227

SOME MINOR STRUCTURES IN THE ANTHRACITECOAL SEAMS OF SOUTH WALES.

[BEING A REPORT OF A LECTURE DELIVERED AT BEDFORDCOLLEGE ON 15TH JUNE, 1940.]

By EMILY DIX, n.se., F.G.S.

[Received 9th December, 1940.]

THE major structural units of the South Wales Coalfieldhave been described by many workers, including Jordan

(1908, p. I) and Strahan and Pollard (1915, p. 76), while much hasbeen written on this subject by officers of the Geological Surveyin the various memoirs dealing with the region. It has beenshown that the dislocation of the strata falls into three systems(Strahan and Pollard, 1915, p. 76) :- .

I. The nearly east-west folds, faults, etc., which traverseSomerset, Devon and the south of Ireland, and whichalso occur in the Vale of Glamorgan, Gower andPembrokeshire.

2. The west-south-west to east-north-east system whichruns for the most part north of the coalfield, but branchesof which traverse the Vale of Keath, Tawe Valley and partof the anthracite region.

3. The faults which range across the coalfield from northto south or from north-north-west to south-south-east.

Mining in South Wales, especially in the anthracite area, isoften very difficult owing to the variation in the Coal Measurestrata caused by the minor structures which affect the coal seamsand associated rocks. It is'possible to find an exceedingly variedseries of structures even within the limits of one colliery, whilethe boundaries of the colliery workings are usually determinedby one of the major structures affecting the Coal Measures.Mining surveyors and colliery managers have stated thatgeologists have not given these minor structures in the CoalMeasures the attention which they deserve.

MOVEMENTS DURING THE DEPOSITION OF THECOAL MEASURES.

Some of the structural irregularities now met with in miningbegan their history with the commencement of the depositionof the Coal Measures. Such movements which affected thedeposition of the strata have been investigated by manyworkers in South Wales, for example, by Davies and Cox (1922,p. 59), who proved that the local variations in the thickness of

228 E. DIX,

strata in East Glamorgansliire and Monrnouthshire arc of sucha nature that they can only be explained as the result of gentlefolds along north-west axes, active at some period between thedeposition of the Nine Feet and the NO.2 Rhondda Seams.Also east-west folds were in operation during Lower CoalMeasure Series times leading to a thickening of the strataalong the synclines and a thinning along anticlines. Davies(1923, pp. 611-622) continued these studies in the area from theCynon Valley towards Rhondda Fawr and, by plotting the thick­nesses of strata between particular seams at different places, hediscovered that the Nine Feet Seam was folded before theFour Feet Seam was deposited, and that the Four Feet Seamwas folded before the No. 2 Rhondda Seam was laid down.Mr. Stanley Jones has given the author data showing that thebottom coal of the Five Feet Seam at the Aberamman Collierywas folded, or the sediments were irregularly squeezed, beforethe Top Coal of the Five Feet Seam was formed and thatsubsequently the Top Coal was' folded' before the Seven FeetSeam was laid down.

Similarly in the area to the west, in the anthracite region,evidence of movements during Coal Measure times can bedemonstrated. D. Farr Davies (1932, pp. 24, 25) illustratesan example of earth movements taking place shortly after theStanllyd Seam was deposited in the Cross Hands Colliery,Carmarthenshire, where there is a bed made up of coal andcarbonaceous shale (rashes), 4 feet 8 inches in thickness on topof the Stanllyd Seam, with 1 foot 10 inches of coal above;this is locally known as the Hwch Vein. In a westerly directiona fault with a downthrow of 45 feet was met with at this colliery;the seam on the downthrow side maintains its thickness, butimmediately above it are 20 feet of clift (blue shale) followedby a thin bed of sandstone, 4 feet of shale and then the HwchVein. This clearly shows that the main movement along thisfault took place after the deposition of the Stanllyd Seam, andsubsequently 25 feet of sediments accumulated on the down­throw side before the Hwch Vein was laid down.

