or absence of silicious particles or finely divided shelly matters, each of which may materially affect the limestone during the process of working, or influence them after being placed in position, and subject to weathering under atmospheric changes. In some localities the beds assume an earthy structure, indistinct in texture, smooth and close-grained, and hold more moistness.
The Lower Ragstone. — Below the fine building beds, or freestone series, are the lower ragstones, which appear to be persistent everywhere over the entire area, and resting upon the fuller’s earth. They consist of numerous and generally well-defined beds of a coarse shelly texture, and hard crystalline limestone exhibiting much false bedding, especially near the base. Many species of mollusca occur in the bottom beds, such as Ostrea acuminata, Terebratula, Ornithocephala, Rhynchonella, Trikitis, Concinna, and Tancredia. These lower ragstones, as before mentioned, rest immediately upon the fuller’s earth, but this member of the oolitic series concerns us only by position, and is in this district west of Corsham and Bradford a most persistent and important zone, between the inferior oolite beds below and the lower ragstones of the great oolite above, and, in some places, very fossiliferous, and varies in thickness from 150 feet to 200 feet. Taking, therefore, as our guide in this district the above three divisions of the great oolite, we are enabled to construct vertical sections to aid us in our determinations as to the position and condition of the few feet of stone profitable to work in the series, or the “freestone beds,” at all times an anxious and im. portant question when seeking for and developing new ground. In this paper I deal chiefly with facts, and therefore give detailed and measured sections of type localities, from which may be determined by comparison the probable conditions under which the beds may occur at intermediate and unexplored stations or localities on the table lands behind such outstanding mural precipices as Farley, Murhill, Box, on the eastern side of the Bradford and Slaughterford valleys, or on the elevated downs at Claverton, Combe, Hampton, Freshford, &c., to the south of Bath, and west of the Bradford Valley, and on the receding flats to the east of Monklow, Farleigh, and Bradford, &c., conspicuous for the numerous quarries opened in the cornbrash and forest marble, the latter of which occurs in detached patches or continuous lines, stretching from Malmesbury on the north, to Chippenham, Bradford, and other localities to the east of Bath, and especially conspicuous near Corsham, Chapel Korap, South Wraxall, and on to Melksham. The most complete section, and which may be regarded as a typical one of the great oolite and forest marble beds of the Bath district is that of the Box Hill and Corshain Quarry workings. No. 1, showing those beds not usually seen or exposed, but which were cut through by the construction of the Box Tunnel, and which we are now extensively working in that neighbourhood. Another exposition of the series is shown at Murhill, on the eastern side of the Bradford Valley, where the three divisions into which the series group themselves may be studied in situ. Also at Upper Westwood, on the opposite or west side of the valley, other sections occur, tending to show the same facts ; and the variable condition and thinning out of the same beds upon the line of Diss, even at this short distance.
The Sections. — The shafts which are constructed along the line of the Box Tunnel, on the Great Western Railway, afford at the several points where they are carried through the beds of the great oolite accurate data for the construction of sections and clear evidence of the succession of the strata comprising the three divisions. I have endeavoured to maintain, as occurring through this district, and being situated considerably to the east of the Bath Valley escarpments, a large area, for the productiveness of that area is estimated by the lie, position, and condition of the building freestones, supposed to occupy the summit of the table land, stretching from the eastern escarpment of the Bradford, Box, and Slaughterford valleys to Yatton Keynell, Biddestone, and Corsham. The Section No. 1 gives accurate measurement and sufficient details to enable a practical observer to determine the series of beds at almost any point over the area above indicated, or even between the westerly extension of the Oxford clay line from Malmesbury to Corsham and Melksham, and the valley escarpments before mentioned. It is not necessary to notice the forest marble or cornbrash, which is foreign to my paper, and which, although usually present, may or may not occur on any special area above the great oolite proper, local conditions, during deposition or subsequent denudations, having removed one or the other, or both ; but everywhere, so far as I know, over the whole table land do we find the coarse shelly limestones, and some finely grained oolite beds belonging to the upper ragstones or highest members of the great oolite. In the typical section No. 1, taken at No. 7 shaft, Box Tunnel, also at the shafts 4, 5, 6, these beds occur, and were cut through when sinking, and were found to be from twenty to thirty-five feet in thickness, before proving the “capping” to the building or “fine” beds below. At Murhill, near Winsley, these upper ragstone beds are about twenty feet in thickness, and are hard, coarse, and fine shelly limestones, highly comminuted in structure, and occasionally colitic. In some localities many of the beds are of considerable thickness, and of regular and even texture, still they are too hard for those purposes for which the softer, fine-grained, whiter, and more easily worked architectural stone below in the second series) are sought for, and to which they are applied ; and again, they are not good weather stones, but rapidly fall to decay on exposure to severe changes of weather. At Upper Westwood, on the south side of the Bradford Valley, opposite Winsley and Murhill, the beds comprising this upper series are thicker and of more even texture, but as weather stones are of little or no value. At Farley Down, overhanging Bathford, this upper series is nearly thirty feet in thickness, composed of coarse shelly limestones at the top, with hard and soft ragstones down to the capping of the fine “building beds” below. At Combe Down and Odd Down the beds closely resemble those of Farley and Box, and approximate in thickness. Thus we may examine detailed sections of the upper series at Murhill, Farley, Westwood, Coombe, and Odd Down, and the Box district generally, but the beds at neither locality are deemed of sufficient value to work for transit as a building stone.
