Essay on the Theories of Coal Formation | Fossil Fuels | Geology

In this essay we will discuss about the theories of coal formation.

One of the important points on which geology speaks with an uncertain voice is Coal-formation. Two main theories have been suggested, but whether either or both are correct further investigation has yet to show.

Fairly early in the nineteenth century we meet with the idea that the coal of to-day was originally vegetation which had been carried down by streams and deposited like any other sediment. Later on attention was drawn to the frequent occurrence beneath coal seams of a layer of rock called Underclay.

This underclay is characterised by the presence of numerous tree-roots (called Stigmaria), which penetrate it, and it became accepted as an ancient layer of soil in which the trees and plants of the Coal period lived and above which vegetable debris accumulated.

These two ideas about coal-formation, the drift theory and the growth-in-place theory, have long held the field. The supporters of the latter pointed to the Stigmaria and underclay beneath each coal-bed, to the absence of sand and clay deposits in a coal seam, to the wide uniformity of some seams, and to the occurrence of vertical tree-stems associated with coal-beds as confirming their view.

Another and absolutely different idea about coal-forma­tion is that the greater part of the carbonaceous material has been derived from without, either having a volcanic origin or being due to vegetable decomposition.

The supporters of this theory call attention to the eruption in past times of chemical compounds, resembling those which occur in coal, which have yielded beds of bitu­minous shale and asphalt, and contend that if extrusion of such matter took place over the floor of lagoons on to which vegetable sediments were brought we should get conditions favourable for coal-formation.

The growth-in-place theory has its variations: some look on coal as the last stage in the alteration of a peat- morass produced by pressure and chemical change; others, pointing out the absence of peat in those warm regions of the earth, whose temperature conditions seem to corre­spond with those of the Coal period, prefer to regard coal as an accumulation of vegetable material derived from a dense forest-growth on an area which was slowly, though not continuously, sinking.

Thus Lyell says:

“In South Wales, where the coal- measures attain a thickness of 12,000 feet, the beds through­out appear to have been formed in water of moderate depth, during a slow but perhaps intermittent depression of the ground, in a region to which rivers were bringing a never- failing supply of muddy sediment and sand.”

There are many geologists now who accept both the growth-in-place and the drift theories as being correct, regarding some coals as having been formed by the one method, and some by the other.

The compositions of coals vary greatly, and the nature of coal deposits is not uniform, and there seems to be no necessity for believing that all coal-beds were formed in the same way.

The continuous occurrence of a bed of coal has been appealed to as an argument against deposition, but evidence has been brought to show that such continuity is not always seen. In the Staffordshire coal-field a very striking feature is the splitting of the seams.

One 30-foot seam, on being traced northwards, splits into ten to fourteen distinct seams by the intervention of sands and shales. To the south we have a 30-foot coal seam; to the north, only five miles away, this seam has become subdivided into thin seams inter- stratified in sedimentary deposits 300 feet thick.

The easiest way to explain such an occurrence is by supposing the whole to be a sedimentary deposit, currents bringing now sand, now vegetable detritus to one spot, while to another no sand was brought at all, but there was a continuous deposit of plant remains.

Another well-known fact is that the coal seams are interrupted by lumps of sandstone. According to the sediment hypothesis these are merely sandbanks piled up by current-action, and after their formation plant remains accumulated and eventually buried them. It is often noticed, moreover, by the tilt of the coal-beds that no coal formed on the top of the banks until the hollows had been filled up.

If plant remains had accumulated on the floor of a forest we should expect to find fragments of the same kind of tree close together, but generally we find a thoroughly well- mixed-up mass of debris. Also we should expect to meet with many large tree- stems and complete leaves and branches. As a rule, however, we get a fine vegetable paste which encloses scattered plant remains.

It is true that we get roots in the underclay, but they do not connect with stems in the coal seams above.

The presence of underclay below a coal seam is not universal, and we also find underclay with roots in it covered by sandstones without any coal.

These are some of the reasons why many geologists are in favour of the idea that coal, at any rate in some places, may be looked upon as a detrital deposit just in the same way as a sandstone or a clay.

If, as Hutton has taught, we are to explain the past by the present, it is of interest to read what travellers have to tell us about regions where plant remains are now accumulating in great quantities. For in our own islands the only great mass of vegetable material which is forming is in peat-bogs, and as peat is not forming in the warmer regions of the world it will not do for us to imagine the Coal period as one when vast peat-morasses extended over this country, because there is no doubt that the temperature of our Coal period was fairly high.