Further evidence of relative movement during Coal Measuretimes in the anthracite area, as in other parts of the SouthWales Coalfield, is shown by the splitting of seams and thethickening of strata between coal seams. It is generally acceptedthat the Big Vein of the Cross Hands area splits, when tracedeastwards, into the Rock and Little Veins of the Ammanforddistrict, where they are divided by from 33 to 36 feet of strata.Again, the thickness of strata between the Stanllyd Seam andthe Rocket Seam above is very variable. At collieries, withina radius of 5 miles, the distance between these seams varies from6 to 36 feet; the variation in thickness of the intervening

A"THRACITE COAL SEAMS OF SOUTH WALES. 229

measures cannot be accounted for simply on the basis of differencein the constitution of the sediments (Nelson, 1937, p. 677).

Robertson has indicated (1932, p. 193) that in the Merthyrdistrict east-west folding took place during the deposition ofthe Coal Measures, and that movements also occurred along thelines followed by the later north-north-west to south-south-eastfaulting. Several belts of contemporaneous folding and dis­turbance traverse portions of the Merthyr area in a directioneast-west, and north-west to south-east (p. 195). Comparisonsof the thicknesses show that there is an abnormality in mostcases both in the development of the coals and of the interveningbeds near the lines of the north-north-west to south-south-eastfaulting, for example, in the case of the seams of the Gellidegand Bluers Group near the Dowlais fault (op. cit., p. 104).

Similarly, Howell (1925, pp. 254, 255) has proved thepresence of minor folding in the Lower Coal Series at variouslevels in the East Glamorganshire and Monmouthshire areas,where the folds are gentle and their axes run approximatelynorth-north-west or north-west to south-south-east or south-east.There are many instances of minor folds in the anthracite areawith axes running north-west to south-east and also north­north-east to south-south-west, or north-east to south-west andalso north-south.

Recently the writer has been investigating some of the minorstructures encountered in coal seams and associated strata atthe Cefn Coed Colliery, near Neath. The plates and text figuresillustrate examples of some of these structures seen in theheadings in the Dulais and the Kine Feet Seams at that colliery,where a third seam, the Brass Vein, is also worked. There are33 to 35 feet of strata between the Brass and the Nine FeetSeams and 75 feet of strata between the Nine Feet and DulaisSeams; all these seams occur in the modiolaris zone.

MINOR STRUCTURES FOUND IN THE NINEFEET SEAM, CEFN COED COLLIERY.

Minor disturbances are particularly prevalent in the Nine.Feet Seam. The roof of the Kine Feet Seam consists of greyshales and was more or less uniform and normal over the wholeof the section, apart from the presence of much slickensiding.The bedding planes were brightly polished and signs of groovingcould be detected. The fire clay floor was quite normal. Theseam, usually about 6 feet in thickness, suddenly expandedto about 14 feet, but the crest of the fold was not proved. Inan easterly direction the seam was missing for about 120 feet,except for small lenticles of coal, which showed evidence ofhorizontal movement. The fireclay floor was normal where the

230 E . DIX,

seam was absent. Similar , but not identical, structures wen:encounte red in all easterly drivagcs in th e Nine Feet Seam .The polishing (slickensides) and grooving which frequentlyoccurred in the roof of t he seam must indi cat e different ialmov ement along the beddin g planes. There is much to suggestth at all th e seams at this coIliery suffered from pressure fromthe west , and it ap pears th at individual seams have yieldedunder horizontal stress in different places-thus th e Nine FeetSeam at the Cefn Coed CoIliery is characterised by these structureswhile th e Brass Vein is relati velv free from th em . In th e TareniColliery to the west, the Brass Vein exhibits th ese peculiar folds,etc., while the overl ying Kine Feet Seam is not affected to anyextent .

