The Geology of Mansfield

THE late Mr. William Jackson, in the course of an address before the members of the Mansfield Mechanics' Institute on the geology of Mansfield, said the stratified rocks which here form the surface belong to the series called "Secondary," and which include the Old Red Sandstone, Carboniferous, Permian, Triassic, Oolite, and Chalk. Our own location being upon the Permian measures, containing the magnesian limestone, it will be seen that we occupy a middle position in the series of rocks. The lowest, and in the order of time the earliest, beds which are accessible to us are the Carboniferous or coal measures, which may be seen at Skegby. They are also present throughout the whole of this district at varying depths, as is evident by the establishment of collieries at Hucknall Torkard, Annesley, Teversal, and at Pleasley, where the coal is 1,600 feet below the surface, the superposed strata belonging to the lower magnesian limestone. As an instance of the perfection to which the science of geology has attained, and the precision wherewith geologists themselves are able to predict the occurrence of underlying beds, it may be mentioned that in the "Memoirs of the Geological Survey of Great Britain," accompanying the quarter sheet map (No. 82, S.E.) of parts of Nottinghamshire and Derbyshire, it is said that "the coal measures occupy but a very small portion of the surface of this county, and occur in the low ground north of Skegby, overlooked by the escarpment of the magnesian limestone. These coal measures are not actually seen on the spot, yet there is no doubt that they are there; not only on account of the level of the soil, indicating clays beneath, but fragments of the lowest beds of the magnesian limestone are found scattered on the fields on a higher level. These coal measures are the eastern extremity of an inlet of the Derbyshire coalfield; yet only some of the highest strata of that field are found near the surface at this particular locality, above all the chief workable coals. Although so small a portion of this district is represented on the map as coal measures, yet they extend beneath all the other measures, and may be reached by penetrating those formations." Subsequent discovery verified the prognostications of the man of science, and now the result is seen in the winning of vast quantities of iron and coal from these Carboniferous measures.

Leaving the coal measures and the interjacent red marls and sandstones, which occasionally crop up between them and the overlying beds of the Permian system, we will direct our attention to those last-named, as they are the formation upon which Mansfield is situated. This system derives its name from Perm, in Russia, where these beds were first discovered. The principal feature of this series are its magnesian limestone rocks and building-stones, of which we have first-rate specimens and vast supplies in our neighbourhood. There are three varieties of these building-stones. The first is a siliceous variety of magnesian limestone called the white freestone, found in quarries only on the south side of Mansfield, and of which most of the dwellings and public buildings of the town are constructed, and is a most excellent stone for the housewright and the mason; and therewith is paved the terrace in Trafalgar Square. Besides being used for building and paving, it is hollowed for cisterns, troughs, &c. The stone has been analysed, and its composition is as follows:—

Silica 51˙40
Carbonate of zinc 26˙50
Carbonate of magnesia 17˙98
Iron, alumina 1˙2
Water and loss 2˙80

In the quarries where the above stone is found may be seen a face between 50 and 60 feet of massive irregular beds of a light yellow coloured, sandy, fine-grained limestone, some of the beds of which are separated by thin layers containing greenish sand, and some of the stone is streaked with green. The top beds are the purest limestone of the quarries, and are burnt for lime. It is from the lower part of the quarry that the white sandstone is obtained. The bottom of this is not reached, owing to the large quantity of water at the foot of the workings, which impedes further excavation downwards. We should infer from the presence of this water that not far below must be a bed of clay preventing the permeation of the water into the lower beds.

The second variety of building-stone is known as the red sandstone of Mansfield. The late Professor Sedgwick visited this formation, and a report of his inspection may be found in the "Geological Transactions," new series, vol. iii. He says: "In the whole range of the magnesian limestone I know of no deposit which can be compared with that here described; and it is more remarkable as it is found in the heart of the formation, and nearly in a line with the finest specimen of crystalline dolomite." This system of beds is about 50 feet thick, and of very extraordinary character. The bottom beds are about 20 in number, and vary from less than one to three or four feet in thickness, but the planes of separation are extremely irregular and not continuous. They are of a dull red colour, and might, without close examination, be mistaken for their red sandstone. The thin beds are much used for building, and the thickest are hewn out into large troughs and cisterns, and in that state are conveyed into all the neighbouring counties. It is also much used for paving-stones; and huge blocks, many of which weigh ten tons and upwards, are constantly being sent into many parts of the country for building materials. The late Sir Gilbert Scott declares this to be "one of the best building-stones in the kingdom." This variety occurs at the Chesterfield Road quarries and at Rock Valley, where a surface of 70 feet is exposed. It underlies the limestone for a considerable distance, and it is generally penetrated in the boring of wells in the north of Mansfield, as was seen eighteen or twenty years ago in sinking Mr. Maude's new well and the new well in the Nursery. It is also seen in Debdale Lane and N.W. of Pleasley Works. Mr. Aveline, F.G.S. is of opinion that these two beds are the same, altered in colour by an excess of iron, and with a slight variation in structure, as the white sandstones of Mansfield.