Lyell points to the Virginian swamps as offering the nearest parallel to what must have existed to give us our coal. The supporters of the drift theory point to the delta deposits of the Mississippi; to Africa, where we are told of vast dense masses of floating weeds which stop naviga­tion up the great rivers; and to South America, which affords similar evidence.

It is to be noted that though one commonly speaks of the Coal period, there have been many periods during which coal has been formed, though none where it has been produced over such wide areas and in such abundance as during the Carboniferous period. We know of coal seams in the Devonian beds in Arctic regions, while in the periods succeeding the Carboniferous there is hardly one which has not some coal in it.

Even in Europe we find Permian coal in France and in Germany, Trias coal in Germany, Jurassic coal (still worked) in Britain, Cretaceous coals are known in America and Japan, while Tertiary coals are found in England, Germany, Hungary, and Russia.

The coal-beds, however, of the Carboniferous age are so thick and cover, even now after thousands of years of denudation, such wide areas, that one may be forgiven when one refers to them as being formed in the Coal period.

Taking our islands alone we find coal being worked in the following areas:

The Coal-Fields of our Islands:

In Great Britain we have the coal-fields of South Scotland, of Durham and East Yorkshire, of Cumberland and of Lancashire. The Mid­land district of England is rich in coal-fields, and Wales has coal-fields in the north-east and in the south. In South England the Somerset and Bristol coal-fields have been worked for long, and now coal-fields are being tapped far below the surface at Dover. In Ireland there is a coal-field near Tipperary.

It has been mentioned that in early post-Carboniferous days earth-movements took place which brought the Carboniferous beds above the sea-level, that the Permian period followed, during which extensive denudation occurred, and that during the succeeding Triassic period still more denudation took place.

The general result of all this was that the Carboniferous beds, that is, the coal-measures above, the millstone grit in the middle, and the carboniferous limestone below, were in many places thrown into a series of arches or troughs and the beds at the top of the arches were removed by the denudation, so that the lowest set of beds, the limestones, were exposed and the accumulations of later ages are now found resting on all three divisions of the Carboniferous beds.

Hence it is that the coal-measures are now found to the east and to the west of the Pennine Chain. The structure here being a huge arch, the central line of which is occupied by limestone and millstone grit. On the one side of this line are the coal-fields of Durham and East Yorkshire, while on the other side we have the coal-fields of Lancashire and that of Maryport and Whitehaven on the Cumberland coast.

In the English Midland district we find a series of up-folds of Carboniferous beds radiating from the broad end of the Pennine Chain, and the coal-measures are found in Leicester­shire, near Nuneaton, in Staffordshire, and in Shropshire.

In many cases the coal-measures, when they disappear under the overlying Permian and Triassic rocks, have been followed beneath that covering.

Farther south there is a basin-shaped arrangement of Carboniferous rocks in Gloucestershire, whose rim is formed by the limestone of the Mendips on the one side and of the Clifton Gorge on the other. Between these the coal- measures are worked in the Bristol coal-field.

It is to be noted that in this area not only did denudation of Carboniferous beds occur in early post-Carboniferous times, but that later on, in Jurassic times, further denuda­tion took place. For we find Lower Jurassic beds resting on Carboniferous strata. Hence in this region there were even more possibilities than in the north that the coal-beds should be washed away.

In South Wales another basin of Carboniferous beds runs east and west, and the important South Wales coal­field has been left at the bottom of the basin, and to the east of it lies the coal-field of the Forest of Dean in a con­tinuation of the same trough.

Scotch Coal:

In Scotland coal is worked along a line of country from Ayrshire on the west coast to Fife on the east.

The rocks to the north and south of this strip are of much greater antiquity, and the newer beds, forming the Carboni­ferous series, have been preserved from denudation by having been let down between two great faults which cross the whole of Scotland in a more or less north-easterly direction from the west coast.

In Ireland the absence of any pronounced folding of the Carboniferous beds has produced most marked results.

Irish Coal:

Speaking generally, the coast of Ireland is occupied, except in the north-east, by rocks older than the Carboniferous, and it is usually of a hilly nature. Inside this saucer-like rim the centre of Ireland-is chiefly occu­pied by a plain of almost horizontal and undisturbed Carboniferous beds, and there seems no reason whatever to doubt that at the top of the Carboniferous beds of Ireland the coal-measures were once upon a time laid down.

