let us go to crayford, near erith, or to ilford, in essex, and take a superficial glance at some of the brickyards found at those places; in particular, let us ascertain a little concerning the earths employed. we find in one brickyard a series of stiff brown or bluish clays, interstratified between sandy clays or “loams,” with thin brownish partings. in another, the loam will become very sandy, and the earth light, with a slight greenish tinge. a third has thin pebble or gravel beds developed, or small stones sparingly scattered in the clays and loams on certain horizons. a fourth contains, in addition to some of the beds above described, a lime-clay or marl1 with small pellets of chalk. it will be noticed on entering the yards that these various horizons,2 or beds, as they are conveniently termed, are disposed in regular lines or layers, more or less horizontal; in other words, the beds are “stratified.” on the face of the working being dug into, it will often be found that these thin beds, a few inches or feet each in thickness, vary in depth, and frequently disappear altogether, or “thin out,” whilst, on the other hand, a bed only a few inches thick may become as many feet, and new beds are found to be developed. a pure sand may in like manner become loamy on being dug into, and, on being further developed, pass insensibly into a stiff clay. and many other changes take place into which we will not enquire at the moment. suffice it to say, that in such brickyards the strata are very locally developed, though it follows from the circumstance of their existence for so many years, that what changes have taken place, to some extent compensate each other, so that the material is still an earth suitable for making bricks. again, certain beds of much economic value may be more persistent than others, both in character and development. having noticed all these things, we perceive a couple of men digging with care into the brick-earth, and presently they bring some objects to us which we have no difficulty in recognizing as the remains of the lower jaw of an elephant’s skull. returning to the spot where they were exhumed, the upper jaw and tusks also are uncovered. to the clay workers these things are well known; in their time they have found many similar skulls of animals in the brick-earth; but they know next to nothing concerning them, or how they got there. another expedition to the same localities may yield the remains of rhinoceros, the musk sheep, grizzly bear, hippopotamus, reindeer, and many other animals. a fine series of the remains of these, obtained from the3 brick-earths of the valley of the thames at several points, is exhibited in the geological department of the british museum (natural history), south kensington, and more or less complete skeletons obtained from the same source may be found in other, and local museums. one of the most interesting points concerning these remains is that so many of the animals represented in the brick-earths are of extinct species—there are no species included in this latter category of precisely similar kinds to animals now living, thus the elephant was different to modern elephants; we know, from remains found elsewhere, that it was clothed with wool. the same also with the rhinoceros. the reindeer no longer lives in this country, being confined to northerly latitudes; whilst the musk sheep is a denizen of the arctic regions, and the hippopotamus is restricted to the tropical or sub-tropical climes. but we might continue for a long time expatiating on the character of the very numerous mammalian remains found in our common brick-earths. what a curious assemblage of animals! it is wonderful to contemplate the time when the reindeer and musk sheep lived side by side with the elephant and rhinoceros on the site whereon london now stands.
that is not all, however. in the same brick-earths and gravels, tools (flint implements), fashioned by the hands of man, are also frequently discovered, and in one place at crayford, the spot whereon flint implements were manufactured has been ’lighted upon. each flake chipped off has been collected and pieced together, and the shape of the original flint has thus been determined. clearly, from this evidence, the earth from which millions of bricks have been made has formed since prim?val man (and with him the animals alluded to)4 inhabited the valleys of the thames and its tributaries. it is interesting, too, to reflect on the circumstance that the materials upon which many of these facts of great philosophical significance are based, have been collected through the instrumentality of the workmen. pal?ontologists are proud to acknowledge that; their debt of gratitude to the intelligent and persevering men can never be fully repaid.
pursuing the matter still further, we discover a quantity of shells, blanched and very frail—they seem to be deprived of much of their original substance, so to speak; their entombment in the brick-earth has taken all the natural colour out of them. studying these, we soon ascertain that they belong to land snails and mollusca which inhabit fresh water. living representatives of the same species are, with few exceptions, found in kent and essex.