The folds are generally of low amplitude, which againsugges ts that small thicknesses of st rata have yielded under

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STRUCTURE. IN "UtE FEET SEAM '!tOuTH SlDE MAIN DRIfT CEFN COED COLL.1E.R'Y,

F I G. 20.

horiz ontal stresses. It should be noted that this feld has anorth-south trend, and it is eviden t that th e pressure resultingin th ese structures in th e Nine Feet Seam came from the west.

The tectonics of this fold (Fig. 2 0) in th e Nine Feet Seam,resembl e th ose of th e reck faults or ' pressure belts' describedby Nelson (1937, p. 677) in th e western part of the ant hracitearea . He indicates that th ese belts, where proved, are somewha tirr egular, but that th ey tend to have a curved lozengeoutline. The width of th ese pressure belts varies up to a provedmaximum of 150 yards, but th ey exceed this distance in length.One of these pressure belts explored in the Lower PumpquartSeam extended for a dist ance of 140 yards. A band of iron­st one 3 inches in thickness and 4! feet above the coal cont inuedconsist ently throughout the disturbed area. After passingthrough this pressure belt, t he coal searn, normally 3 feet in

ANTHRACITE COAL SEAMS OF SOUTH WALES. 231

thickness, gradually increased in thickness up to over 13 feetalong the length of the disturbance.

It appears that in such disturbances, as pointed out byNelson (1937, p. 678), the fireclay floor remains normal and runsuniformly across the disturbed ground; the roof shows littleor no deformation, except gentle flexures, but the beddingplanes are finely polished (slickensided) and sometimes striatedand furrowed, owing to horizontal movement. The disturbanceis accompanied by abnormal thickening of the coal and softshales along the fringe of the belt remote from the direction ofpressure (in the case of the Nine Feet Seam, Cefn Coed Colliery)from the west. The attenuation or total displacement of theseam is brought about by the gradual lowering of the strongroof with a natural bending of all the individual layers.

DI~TURBED COAL IN TOP COAL OF NINE FEET SEAMIN ONE HEADING CErN COED COlLIER'i

FIG. 21.Vertical Scale 1'" = l ' (approx.)

Another feature of these rock faults or 'pressure belts(see p. 234), is that their trend is usually almost coincidentwith that of the normal faults-north-south or north-north-westto south-south-east-that is, they have the same general trendas the slips. It seems evident from the instance of the fold in theNine Feet Seam described above, and from the example givenby Nelson (1937, pp. 677-8), that anthracite could flow underpressure, so that the coal has been removed from between twocompetent layers and has accumulated to form a thick massof coal in the area away from the direction of pressure. Even

PROC. GEOL. Assc c., VOL. LII, PAf-T 3, I<)4I.

232 E. DIX,

so, the banding of dull and bright coal appears to be preservedin the accumulated mass.

Fig. 21 illustrates another structure seen in the NineFeet Seam, at the Cefn Coed Colliery. In this heading, witha north-east to south-west trend, the bottom coal is 4 feet 6 inchesin thickness and consists of good coal except for the presence ofslips at rather frequent intervals of 6 or 7 inches. Cleaved coaland carbonaceous shale rest on a smooth upper surface of thebottom coal. This material arches upwards towards thesouth-west and passes into a fold; this fold has a north-westto south-east trend. Some excellent, small, normal faults couldbe seen, while the fold itself was replaced by contorted coal andcarbonaceous shale and packed, cleaved, slickensided carbona­ceous shale. The under surface of the overlying roof of theseara, which was made up of blue shale with coal lenticles,was highly polished and slickensided. This appears to be adear example of folding and faulting in the incompetent softertop coal and the carbonaceous shales between the competenthard roof of the seam and the strong coal of the bottom coal.Strahan (1897, p. 66) has illustrated similar folding in incompetentbeds (the parting between two coal seams) at Tirbach, Banwen,and recently Mr. C. ]. Davies, of Garnant, has informed thewriter that there are numerous examples of such structures inthe western part of the anthracite field (see Fig. 22). Sometimesthe rashes or carbonaceous shales between the good coal andthe roof exhibit these puckerings, while folds sometimes occurin the fireclay floor (Fig. 23). More rarely the folds occurin the anthracite itself. Kelson (1937, p. 676) has drawnattention to flexures in the Middle Pumpquart Seam of theAmmanford district, while J. H. Davies (1927, p. 108) hasnoted the prevalence of folding in incompetent strata associatedwith coal seams, for example, in the Big Vein, Brynamman.