The Second or Middle Series. — Succeeding the ragstones above mentioned, and commencing the second series, there appears to be everywhere a peculiar bed extending over a large area, termed the “cover,” or capping, varying in thickness, but generally hard in texture ; this forms the roof, or ceiling, to the fine economical building freestones below, and over which it lies, and is a marked feature in extensive underground workings, both for its horizontal extent, application, and importance as protection to the workmen, and as commencing the second series, or middle beds, which occur between the “ upper and lower ragstones.” At Bradford, Westwood, and Murhill this bed is a coarse, shelly, hard limestone ; at Corsham and Box, a closer-grained and tough rock. I associate it with the building freestone, or fine beds below it, rather than with the ragstone above, from its persistency and the constancy of its conditions. Succeeding this is the true Bath stone, or fine freestone, and which I believe occupy, with minor differences, the same position or horizon over the whole of the Bath district. This second, middle, or freestone series are as a group from twenty to thirty feet in thickness, and are coloured chrome-yellow in all sections, and those beds worked for transit are usually evenly grained in texture, regularly bedded, yield well to the saw, are non-fossiliferous, and give evident proof of having been accumulated or deposited in a somewhat deep and tranquil sea, or away from any littoral or wave disturbance, and which the almost total absence of organic remains still further tends to confirm or demonstrate. It appears from observation, and the correlation of measured sections, and conditions observable underground, that the fine-grained regular beds thin away in a south-eastern direction, or upon the line of their general dip, a fact clearly determinable on examining the sections exposed in the valleys. Indeed, it cannot be doubted but that the great or Bath oolite as a group, in this neighbourhood, exists under extremely irregular conditions, and dies out and disappears in the form of a lenticular or wedge-shaped mass, to the east and south-east. This circumstance, causing the building freestones to thus vary in their relative thickness as we proceed from the western part of the area to the east and south-east, and the removing of much of the exposed belt comprising the oolitic series between Bath and Bradford, on the line of their strike, north-east and south-west, caused, it would appear, by the extreme denudation of the Bath and Bradford valleys, and the westerly extension of the cretaceous series from Melksham to Westbury, Frome, and Warminster, are due, perhaps, to physical conditions connected with the eastern extension of the Mendip axis, and the little understood, deeply-seated, but undoubted position of the Palæozoic series, between Frome on the south, and Bath and Wickwar on the north, or along the eastern edge of the Bristol coal-field ; but under any circumstances the extension or invasion of the cretaceous series in the east, the narrowing of the exposed oolitic series above-mentioned, and the mechanical arrangement of the rock structures themselves, evince and determine lccal deposition to have gone on under continued oscillation of the land at the time of the deposition of the great colite series. It is to this second grouping, therefore, or the middle series, which exist between the upper and lower ragstones, that we must assign the workable beds of freestone now extensively quarried in the Bath district.