putting all this evidence together, we come to the conclusion that the brick-earths alluded to accumulated in the channel of a river; they are found above the present level of the thames, for the simple reason that they have been elevated into that position partly by earth movements and partly by the channel of the river being cut deeper by natural causes, of which abundant proof will be adduced. the snails were washed down from the land by freshets, or caught by the river in flood; the elephant, rhinoceros, hippopotamus, and musk sheep were overcome, perhaps, by floods, drowned, and subsequently covered up by the mud of the swollen current. we can imagine that the savage hunter, in his canoe, attacking the animals swimming in the river, loses his tomahawk, or his frail bark may be upset, and he is striving to gain the shore for dear life. or, it may be winter time; the river is frozen over, and he is cutting5 a hole in the ice with his flint chisel wherein to fish; his hands are benumbed, and he loses his grasp of the tool; it falls into the water, to be discovered in the brick-earth by one of our intelligent friends. truly, the revelations of the brickyard enable us to construct a picture of one of the most interesting phases of the past history of the earth.
we have given an outline of the evidence upon which certain brick-earths in the thames valley are proved to be of fresh-water origin—to have accumulated in quiet reaches of the river, and at other convenient spots along its course—but we have used that as an illustration only; phenomena of precisely the same character are manifested in nearly all river valleys in this country, especially those in which the bottom of the valley has only a slight gradient down to the sea.
the brickmaker may ask: what is the practical bearing of these observations? what difference does it make to us whether the earths we use are of fresh-water, lacustrine, or marine origin? all the difference in the world, from the points of view of structure, composition and suitability of the earths, and especially of their distribution over the face of the country. how much easier it is to value an extensive brickmaking property when you feel perfectly certain as to whether the face of earth as shown in the pit will die out on being worked into for a few yards, or whether it will be persistent throughout the whole of the property to be valued. better still, when your knowledge enables you to state definitely whether the quality of earth now being worked in a pit is likely to continue the same, or whether it will get better, or worse. the disposition of the earths, in some instances, is so clear that no brickmaker with an eye to business could fail to trace their6 extent over his property. but this is not often the case, for the earths being used are for the most part covered by a superficial mantle, or overburden, which masks the true character of the beds beneath. a very slight acquaintance with the principles of geology overcomes these difficulties as a rule; and we are about to lay down the elements of these principles, so far as they apply to the immediate subject in hand. by seeing why it is the beds of brick-earth vary in structure and composition we shall be in a better position to make forecasts of their general behaviour.
in regard to fluviatile deposits, it goes without saying that every river flows along a general depression more or less pronounced, called a valley, and that this valley is bounded physiographically by a ridge, except in the region of its entrance to the sea or lake, or, if a tributary, of its joining a main stream. the watershed of a river and its tributaries includes and comprises what is technically termed the “river basin.” all valleys are, in the end, the result of denudation taking place in them. in other words, on the birth of a valley a very slight depression or other physical feature determined its general direction for the time being, but the little rivulet once being formed proceeded, through the medium of the “agents of denudation,” to carve out its channel more clearly, and eventually to eat into the rocks over which it flowed, until a large valley had been formed. the “agents of denudation” in river valleys may be summarised as rain, snow, ice, heat, and wind, and their general effect on rocks is called “weathering.” we need not stop to enquire into the precise methods adopted by these agents in accomplishing their work; it suffices at present to say that the rock destroyed or broken up is7 removed by the running water constituting the rivulet, stream, or river, as the case may be. some of the material is chemically dissolved in the water, whilst another and larger proportion is taken away in suspension, or is said to be dealt with mechanically by the river. the agents of denudation do their work very slowly, as a rule, and yet no one who stands on london bridge and contemplates the swollen stream laden with muddy sediment passing under it after a few days’ rain, could say that they are not doing their duty effectually. to give some idea of the quantity of sand, gravel, and mud removed from the land through the medium of rivers, we may remark that the mississippi discharges into the gulf of mexico annually a mass of earthy matter equal to a prism 268 feet in height with a base area of one square mile. in regard to denudation by chemical means we may say that the thames carries past kingston 19 grains of mineral salts in every gallon of water, or a total of 1,502 tons every 24 hours, or 548,230 tons every year; this is not taking into account the muddy sediment, gravel, &c., annually sent down to the nore, which must be infinitely greater in quantity.
enough has now been said to show that stupendous quantities of mineral matter derived from the destruction of the land are sent down to the sea by natural agencies, and we may at once state that a very large proportion of this, which finds a resting-place in and about the mouths of the rivers and their backwaters, is material suitable for brickmaking at places where it is obtainable. enormous quantities of muddy sediment, sand and gravel, however, never reach as far as the sea with great rivers. this material is arrested at sundry convenient spots, and, as a rule, forms excellent brick-earth.
fig. 1.—formation of brick-earth in a river valley.
see fig. 1, which represents part of a river of slow8 current with three bends, a, b, c. the water is flowing in the direction indicated by the arrows; and it is part of the mechanics of such a river that in rounding a bend its velocity is greatest (and its eroding power also) at the outer portions of the curves approximately indicated by the arrow points. the water “wheels round” such portions of the curves, and “marks time” at the points x x x, and, indeed, its progress may be altogether arrested for a time at the latter places. now the transporting power of a river is its velocity, and, naturally, the greater the velocity, the coarser will be the fragments or particles of rock carried along. it is interesting in this connection to quote the figures calculated by mr. david stephenson, giving the power of transport of different velocities of river currents:—
ins. per
second. mile per
hour.