The third variety of building-stone is the crystallised magnesian limestone, or dolomite, found at Mansfield Woodhouse and along the cutting of the Worksop Railway, as well as at Bolsover Moor. Speaking of this group as observed at the Woodhouse quarries, Mr. Aveline says: "In these quarries may be seen a massive, yet irregularly bedded and beautifully crystalline limestone of a very fine yellow colour, speckled with black." These specks are in all probability due to the pressure of magnesia. Continuing, he says:—"The crystals often radiate from a centre, but in other parts it is more ampherous and compact, and is then a harder stone. This rock, from its hardness and composition, forms a most excellent and durable building-stone, as Southwell Cathedral and other buildings of great age will testify. The foundation and lower parts of the Houses of Parliament at Westminster were built with it. It was also used for the Martyrs' Monument at Oxford." As much as 50,000 feet of dressed stone was sent herefrom to Westminster. We cannot say much in favour of its durability if this be the stone which shewed signs of decay before the building had been finished a dozen years. Perhaps the excess of crystals of lime and magnesia in the stone may be the cause of its tenderness, efflorescing, and crumbling; and it may not have been sufficiently consolidated under pressure. It is, however, said to stand much better than the stone of the same series from Anston quarries. By contrasting its composition with the analysis already given of the Mansfield white sandstone, it will be seen that the latter ought, by its richness in silica, to be much the better material for the external portions of buildings. The following is an analysis of dolomite from Mansfield Woodhouse :—

Carbonate of lime 51˙65
Carbonate of magnesia 42˙60
Silica 3˙70
Water and loss 2˙5

The amount of magnesia contained in this stone is very remarkable. At Bolsover, the amount of carbonate of magnesia reaches forty per cent., and at Woodhouse about the same, including two-and-a-half per cent, of carbonate of manganese.

Near to Sutton-in-Ashfield there are some large quarries of yellow limestone used for building-stone, road repairing, and lime-making. The stone of these consist of both hard and soft beds, varying from a coarse to a finegrained rock. The higher beds of the series are flaggy. From Sutton-in-Ashfield eastwards across the strike of the limestone, the beds are chiefly of a very coarse grain and a yellow colour, the dip being not more than two per cent, which is about the inclination of the ground; wherefore, a person in walking from Sutton to its railway station keeps nearly on the same bed, except when he descends into slight hollows. The thickness of the limestone cannot be more than 30 or 40 feet; but below the limestone there are bands of shale, sandstone, and limestone; and, as these beds occur within the area above the coal measures on the north of Skegby, although there is no actual section exposed, yet a section may be calculated from one just outside the district. On the road from Skegby to where the hill has been cut down, a considerable section of the rock is exposed. The highest beds are yellow limestones getting sandy at their base, and passing downwards into beds of soft sandstone of a brown colour. Interstratified with these sandstones are bands of hard, compact limestone containing fossil shells, whilst the sandstone is full of fossil wood. These beds might be mistaken for coal measures were it not for the bands of limestone, the fossils found in which are of the Permian series. There must be about 20 feet of these beds exposed, but their junction with the coal measures is not seen, although it cannot be far below. (Geol. Sur., 82, S.E., p. 5).

The Permian group supplies very few fossils. Some have been found in a limestone cutting between Skegby and Tibshelf, also at Stony Houghton and at Bolsover, yet are rarely, if ever, found in our own quarries. The limestone bands yield the following:—Bakewellia ceratophaga, Axinus (schizodus) truncotus, and Pleurophorus costatus, whilst the sandstone contains only fragments of plants which are indeterminable ; sometimes lead and barytes in the limestones, as at the railway cutting by Bleakhills. Barytes (sulphate of baryta), or "heavy spar," is the heaviest known earth (sp. gr. 4), and is found plentifully in Derbyshire— where it is called cawk—and in other counties.

Its chief use seems to be a "moral application" in the shape of weightener for soap, the colour and texture facilitating the adulteration. It need hardly be stated that, as it is perfectly insoluble, it is worse than valueless, as it makes the soap hard and latherless, and hinders its detergent functions.

The most remarkable thing about the magnesian limestone over this district is its frequent change in character, the different kinds of strata being found over very limited areas. The white sandstones, although of considerable thickness, are only found, as has been stated, in the Mansfield quarries; and, although there is every reason to suppose that they are near to the bottom of the series, no beds like them occur elsewhere along the western outcrop of the magnesian limestone; for, in a very short distance, another set of beds appear to take the exact place of the white sandstone. Over the whole area it is exceedingly difficult to find out the true dip, or even in many cases to know to what part of the series certain beds belong, whether they be above or below certain other beds about a mile distant. The thickness of the whole series is likewise very uncertain, there being no data whereon to depend; yet there can be no doubt that the thickness is greatly augmented in a northerly direction. It is probable that at Sutton-in-Ashfield it is not more than 40 feet thick; not, however, including the shales. In the Mansfield quarries, beds between 60 and 70 feet thick are laid bare; and at Cresswell Crags the thickness may exceed 100 feet, without counting any shales or marls that may be below.

