Chapter II: Clay and Associated Rocks

Clay, as already mentioned, is geologically a rock and not a mineral, and belongs to the important group of sedimentary rocks which have been derived from the igneous or primary ones by processes of weathering, suspension in water and subsequent deposition or sedimentation.

Whatever may be the primary origin of clay, its chief occurrence is in geological formations which have undoubtedly been formed by aqueous action. The materials resulting from the exposure of primary rocks to the action of the elements have been carried away by water--often for long distances--and after undergoing various purifications have been deposited where the speed of the water has been sufficiently reduced.

In some cases they have again been transported and re-deposited and not infrequently clay deposits are found which show signs of subsequent immersion at considerable depths and have every appearance of having been subjected to enormous pressures and possibly to high temperatures.

Some clays have only been carried by small streams and for short distances; these are seldom highly plastic and resemble the lean china clays and kaolins. Others have been carried by rapidly moving rivers and have been discharged into lakes or into the sea; they have thus undergone a process of gradual purification by elutriation, the sand and other heavier particles being first deposited and the far smaller particles of clay being carried a greater distance towards the centre of the lake or the quieter portions of the ocean. The nature of such deposits will, naturally, differ greatly from each other, the materials at first associated with the clay, or becoming mixed with it at a later stage, exercising an important influence on its texture, composition and properties. If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without destroying the nature of the clay. In addition to this it must be remembered that the land is continually rising or sinking owing to internal changes in the interior of the earth, and that these subterranean changes bring about tilting, folding, overturning and other secondary changes, which, later, cause a fresh set of materials to be mixed with the clays. Further than this, the action of the weather, of rivers and of the sea never ceases, so that a process of re-mixing and re-sorting of materials is continuously taking place, and has been doing so for countless ages. It is, therefore, a legitimate cause for wonder that such enormous deposits of clays of so uniform a character should occur throughout the length and breadth of Europe, and practically throughout the world. For although the composition of many of these beds is of a most highly complex nature, the general properties such as plasticity, behaviour on heating, etc., remain remarkably constant over large areas of country, and the clays of each geological formation are so much alike in different parts of the world as to be readily recognized by anyone familiar with the material of the same formations in this country. Considerable differences undoubtedly exist, but these are insignificant in comparison with the vastly different circumstances under which the deposits were accumulated.

Leaving the consideration of the modes of formation of the various clay deposits to later chapters (III and IV), it is convenient here to enumerate some of the chief characteristics of the different clay deposits and their associated rocks. In this connection it is not proposed to enter into minute details, but rather to indicate in broad outline the chief characteristics of the clays from the different deposits. This general view is the more necessary as clay occurs in each main geological division of the sedimentary rocks and in almost every sub-division in various parts of the world.

The =Precambrian, Cambrian, Silurian and Devonian= 'clays' are chiefly in the form of shales or slates, the latter being clays which have undergone a metamorphic change; the latter resulted in the production of a hard and partially crystalline material with but little potential plasticity and therefore of small importance for the ordinary purposes of clay working.

_Slates_ are distinguished from shales by their splitting into thin leaves which are not in the plane of original deposition, but are due to the deposited material being subjected to great lateral pressure. The re-arrangement of the particles thus produced has imparted to the material a cleavage quite independent of the original lamination.

The shales in these formations are occasionally soft and friable and are then termed _marls_, but this name is misleading as they contain no appreciable proportion of finely divided calcium carbonate as do the true marls[8].

[Footnote 8: Readers desiring more detailed information on the occurrence of the clays mentioned in this chapter should refer to the author's _British Clays_ (No. 2 in Bibliography).]

The clays in the =Carboniferous Limestone= are not, as a whole, of much importance, but the occurrence in this formation of pockets of white refractory clays in Staffordshire, North Wales (Mold) and Derbyshire is interesting, especially as these are used for the manufacture of firebricks and furnace linings. These clays are highly silicious and in composition are intermediate between the Yorkshire fireclays and ganister. Their origin is uncertain, but it is generally considered that they have been produced by the action of the weather and streams on the shales and grits of the Coal Measures which formerly occupied the higher ground around them, though Maw (16) states that 'it is scarcely open to question that they are the remnants of the subaerial dissolution of the limestone' (see 'Fireclays,' Chapter V).

In the =Upper Carboniferous System= the clays are highly important because of their general refractory nature, though they differ greatly in this respect, some red-burning shales of this formation having no greater power to resist heat than have some of the surface clays.

Those of the Coal Measures are of two main kinds--shales, or laminated rocks which readily split along the planes of deposition, and unstratified underclays. The _shales_ usually occur above the seams of coal and are either of lacustrine or marine origin, differences in their fossils and lithological character supporting one origin for some deposits and the other for the remainder. Some of them are fairly uniform in composition, but others vary so greatly in their physical characters, that they are divided by miners into 'binds' or relatively pure shales, 'rock-binds,' or sandy shales, and sandstones. They also vary greatly in thickness in different localities, and whilst they form the main feature in some districts, in others they are replaced by sandstones.