The Lower Ragstones. — These are an extensive series of rather fine and hard, as well as coarse and shelly, limestones. The lowest beds of this series being usually finer in texture than the upper, and when exposed, are generally from thirty to forty feet in thickness. Nowhere in this neighbourhood are finer sections to be seen than at Murhill, on the north side of the Bradford Valley, and Upper Westwood, on the south side. The beds comprising this division usually occur, or are exposed, in the escarpments of the denuded valleys or the projecting downs above. Masses of the thicker and fine-grained beds frequently occur on the inclined slopes of the valleys, owing to or rising from frequent slips or slides over the fuller’s earth upon which these lower ragstones immediately rest. It is, therefore, in the narrowing of the valleys and abrupt cliffs that this series of the great colite are best exposed. The chief economical value of these beds is confined to local purposes, being utterly unfit for architectural work or exposure to atmospheric influences. The stone used in the construction of the aqueduct conveying the canal over the river Avon, at Avon Cliff, came from the beds of this series at the Westwood Quarry ; and although in situ the stone appears of fine texture and quality, yet it rapidly decomposes on exposure, and the stone work of the Avon Cliff aqueduct is a perishing evidence of its non-durability. At the Box and Corsham quarries these lower beds, though not observable at the surface, are, nevertheless, forty-three feet in thickness, and are chiefly composed of the fine-textured oolitic limestones, but are not worked, as they are of no value in a commercial point of view.
On the Mode of Working the Bath Freestones. — Having endeavoured to determine the horizon of the workable beds of oolite and the relations they hold to the ragstones, or shelly series-recognised above and below these freestones, I will endeavour to describe shortly the mode of opening, working, and extracting the rock; a matter of no little importance, when we consider that more than 100,000 tons of the Bath freestone is annually removed from its original position in this neighbourhood, and forwarded to various parts of the United Kingdom. In working for stone, the first question to determine is, whether the stone shall be reached by open or underground workings, and this must depend upon the presence and conditions of the upper ragstones (and forest marble, where they exist), as they must of necessity be passed through, unless the stone can be reached by tunnelling on the face of an escarpment, where the beds are vertically exposed, or by driving a level to cut the beds ; but if the desired beds are not too much covered, open workings are resorted to. Few persons travelling from London to the West of England, vid the Great Western Railway, through the Box tunnel, have any conception, on passing through it, that around and over them are large and extensively worked mines, from which the well-known Corsham and Box freestones are taken, or as they shoot from the tunnel-mouth into the Bath-hampton, Bath-eastern, and Bradford valleys, that it is the seat of so much quarry industry, having for its object the working of the Bath freestone. In describing the particular mode of getting the stone, I will take for my type the Corsham Down and Box Hill workings. I do so, because these mines have had more thought and attention bestowed on them than any others in this neighbourhood, and because they are the most extensively developed. It is believed that the Box and Corsham locality has been worked for stone, with more or less activity, for three centuries, but it was not demonstrated that so large an amount of good workable freestone existed in the district until the fact was evidenced by the cutting of the Box tunnel, which at once exposed the beds, and showed that to the north and north-west of the tunnel, on the strike of the beds, there existed what we may practically call an inexhaustible supply of valuable freestone. The cutting of the Box tunnel having opened to view this fact, gave an impulse to the previously limited mining operations of the district. The chief operations are situated on the north side of the tunnel ; the reason of this is, that the rock is found sounder in this direction, and the stone more even in colour, and more regular in quality and texture, than to the south or dip of the stone. The entrance to these workings is driven from the Corsham or eastern end, immediately contiguous to the mouth of the Box tunnel, and it is here that the railways of the underground workings join the Great Western Railway on the same level. The chief or main road through the workings is carried from this point due west, in a direct line towards the Box Hill escarpment, a distance of one mile and six-eighths ; rising with the strata, for the purpose of keeping on the floor of the workable beds, thus making an incline to the west of about 1 in 40 ; and as the rise to the north is about 1 in 60, advantage has been taken of this, and the works so laid out, that much of the stone can be run on trollies without draught power—that is to say by gravitation—to the loading platform, where it is transferred from the quarry trollies into the railway trucks, which are taken into the mine to receive it. To economise and facilitate the operation of loading, the platform stands on a level a few inches higher than the sides of the railway truck, into which the stone has to be loaded, and by the upper level