3 = 0.170 will just begin to work on fine clay.
6 = 0.340 will lift fine sand.
8 = 0.4545 sand as coarse as linseed.
12 = 0.6819 will sweep along fine gravel.
24 = 1.3638 will roll along rounded pebbles 1 inch in diameter.
36 = 2.045 will sweep along slippery angular stones of the size of an egg.
9 these figures2 have greater interest for us than in the connection at present used, as will be noticed hereafter. we have seen that in rounding the bends (fig. 1) a, b, c, different portions of the stream possess different velocities. we know it is charged with sediment and stones all the time. the tendency, therefore, will be for the large stones and coarse detritus to go round the outer side of the bend, to bombard the banks near the points shown by the arrows, and to erode the channel deepest in those situations; whilst a goodly proportion of the fine muddy sediment will find its way to the quiet and shallow parts near x x x, and in course of time become deposited there, whilst the main course of the stream is eating its way and shifting its course as indicated by the dotted lines a a. this action proceeds, it may be, until the course of the river becomes straighter, as shown by the dotted lines b b, when the whole of the loop b d is abandoned, its former course there being evidenced by pools of water and irregular heaps of gravel, sand and mud. the reader will now see that the whole of the land marked x x x has been formed of sediment brought down by the river, and in the majority of cases such fine silt and sandy mud or clay is specially suitable for brickmaking—many of our largest brickmakers obtain their material from such a source. it should be observed that the valley, as shown between the lines v v, may be two or three miles in width, and it is often much more, so that the actual amount of land made by the river at x x x may be several thousands of acres in extent.
now as to the practical application of the foregoing observations. in the first place, it will be seen that10 such deposits of brick-earth as are made in this manner cannot be very thick, their total thickness perhaps, resting on the bottom of the valley, not being more than 20 feet, and it is frequently much less. the next thing to be noticed is that they must be very variable in character, a bed changing perhaps every 100 feet or so horizontally, and more often every few feet. individual beds must of necessity be very irregularly developed under the circumstances. the velocity of the stream being greater at certain seasons of the year than at others, we frequently find some such section as the following developed:—
fig. 2.—section of fluviatile brick-earth.
11
a = mould and soil, of no use to the brickmaker.
b = sandy clay, with a large proportion of sand; useful for moulding or incorporating with the “fat” clays below for brickmaking.
c = gravel bed, lenticularly developed; suitable for mending roads, paths, &c.
d = sandy clay; similar to b.
e = thin bed of marl, with a fair proportion of lime.
f = sands and small pebbles, irregularly stratified (false-bedded).
g = laminated sandy clay.
h = stiff clay; can be mixed with f and passed through the pug mill.
i = sand; an irregular bed of very local occurrence.
j = gravel bed, with much sand.
the above is typical of deposits accumulated in river valleys; it is different in character to deposits laid down in the sea (as will presently be described); the section exhibits very different classes of brick-earth also, and yields a totally different kind of brick to that obtainable from brick-earths of marine origin. the importance of the question of origin of a brick-earth, therefore, is just beginning to dawn upon us. many rivers are noted as having throughout a long period of time wandered from one side of the valley to the other (by the process depicted in fig. 1) several times, in which cases the brick-earth sections relating to them are liable to still greater variation. the reader would perhaps be very much astonished to find how much is known concerning peregrinations of that description in regard to particular localities, by competent authorities—usually field geologists.
we come to another important point in regard to river deposits. the ceaseless flow of the river, and the abrading action of the large stones rolled along at the bottom of its channel, tend to cut the latter deeper12 and deeper, and we have excellent evidence that most english rivers once flowed at a greater elevation in their valleys than they now do. in consequence of this, the brickmaker may find his pit somewhat higher than the neighbouring river, which at an earlier stage of its existence made his brick-earths. to a certain extent, small earth movements, as previously explained, are also undoubtedly responsible for many of these brick-earths now being at a considerable elevation above the surface of the river. this phenomenon is illustrated in fig. 3.