The _underclays_ are so called from their usually lying beneath the coal seams. They are not noticeably stratified and vary greatly in character from soft unctuous materials to hard, sandy rocks. In composition they vary enormously, the percentage of silica ranging from 50 per cent., or less, to as high as 97 per cent.

The mode of formation of the underclays is not certainly known. They do not appear to be soils or of terrestrial origin, but according to Arber (24) correspond closely to the black oozes of marine and semi-marine estuarine deposits of tropical swamps, or to the muds surrounding the stumps of trees in the buried forests of our coast-lines. They thus appear to be quite distinct from the shales above them, both in origin and physical characters. The more silicious portions, known as _Ganister_[9], possess comparatively few of the characteristics of clay though used, like all the more refractory clays of the Coal Measures, for all purposes for which fireclay is employed. The term _fireclay_ is, in fact, frequently applied to all the refractory deposits in the Coal Measures, without much regard to their composition (see Chapter V).

[Footnote 9: The Dinas rock used in the Vale of Neath (Wales) is an even more silicious material found in the Millstone Grit immediately below the Coal Measures. It is largely employed for firebricks.]

Valuable Coal Measure clays occur in enormous quantities in Northumberland, Durham, Yorkshire, Nottinghamshire, Derbyshire, Staffordshire, near Stourbridge, in Warwickshire, Shropshire, North and South Wales and South West Scotland. In Ireland, on the contrary, the Coal Measure clays are of little value except in the neighbourhood of Coal Island, co. Tyrone. The position of the 'Sagger Marls' of North Staffordshire (Keele Series and Etruria Marls), relative to the 'Farewell Rock' or Millstone Grit, is shown in fig. 8 in which the horizontal lines represent coal-seams and ironstone veins.

Fig. 8. Coal Measures sequence in North Staffordshire.]

The dissimilarities in the fossils of the Coal Measure clays and shales in the Northern and Southern Hemispheres suggest that there is a considerable difference in their formation, but the number of clays and shales which have been examined is too small for any accurate conclusion to be drawn.

For many industrial purposes, particularly for the manufacture of refractory goods, the clays and shales of the Carboniferous System are highly important. The less valuable burn to a reddish colour, often spoiled with many grey spots of ferrous silicate derived from the pyrites in the clay, but the purer varieties burn to a delicate primrose or pale buff tint and are amongst the most heat-resisting materials known. The Coal Measure clays of Yorkshire are particularly esteemed for their refractory properties; for the manufacture of glazed bricks and for blocks for architectural purposes somewhat ambiguously termed 'glazed terra-cotta.' The inferior qualities are largely used for the manufacture of red engineering bricks, some of them competing successfully with the more widely known 'blue bricks' of Staffordshire.

The Coal Measure clays of Shropshire are noted for the manufacture of red roofing tiles, especially in the neighbourhood of Broseley.

Agriculturally, the Coal Measure clays are usually poor, but are occasionally of good quality. The shales produce heavy, cold clays and the yellow subsoil produces soils of a light, hungry character so that the two should, if possible, be mixed together.

=Permian clays= are of little value except for the manufacture of red building bricks. The Nottinghamshire Permian clays make excellent roofing tiles, flower pots and red bricks.

Agriculturally, the Permian clays are a free working loam yielding large crops of most of the ordinary farm products.

=Triassic clays= are of great importance in the Midlands, those upper portions of them known as the Keuper Marls being much used for the manufacture of bricks.

They are specially known amongst clayworkers as the material from which the Midland red bricks of Nottinghamshire and Leicestershire and the Somersetshire tiles are prepared.

=Jurassic clays= are an important group, of marine origin, occurring in close association with limestone. For this reason they form a valuable source of material for the manufacture of Portland cement, but are of less value to the brick and tile manufacturer. The Jurassic System contains so large a variety of clays, of such widely different ages and characteristics, that no general description of them can be given in the present volume.

The '_Lias clays_'--the lowest of the Jurassic formation--are chiefly dark, bituminous shales, including the 'alum shales,' and are often seriously contaminated with pyrites and ironstone. When carefully selected they may be used to advantage in the production of most red articles such as bricks, tiles, chimney pots, etc. They shrink less in the kiln than do most clays, and are easily fusible on account of the lime they contain, but on the whole this formation is of great value for the manufacture of the articles just mentioned.

Agriculturally, the Lias clays are laid down for grass, but the lighter soils are useful for arable purposes.