But, owing to the one fact that the beds have remained almost horizontal and have not been bent into folds like their British equivalents, denudation when it got to work has slowly and by now almost completely removed all traces of these coal-bearing beds. A few relics remain, usually on the higher grounds, and so we do find coal-beds in Leinster, but these coal-measures now cover only a very small area.

Kentish Coal:

Within recent years England has been startled by the news that men were boring for coal at Dover, and after many years of patient work the reward has come in the discovery, deep down under a thick covering of newer rocks, not only of the coal-measures, but of workable seams of coal in them.

A geological theory, which has often been tacitly assumed in the preceding pages, is that when older rocks disappear beneath newer ones they continue beneath them, though they may not be seen again at the surface for many miles. The obvious way to prove this theory is to dig down at some spot through the surface-beds and see whether one can find the older ones again below them.

Putting down a bore is, however, a costly matter, and, unless there is a likelihood of getting some return for the money spent, no one is likely to incur the expense.

The two things for which men in this country find it worth while to bore are water and coal, and our knowledge of rocks underground has been largely advanced by the attempts which have been made to bring up these two materials from great depths.

The coal-measures at the edges of our coal districts often disappear under newer rocks and coal has been extracted from beneath the latter for many years. All the surface rocks of the south-east of England are newer than the coal-measures, and if the coal seams were in regular continuous beds below these newer rocks any one digging down in the south and east of England would be bound to find coal.

But the matter is by no means so simple as this. As we have seen, since the days when the coal was deposited, England has been raised up and lowered down several times, her rocks have been bent into great folds and often shattered by faults, and certain parts have been subjected to an intense amount of denudation.

In consequence of all these causes her coal-fields are no longer continuous; they are separated sometimes by older rocks rising from below, sometimes by newer rocks covering them from above, and sometimes the newer rocks pass from coal-measures on to older rocks and then on to coal- measures again. Consequently digging for coal may be a mere speculation.

It is as though one were playing a game with five-pound notes and a blanket. Suppose a large blanket were cover­ing five five-pound notes and one could pay two pounds and be allowed one stab with a knife, and if one cut a note one could have it, and if one did not cut one the two-pound stake was forfeited.

Then the game would have some correspondence to boring for coal through the overlying covering of newer rocks. Nevertheless a knowledge of geology does make the searching for coal slightly less hap­hazard than would be the case if there were a complete ignorance of that science.

We have seen that the older rocks in the south of our islands in post-Coal-measure times were bent into a series of east and west folds. One result of this is that we find the coal-measures of South Wales and the Forest of Dean in an east and west trough, but these ancient primary rocks are covered by a thick blanket of newer rocks all the way east­wards from the Severn to Kent.

Godwin-Austen in 1855 suggested an underground connection between the coal-measures of South Wales and those which rise to the surface in France near Calais. For, eighteen miles south of that town, there are the old coal- works of Fergues, and an artesian well sunk at Calais passed into coal-measures at a depth of 1100 feet. He considered, moreover, that the most favourable places for reaching the coal-measures would be in the south of the Wealden area.

In 1866-71 a coal commission was in existence, and Prestwich, in an elaborate report, came to the conclusion that the coal-measures of North France and South Wales were represented in South-East England by long, narrow, east and west troughs far below the surface.

He mentions that in a boring at Kentish Town in London after 1100 feet of newer rocks, red sandstones, probably of Primary age, had been met, that at Harwich after 1000 feet of newer rocks had been bored through Primary rocks of a slatey nature were passed through, and that at Calais about 1100 feet of Tertiary and Secondary rocks had been proved to rest on Carboniferous strata.

At this time the Somersetshire coal-field was known to continue eastwards under a covering of newer rocks and coal had been actually extracted over an area of many square miles from beneath this overlying layer.

Prestwich concludes that it is probable that, in the area of the south-east of England included between Harwich and London, the Primary rocks would be found at a depth of about 1100 feet, and that between London and Bath the coal-measures, if found, would not be too deep to preclude profitable working.

The Battle Boring:

In 1872 a committee called the Sub-Wealden Exploration Committee was appointed and put down a bore near Battle. This passed through 1900 feet of Secondary rocks and never got down to Primary ones at all, and so pointed to the inadvisability of making another trial boring in the southern area of the Weald.