fig. 3.—section across a river valley, showing formation of terraces of gravel and brick-earth.
this type of disposition of fluviatile deposits is of common occurrence. we will assume that the valley is carved out of clay (shown by horizontal lines and dots). on both sides of it, and at the same relative heights, are two masses (marked 1 and 2) of brick-earths and gravels running along so as to form two distinct broad terraces. these beds were laid down when the river, in flood, though occupying only a small portion of the valley, was approximately of the height shown by the dotted lines a b. denudation has been hard at work, however, since then, and only vestiges of these beds clinging to the sides of the valley, as shown, remain. at a later period, and coming on towards modern times, the13 broad expanse of beds (comparable in disposition with those depicted in fig. 2) some miles in width, marked 3, were laid down, and we notice the river channel, as it now is, cutting its way through them. thus it comes to pass that brickyards may be situated in terraces one above the other; and what is much more important, the brick-earths may vary very widely in quality along these horizons, those in 1 differing from 2 and both from 3. the brickyards may be quite close to each other, and to the unscientific eye the earths are of similar appearance, but they do not yield the same class of brick, and no one seems to trouble to enquire the reason why. these differences have resulted primarily from the materials having been derived from other collecting grounds, other watersheds, than those comprised within the basin of the river as at present constituted. they are the inevitable accompaniment of the evolution of the river system, and throw light on successive phases of the past history of the stream and its tributaries. for us, as we have seen, they possess considerable practical value of the first importance in selecting the site for a brickyard.
apart from differences of the character just described, serious alterations sometimes take place on these brick-earths being traced higher up the valley, and indeed an excellent brickmaking material may become absolutely worthless in that respect, for the reasons about to be explained. the reader will agree that neither stones nor sediment can travel up a valley, and he will understand that no sediment can be found in the valley earths other than that derived from the destruction of rocks within the watershed of the river system, to which the valleys belong, or did belong, at the time the earths were formed. we desire to put the case in a very simple light, so as to be clearly comprehended. let us contemplate fig. 4.
fig. 4.—map shewing river basin, with geological formations depicted.
14 here we have represented a river basin, the limits (watershed) of which are indicated by a sinuous dotted line. three geological formations are found therein; in the upper reaches of the main river is a series of clays marked a; a large tract in the middle, b, is sandstone; and the lower part, c, is occupied by limestone. seeing that nothing but clay crops out in the part a, it follows that the deposits of the river in that region must be principally of an argillaceous character, to the point a. on flowing over the sandstone b, the main stream, already charged with clay15 particles, will be mixed with sand; the proportion of sand increases as the first large tributary (b) to the east is encountered, and is considerably augmented as the still more important tributary (b) to the west enters it. the superficial deposits in the valleys of the area b will likewise be very sandy and perhaps gravelly at b b, but at c c1 the sands and gravels will be mixed with much clay. on passing over into the area c, much carbonate of lime is added, though the larger proportion denuded from the rocks is taken away, chemically, in solution. nevertheless, nodules of “race” (lime concretions), limestone pebbles, and perhaps chert and flint gravel will come upon the scene at about the point marked e. at d the deposits would principally consist of gravel and impure marls. to sum up, the clays at a would no doubt be too stiff of themselves to make good bricks; similarly the beds at b b would be nothing but sand, though these might be made, with a little judicious treatment, into a species of fire-brick; at c we should find alternating loams and clays suitable for turning out fair bricks; at c1 the beds would be more variable in character and more locally developed; they would consist of thin beds of sand, clays, loams and gravels (principally sandstone fragments), which as a whole might be made serviceable, though difficult to deal with; nothing of much use to us would come from point d, nor bordering the tributary running over c; there would be too much lime present, though a trade might be started in basic bricks should there be any demand for them in the neighbourhood; this, however, would only pay under extremely favourable conditions. at e there may be a mixture of all the foregoing deposits, and providing the beds above were easily weathered and thick beds of loam were thus fairly well developed,16 good sites for brick-earth might be found. the point e might possess this advantage over the other sites mentioned, viz., that marls would no doubt be present, and thus no necessity should arise for grinding lime to be incorporated with the brick-earth; the only danger would be that lumps of limestone might be too numerous—especially if c were a hard limestone.
the general character of the deposits might be slightly modified by mineral matter brought up in springs and thrown down at convenient spots.