The '_Oolitic clays_,' which are also Jurassic, usually contain limestone in the form of nodules, but are nevertheless important. They form a broad belt above the Lias from Dorset to Yorkshire, and include the blue clays of the Purbeck beds, stiff blue bituminous Kimeridge clays, the irregular, sandy Coral Rag clays, the famous Oxford clay (from which the Peterborough and Fletton bricks are made), the Kellaways blue clay, and the Fuller's Earth deposits.

The '_Kimeridge clays_' are dark, stiff laminated clays, closely resembling gault, and are much used in the West and Midlands for brickmaking. A well-known deposit of this character has long been used at Pickering in Yorkshire, but the most typical deposits are in Huntingdonshire. The Kimeridge clays contain a bituminous shale, or Sapropelic Coal, which evolves a characteristic odour on burning.

Agriculturally, the Kimeridge clays resemble gault and are difficult to work as arable land, though they form first-rate pasturage.

The '_Oxford clays_' are valuable for brickmaking when their use is understood, but to the uninitiated they are very troublesome. Their colour is dark blue or grey and they are usually stiff or somewhat shaly in texture with layers of variable composition. The closely associated Cornbrash (limestone) is a source of trouble unless great care is taken in the selection of the material. 'Oxford clays' are not infrequently traversed by seams of poor coal or by oil-shales.

Agriculturally, Oxford clay is difficult to work and, while much of it is valuable, large portions are poor and cold. When well exposed to frost it is made much lighter, but even then is not very suitable for wheat and autumn sown crops.

The '_Kellaway blue clays_' are often included in the Oxford clays, though they form irregular bands above them and are of fresh-water origin, whilst the Oxford clays are marine deposits. They are chiefly used commercially for domestic firebricks near Oundle and Stamford.

=Cretaceous clays= occur, as their name implies, in association with chalk. The chief clay in this System is the _gault_, a stiff, black, calcareous clay of marine origin chiefly used for brickmaking. When used alone, gault burns to a reddish colour, due to the iron present, but if, as is more usual, it is mixed with chalk, it burns perfectly white. Some gaults contain sufficient chalk to render the addition of a further quantity unnecessary.

Agriculturally, the Cretaceous clays form good arable soil where they are not too exposed, but they suffer from drought.

The '_Wealden clay_' is a stiff yellowish grey or blue clay extensively used for brickmaking in Kent, Sussex and Surrey. It has been subdivided by geologists into a number of other clays, such as the Wadhurst, Fairlight, etc., but the differences between them lie more in the fossils occurring in them than in the characters of the clays themselves. They are usually contaminated with ironstone, gypsum and some limestone.

Agriculturally, the Wealden clay produces stiff, yellowish soils of a wet and poor character, but sometimes loams of a highly productive nature occur.

The =Tertiary clays= include all those deposited after the Chalk and previous to the close of the Glacial period. They are usually mixed with sand and gravel, and though the deposits are often thin and irregular they are the most generally important of all clays. They vary greatly in character; some, like the London clay, being almost useless unless mixed with other materials, whilst others like the ball clays of Devonshire and Dorset are amongst the purest and most valuable of the plastic clays. The Tertiary clays are divided by geologists into Pliocene, Miocene and Eocene formations; of these the first are commercially unimportant and the second do not exist in Great Britain. At one time the Bovey Tracey clays were considered to be Miocene, but they have recently been classed as Oligocene by Clement Reid.

Agriculturally, the most important of the Tertiary formations is the Eocene, particularly near London, though it is much covered by sand or gravel. The _London clay_, which produces a heavy brown soil, is of slight value, though when properly drained it produces good crops of wheat, beans, and cabbages and other market-garden produce. For this purpose it is greatly improved by the addition of lime and of town manure. The South Hampshire Eocene beds of clay are cold, wet and of small agricultural value.

The Eocene clays are composed of a variety of clays, many of which are only distinguishable by the different fossils they contain. The most important are the Reading clays, the London clay and the Bagshot clays.

The _Reading clays_ extend over a considerable area in the South of England and are most valuable near the town from which they derive their name. The best qualities are mottled in a characteristic manner and are particularly suitable for the manufacture of roofing tiles and small terra-cotta--an industry for which Reading is famous.

The _London clay_ is always a treacherous material and is best avoided in the manufacture of bricks and other articles except under highly skilled technical advice.

The _Bagshot clays_ in Dorsetshire are famous for the ball and pipe clays shipped from Poole, whilst at Bovey Tracey and in several parts of Devonshire equally valuable ball clays are found and are shipped from Teignmouth.

These _ball clays_ are of variable composition and colour and require careful selection and testing. They are closely associated with sands, but the lower beds of clay are remarkably stiff, plastic and white-burning. The colour of the raw clay varies from a pale yellow to a dark brown or even to black, but this is little or no criterion of the colour of goods made therefrom, as the colour is due to carbonaceous matters, 4 per cent. or more carbon being usually present.