During the next eleven years many bores were put down, in and near London, and these revealed the presence under London of a buried ridge of Primary rocks over which the Wealden rocks were absent, and the oolites thin, the older rocks coming to within 1200 feet of the sea-level.

This fresh information made it clear that the best place for a new attempt to hit the coal-measures was in the region of the North Downs where both Wealden and oolitic beds thinned off against the old ridge of Primary rocks.

The Dover Boring:

In 1886 Professor Boyd Dawkins recommended the foot of Shakespeare’s Cliff at Dover as the best place for a new boring. The work here was con­tinued until in 1891 a workable seam of coal was proved. The coal-measures were struck at a depth of 1100 feet, and the bore-hole was continued for nearly another 1200 feet through the coal-measures without getting below them. In this thickness thirteen seams of coal were passed through, though several of them were too thin for profitable working.

This discovery proved the absolute correctness of Godwin-Austen’s theory in 1855 that there were coal- measures below the surface in Kent. It is true that his idea of the ease of getting to them in the South Wealden area was shown to be incorrect, and that it was along the line of the North Downs that success had attended the efforts of a later generation, but his main contention was triumphantly vindicated.

In 1897 a bore was put down at Ropersole, eight miles from Dover, and struck coal-measures at 1180 feet below sea-level. Again in 1901 a boring at Ellinge, five miles north-west of Dover, proved coal-measures at 1286 feet below sea-level.

At Brabourne, however, in 1899, a bore proved that the secondary rocks did not rest on coal-measures for after 1921 feet the bore entered Old Red Sandstone, which is an older rock than the coal-measures and shows that denuda­tion has removed the latter here before the Secondary rocks were laid down.

Knowing where the Old Red Sandstone is found in Somerset and North France, this knowledge of the occurrence of Old Red Sandstone below Brabourne is of advantage, as it enables us to give a very good guess as to the southern edge of the Kentish coal-field.

As coal is now being raised in Britain from depths of about 4000 feet, it is clear that the depth at which the Kentish coal is found presents no great difficulty in the way of its extraction. In the current year the first barge-load of Kentish coal has been despatched to London.

Deep borings in the south-east of England have now given us considerable information about the older rocks on which the Secondary rocks are resting. And it seems clear that just as in the west the older rocks are bent into a trough in South Wales, and into an arch in the Mendips, so the older rocks under North Kent form a trough or Syncline, and under South Kent an arch.

Under Tottenham Court Road, in London, and under Streatham, south of the Thames, is Old Red Sandstone, and this rock is also found below Erith to the east of London and below Richmond to the west. Beds of this age probably continue from Streatham under Maidstone and Ashford as they have been proved below Brabourne farther to the south-east.

But in the lap of this trough in the region of Dover the coal-measures have been proved, and also at Ellinge and Ropersole some miles to the north-east.

To the north of London, as one passes away from the middle of the trough, older and older beds come up to the base of the Secondary rocks. The Old Red Sandstone has been proved at Kentish Town and Turnford Green, Silurian beds of Wenlock age at Ware and at Bury St. Edmunds, while beneath Harwich Cambrian locks have been found.

There seems no doubt that in a very few years the south­east corner of Kent will be converted into a big coal-mining district. If this turns out to be the case it will be a most remarkable tribute to geological science.

As one wanders amongst the mountain districts of England, Wales, Scotland, and Ireland, one repeatedly comes across old workings in black shale and is told that they were made in the hopes of finding coal. If those who were responsible for this expenditure had only consulted a geologist before venturing on their efforts, they would have been told at once that those efforts would end in failure.

For these black shales, though at a distance they may look like coal, can be proved, either by their own fossils, or by those in the beds above and below them, to be of a very ancient date, far older than the Carboniferous period.

They were generally laid down beneath a deep sea where no vegetation could accumulate, and the only point in which they bore any resemblance to coal was their colour.

It is not too much to say that hundreds of thousands of pounds have been fruitlessly spent in these trials for coal, a waste which a moment’s conversation with a geologist would have prevented.

The Kentish coal-field shows a different picture. Here the geologist foretold the presence of coal more than a 1000 feet below the surface. The trial borings were put down under the eye and with the advice of a geologist, and success is beginning to attend the experiment.

But in spite of the discovery of a new coal-field it is easy to see that in time our coal will be used up. As so much of our commercial activity depends on coal it is obvious that we should be careful of its consumption. At the present rate of extraction it seems that we have enough to last for between one hundred and two hundred years.