Folded anthracite is particularly well developed in certainareas worked in the Nine Feet Seam at the Cefn Coed Colliery.These folds are generally confined to the top coal. Similarfolds also occur in the Dulais Seam, although they are lessfrequently developed than in the Nine Feet Scam. These foldsexhibit all the features of alpine tectonics on a miniature scale.They range from slight flexures to symmetrical folds, asym­metrical folds, recumbent folds, and small nappes, which havebeen carried forward along thrust planes. (PIs. 16, 17.) Thebedding planes of the coal are often markedly slickensided andhighly polished, and many of the folds exhibit movement alongthe bedding planes almost at right angles to the axes of thefolds. The detailed mechanics of the folding require furtherinvestigation. (These ready-made fold models should find a.home in all University and Museum Collections.) Small normalfaults are often found in association with these folded beds.

PROC. GEOL. Assoc., VOL. LII (1941). PLATE 16.

2

3

FOLDING IN AliTHRACITE.

[To face p. 232.

ANTHRACITE COAL SEAMS OF SOUTH WAL E S. 233

. The ch.a~a~teristic .band~ng of bright and dull coal (some­hmes . exhibiting durain, vitrain, clarain and fusain) can beseen Ill. these folded sJ?Ccimens, indicating th at th e stressesresponsible for the foldmg were not sufficient to destro y theoriginal banding of the coal (see PI. 16). Where the folding has

BLUE SHALE.

COAL.

FIRECLA'f.

FI G. 2 2.-DIAGRAM TO ILLUSTRATE FOLDING IN S OFT SH ALEBETWEEN TH E COAL AN D THE H A RD ROOF OF A S EAM.

8l..UE SHALE.

FIG. 2 3.-DIAGRA~1 TO I LLUSTRATE F OLDING I N TilE FIRECLAYFLOOR Ul'DE RNEAT H A COA L S E AM.

been particularly intense, the coal becomes friable, and passesinto powder (culm), similar to the extensive powdered coal orculm developed on such a large scale in Pembrokeshire. Thissuggests that under these conditions the anth racite lost it splasticity and became crushed to powder.

MINOR STRUCTURES FOUND IN THE DULAISSEAM, CEFN COED COLLIERY.

Rem arkable structures are sometimes encountered in theDulais Seam at the above colliery. Some variations in thecoal faces of thi s seam were examin ed by the author. TheDulais Seam usually consist s of 4 feet 6 inches of bottom coal,showing the characteristi c slips. overlain by a parting ofcarbonaceous sha le (about 6 inches) and above this parting,

234 E. DIX,

a top coal, about I foot 8 inches in thickness. The roof ofthe seam generally consists of grey sandy shales. The partingbetween the bottom and top coals varies in thickness from6 inches up to 3 feet, and varies in lithology from fireclay tocarbonaceous shale or sandy shale, while occasional lenticlesof hard sandstone are found. In another section the top coalwas normal except for the presence of very frequent slips whichare often well developed in disturbed areas of any coal seam.The bottom coal was g feet thick and it exhibited minor foldsand faults and some excellent examples of miniature overthrustfaults. In an adjoining coal face, the top coal had a normalthickness, but the slips were curved. Here the parting wasparticularly interesting-it consisted of sandy shale and somecurrent-bedded sandstone, and contained a lenticle of coalwhich showed a fold with a north-west to south-east trend,while the coal was also faulted. The bottom coal was 5 feet inthickness and showed several irregular fractures. The occurrenceof this fold in coal in the parting between the top and bottomcoals requires explanation.