The 'blue' and 'black' ball clays are the most valued by potters, but the quality is usually ascertained by a burning test.

The value of these ball clays both in Devonshire and Dorset is due to their comparative freedom from iron and alkalies and to their remarkable unctuousness and plasticity. They are, therefore, largely used in the manufacture of all kinds of earthenware of which they form the foundation material.

In composition, ball clays appear to consist chiefly of a hydro-alumino-silicate corresponding to the formula H4Al2Si2O9, and in this they very closely resemble the china clays (kaolins). The latter are, however, but slightly plastic whilst the ball clays are amongst the most plastic clays known. The china clays are also much more refractory than the ball clays owing to the somewhat larger proportion of alkalies in the latter.

_Pipe clays_ are an inferior quality of ball clay; they contain rather more iron and alkalies and considerably more silica. For this reason they can only be used for cheaper wares where colour is of less importance and where their excessive contraction can be neutralized by the addition of other substances such as flint.

The =Boulder clays= occur in a blanket-like covering of Drift which lies over the greater part of Northern and Central England, and over a considerable portion of Scotland and Ireland. They are a product of the Ice Age and, whilst varying greatly in character, may usually be distinguished by the occurrence in them of rounded stones and gravel, some of the former bearing clear indications of glacial action. The boulder clays are largely used for the manufacture of building bricks, but the strata in which they occur are so irregular that very careful supervision of the digging is necessary. In some localities these clays form beds 12 ft. or more in thickness and relatively free from gravel; in other districts the clay is interspersed with lenticular deposits of gravel or sand (commonly known as 'pockets'), and if these are mixed with the clay considerable difficulty in manufacture may be experienced. The total thickness of the drift deposits is often very great, as in the cliffs at Filey (fig. 11) which are 200 ft. high.

The boulder clays--considered apart from the stones, gravel and sandy materials occurring with them--are usually red-burning, stiff and very plastic, but the gravel, sand and crushed stones mixed with them in the formation of the material usually render them of medium plasticity. By careful washing, most boulder clays may be purified sufficiently to enable coarse brown pottery to be made from them. Clean deposits of sufficient size to be worked without any purification are occasionally found. Usually, however, the boulder clay formation is somewhat treacherous as it is difficult to ascertain its nature; boreholes are apt to be quite misleading as the formation is so irregular in character.

Agriculturally, drift or boulder clays are poor soils, but by judicious management and careful mixing they may be made more fertile. Where it contains chalk--as in Norfolk and Suffolk--boulder drift forms an excellent arable soil.

=Pleistocene or Recent clays= are amongst the most important brickmaking materials in the South of England. They are of remarkably varied character, having been derived from a number of other formations. Usually the deposits are somewhat shallow and irregular in form, but beds of considerable thickness occur in some localities.

Agriculturally, they are of considerable importance.

Most of the =brick earths= used in the south-east of England are of Recent formation, those of the Thames Valley being of special importance in this connection, particularly where they are associated with chalk; thus forming natural _marls_ or enabling artificial marls to be produced.

The brick earths--in the sense in which this term is used in the south--comprise three important types of clay: (_a_) _Plastic clays_ not particularly differentiated from those already described, (_b_) _Loams_ or sandy clays which are sufficiently plastic for satisfactory use, have the advantage of shrinking but slightly in drying, and are largely used in the manufacture of red facing bricks and as light soils, and (_c_) _Marls_ or calcareous clays, used for the production of light coloured or white bricks, the chalk they contain combining with any iron compounds present and, at the same time, reducing the contractility of the clay. On burning, they form a cement which binds the particles into a strong mass. These are the 'true marls' or 'malms' composed of clay and chalk and must not be confused with the so-called marls of Staffordshire and elsewhere which are almost free from lime compounds. There is, at present, no definition of 'marl' which is quite satisfactory; a maker of London stock bricks understanding by this term a clay containing at least 10 per cent. of chalk; a maker of white Suffolk bricks a material containing at least twice this amount; an agriculturalist any soil, not obviously sandy, which will make his clay land less sticky; and many geologists any friable argillaceous earths. A general consensus of opinion is, however, being gradually reached that the term 'marl' should be limited, as far as possible, to clays containing calcium carbonate in a finely divided state.

=Alluvial deposits=--which are also of Recent formation, though still of sufficient age for skeletons of mammoths to be found in them--are of so variable a nature as to render any brief, general description impossible. Many of them are so contaminated with sand and crushed limestone as to be useless for manufacturing purposes and of small value agriculturally, but others are important in both these respects.

Further details of the occurrence of clays in the various formations described will be found in the _Maps and Memoirs of the Geological Survey_ and in the author's _British Clays_ (2).