narrow-guage tramways this platform is placed in direct connection with the whole of the headings or workings ; and by its lower level broad-guage railway it is connected with the Great Western Railway. By this loading arrangement, we are enabled to load off into railway trucks from thirty to forty tons in the hour. One uniform system of getting or working the stone prevails throughout the quarries, and this system is an inversion of the mode of working coal. The coal-miner undercuts his coal, that the mass may fall and break, but building-stone so worked would make a valueless rubbish heap. The freestone miner or quarryman has to commence his operations at the roof of the stone. This picking operation is effected by means of adze-shaped picks, on the heads of which longer handles are inserted as the work proceeds, and the men thus make their driving a distance of six or seven feet back into the rock. The width or span of these stalls must of course depend on the soundness of the rock. In the Corsham workings, they can, without danger, be driven a width of from twenty-five to thirty-five feet. In the Box quarries, where the rock is not so sound, and the capping bed, before referred to, not so regular, the drivings are limited to from twelve to twenty feet. This is, of course, regulated by the space that may be safely opened without danger to the working beneath. It must be evident that the removal of eight or nine inches of the rock immediately under the ceiling deprives the overlying strata of the support of this area of stone, as effectually as its removal throughout, from roof to floor, would do, and any tendency to settle or drop is at once determined and any risk of life thus guarded against. Another process, by a fresh agency, is now called into exercise, for the cutting of the rock into blocks of required dimensions; for this, a one-handled saw is used. These saws are worked in lengths of four, five, six, and seven feet, and are made broad, rather I should say, deep, at the head or extreme point, so as to insure the saw sinking to its work at that point. The saw is worked at first horizontally, dropping a little as the cut goes on; and after the rock is thus opened down to the next natural parting, and the block thus separated laterally from the parent rock, levers are introduced into the bed or parting at the bottom of the block, and these levers are weighted and shaken till the block is forcibly detached at the back. It is then drawn down by crane power, and the broken end and the bed dressed with the axe, so as to make the block shapely ; it is then placed on a trolly, and allowed to run to the loading platform. After the first block is removed, it will be evident that the workmen have then access by that opening to the back of the bank of stone, and they avail themselves of this to work the saw transversely, which, separating the block from its back or hinder attachment, renders all further breaking off unnecessary, so the first block of each face is the only stone broken from the rock. To each face or heading of work, a ten-ton crane is erected in such position as to command the whole face. These cranes are now constructed telescopically, so as to accommodate them to slight variations in the headings, arising from differences in the depths of the valuable beds, and the expense otherwise attendant on frequent alteration of the crane is thus avoided, and the periodical shifts from old worked-out to new localities are effected with less trouble and loss of time. Sometimes after a block of freestone has been loosened in situ, a Lewis bolt is let into the face of the block, the chain of the crane attached to it, and the block is then drawn out horizontally. By the removal of the first stratum a sufficient space is obtained to allow the workmen an entrance under the roof; and vertical cuts are again carried down through the next bed to the parting below, and a transverse cut readily made ; meanwhile, the cutting is continued in the picking bed, the upper layer removed as before, and everything below this point quarried away, with all the sides of the block sawn, except the bed on which it has rested, and those abutting on the natural joints ; hence each block comes out ready to pass into the hands of the builder, sculptor, or dealer, and this with much less cost and loss in waste than formerly attended blasting and other powerful but rough modes of extraction. The continued repetition of these several operations produces a terrace or step-like profile in the workings, extending from the highest to the lowest of the beds worked, and thus they present themselves to the view.
Professor Phillips said such a paper as this was of the utmost practical importance in connection with science.
Professor Ansted pointed out that the Bath stone, when carried for building purposes to a distance, was exposed to rapid destruction by the action of the atmosphere. He attributed this to the manner in which the stone was quarried. It had been observed that this did not occur with the stone that was used in the immediate neighbourhood of Bath. This, no doubt, was attributable to the fact that the stone was not taken away from its own atmosphere, as is done. He would suggest, therefore, to the quarry-owners, that all the stone to be sent to a distance should be exposed to its own atmosphere for some considerable time, until it had became seasoned, as it were. He believed that if this were done, the stone would be as durable everywhere as it was in the immediate neighbourhood of Bath.