SLIPS.One of the most interesting features of the coal seams

themselves is the development in them of a series of jointscommonly known as slips. These occur in all seams of SouthWales (see Fig. 24) at a distance varying from 6 inches to 6 feet,but usually at intervals of It to 2 feet. The distance varieswith the character of the seam. As a result of these joints, thecoal occurs in diagonal layers throughout the coalfield. Thedirection of the working face is usually determined by the slips,the face being parallel to the strike of the slips. As a rule,working in the opposite direction to the dip of the slips(on the back) is preferred, as the coal overhangs if the seamis worked in the direction of the dip of the slips, on theface (see Figs. 21, 24). When the coal is worked on theface it usually comes away readily from the roof.

In general (especially in the east of the coalfield) there isno displacement, crushing nor slickensiding along the slip faces,but, in some parts of the anthracite field, movement has occurredand displacement varies up to 2 inches; slickensiding also cccursin some cases. The strike of the jointing (slips), except inproximity to structural disturbances, is parallel to that of thenormal faulting, that is, almost north-south in the west of thecoalfield and north-north-west to south-south-east in theMerthyr district and the east of the coalfield. Robertson statesin the Merthyr memoir (p. 2Ig) that the angle of dip of the mainslips with reference to the bedding planes in areas of uniformdip of the strata, is 55 to 57 degrees to the west. In all normal

PROC. GEOL. Assoc., VOL. LII (1941). PLATE 17

FOLDING IN ANTHRACITE.

[To face p. 23+.

ANTHRACITE COAL SEAMS OF SOU TH W ALES. 235

ground, in the South Wales Coalfield , these slips' dip ' fromeast to west. ]. H . Davies has collected data concerning theslips in collieries in t he Ystalyfera and Gwauncaegurwen districts(north of Swansea), and he sta tes (1927, p. 93) that th e hade ofthe slips t o the vertical varies in this area from 25 to 42 degrees.Mr. C. .J. Davies has informed the writer that th e angle of the

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FIG. 2 4.- D IAGRAM:>IATIC S E CTION OF A NOR:>IAL C OAL SEAMS HOWIN G SLIPS.

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FIG. 25 .-DIAGRAM~IA1.'IC S ECT ION OF A C OAL SEAM SHOWING SLIPSPENETRATING TH E R OOF AND FLOOR OF A COAL SEAM.

slips to the vertical in the western part of the anthracite field(Carmarthenshire) varies from 40 to 60 degrees.

In some cases it can be demonstrat ed that the slips becomecloser together as disturbances are approached, but th ey remaininclined at the same angle. Slips in the Hughes Vein, Yn ysfechanColliery, hade from 25 to 32 degrees in the bottom and t opcoals, which are about I foot 2 inches and 1 foot 8 inches in

E . DIX,

thickness respect ively. These coals arc separa ted by a thi ckband of ' stone ' which is pract ically free from slips, but the facesof both coals continue in the same plane, although the' stone 'intervenes (J. H. Davies, 1927, pp . 93- 4 ; comp are Fig. 26).

Occasiona lly in the area north of Swansea and in Carmarthen­shi re, slips occur which traverse the roof, seam and floor of theseam, and examples are known where each slip represents asmall normal faul t with a displacement up to about 2 inches

E W.

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SS'Sp;:,s)COAL.

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FIG. 26.- D IAG RAM TO ILL USTRATE S LI PS I N T HE BOTTO M Al'D TopCOALS, B UT ABSE NT I N THE HARD PART I NG B ETWE E N THE BOTT OM

AND T op COALS IN A COAL SEAM .

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F I G. 27.-D IAGRAM TO ILLUSTRATE SLIPS (N OR MAL FAULTS ),PENETRATING TH E ROOF AND FLOOR OF A COAL SEA M.