The Permian marls and sandstones consist of red and white sandstone which overlie the lower magnesian limestone somewhat noncon-formably, that formation appearing to have been disturbed before the marls and sandstones were deposited over it. These are opened out in several places for the purpose of brick-making, both sand and marl being ground up together. Some of these brickyards are on the western and southern sides of the outlier of sand and marl between Mansfield and Sutton-in-Ashfield, where the beds are deep red clays, with thin bands of soft sandstone, either white or red, lying flat. Immediately under the marls are crystalline limestone. These marls and sandstones seem to have been deposited in patches and generally of very shallow depth; sometimes the one preponderating over the other without any regular system of stratification. A good section of these sandstones may be seen in a lane between Woodhouse Hill Farm and Holbeck Woodhouse, where there are between 30 and 40 feet of soft red sandstone in thick beds, except about six feet in the centre consisting of thin alternations of sandstone and marl. The sand is found in all conditions between actual sand and a more or less consolidated stone. Following the series in a north-easterly direction, we find brickyards in the marl beyond Woodhouse Hall; and, at the gardens west of Welbeck Abbey, red sandstone in hard flag-like beds has been quarried; whilst the lake in the Park is on marl.

The Triassic or new red sandstone series occupies very nearly three-fourths of the area of this district (North Notts, and East Derbyshire) indicated by the Geological Survey. We shall notice the lowest members of the group, consisting of the lower red and mottled sandstones, or Lower Bunter bed. This formation is of no great thickness, and for the most part occupies the lower part of the slope that descends from the Forest lands of Sherwood. It is, as its name implies, a red sandstone, semi-consolidated and generally made of very fine grains. Some of it is so fine and so free from earthy matters that it makes a most excellent moulding sand, for which Mansfield is celebrated over all the world. It is obtained from the side of a hill on the south-east of Mansfield; and it also occurs equally good in a field north of the town. Another peculiarity of this sandstone is that it is quite free from the pebbles that characterise the formation overlying it.

The overlying formation, called the Pebble and Conglomerate beds, occupies nearly the whole of the Forest lands—to Bilsthorpe in one direction and to Ollerton in the other—and is known also as the Upper Bunter bed. Its chief character is the immense deposit of pebbles, sometimes forming a hard conglomerate, as at Blidworth (of which the celebrated stone there is an example), and more generally lying loosely mixed with the unconsolidated sand, as may be seen in the cuttings on the Mansfield and Southwell Railway. Both divisions of the Bunter beds may be examined at the pits near Mansfield whence the moulding sand is obtained. The pebbles are of all sizes, and are derived from various older formations, and to a great extent consist of quartz—white, red, green, and other colours. These beds occupy the whole of the Forest lands; and it is owing to its poor, sandy and gravelly soil that the Forest of Sherwood existed so long as woodland or common.

Over the whole of the formations already described, and covering them to varying depths, is a vast expanse of gravel, sand, and pebbly shingle derived from many kinds of rocks; but the greater part of the deposit appears to be made up from the pebbles and the sand of the new red conglomerates. This deposit is known as the drift, the boulders of which have been broken off from the hard rocks of all previous formations, and subsequently water-worn into their present rounded state. The foreign pebbles composing this drift are often found mixed with fragments or pebbles of the rock on which they lie; or if the formation below be marl, then they are mixed with a deposit of clay. Over the country where the pebble beds extend— reaching, as they do, from Hull to Derby—it is not easy to distinguish the recent superficial formation from the older deposit of sand and pebbles, for on the surface they are usually mixed together. The origin of the various drifts is involved in great obscurity, but there can be no doubt that they have been driven by torrents of water, and so at last where they fell there they lie. The epoch generally ascribed to the drift is geologically a modern one, being probably connected with the upheaval of the vast mountain chains of the Alps, Pyrenees, Himalayas, and other gigantic elevations, which took place long after the deposit of the newest strata which has been mentioned. And such upheaval must have caused a great alteration in our climate, wholesale destruction of species, and vast physical changes whereof we can form no adequate conception.

The foregoing very accurately describes the geology of this district. More exhaustive details are given in a paper written by Mr. Jackson, but this is quite sufficient for the present purpose. It may be added that another geologist who has made a survey of the district gives it as his opinion that there is beneath the town and surrounding neighbourhood a valuable bed of the best quality of coal obtainable; so that Mansfield has every prospect of becoming an important town in the no very distant future.