(Fig. 27). In th e Lower Vein, Ystalyfera (Davies, 1927,P· 94), .the slips are narrow and cur vilinear , an d as a result the coal iswon in relatively small lumps. Very rarely, in abnormal ground,the slips of the bot tom coal of a scam hade in one direction,whil e those of the top coal hade in the opposit e direction .Sometimes false slips occur in disturbed ground obliteratingthe norm al slips. At the Cwmgorse Colliery, Davies (1927,p. 93) notes that in one district the slips t raverse an an ticli neand syncline without any change in directi on or inclination .

ANTHRACITE COAL SEA;\IS OF SOGTH WALES. 237

The slips are sometimes lined with calcite or ankerite (forexample, the Four Feet Cornish Seam at Ystradgynlais-e­Robertson, 1932, p. 219) and occasionally with powdered coal.

It is interesting to note the variation in the extent to whichthe slip faces have been developed. In some cases they areinconspicuous, while in others the coal simply" jumps" off them..Generally the slips do not penetrate the roof of the seam andthe coal comes away, leaving a flat even surface on the under­side of the roof. Usually the movement of the slips over oneanother has been very slight, and when striae occur theyalways indicate movement along the full dip of the face, whichsuggests that vertical pressure may have played some part intheir origin or that the slips were produced as the result of move­ments of elevation or depression or a combination of both,assisted by twisting stresses. When were the slips introduced?There may be some connection between the vertical jointingknown as cleat (see below) and the slips, because as Mr. \Y. D.Ware has observed at the Cefn Coed Colliery, where the one iswell developed, the other appears to be less frequent. Thestrike of the slips is coincident with that of the normal faultingand they may have originated at the same time.

BACKS OR RIDERS.Another type of structure commonly met with in the anthracite

area as well as in other parts of the South Wales Coalfield, forexample, in the Merthyr district (Robertson, 1932, p. 220), isknown as a ' Back' or 'Rider.' They are crush-fracures andfrequently are inclined at an angle of about 45 degrees to theroof and the floor of the seam. They are much more irregularthan the slips in development, in direction and inclination.They generally occur, however, in two more or less distinctsets striking at right angles to one another, and the strike ofone of these sets frequently forms an angle of 35 to 45 degreeswith the strike of the slips. There is usually much powdery coaland sometimes soft shale, occasionally contorted, in the Backs.Both the Lower Pumpquart and the Nine Feet Seams are proneto these structures.

CLEAT.If one examines a piece of ordinary bituminous coal, several

facts can be notified. The coal is laminated, it divides easilyalong the bedding planes, and it also breaks into rectangularblocks by well-marked joints at right angles to one another.One set of joints runs in a series of short steps of rarely morethan an inch in extent across the end of the block, giving anirregular appearance. The other joint is longer and gives a

E . »ix.

well-m arked face, along which sparr y films of ca lcite, ankerite ,ctc ., occur in th e better class coa ls, and occasional films ofbr assy-looking pyrites in th e more sulphurous types of coal(Ken dall , 1914, p . 49 ; 1922, p. 71 ; 1924, p . 130; K endall andBriggs, 1932-3, p . 164; Davies, 1927, p. 95 ; North , 1931, p. 104 ;R aist r ick an d Mar shall, 1939, p . 42). These fractures are kn ownas cleat . Kendall has shown th at the direction of the majorfr acture, namely. th e face or ba rd of the coal , is remarkabl yconsta n t in directi on varyin g within a few degrees of north­west and south -cas t (1924, p. 131).

Clea t is characteristic of coa ls in all the coalfields in GreatBritain, but it is mu ch more clearlv seen in the bituminous sea msof coa l in Sou th Wales t ha n in t he an thracitic coals. Robertson(1932, p . 220) states in a mem oir dealing with the Merthyrdistrict that "The ordinar y fine cleavage or cleat of coa l isnot so well developed in the seams of this district as in the morebituminous coals, and it becomes st ill less apparent westwards.It does exist, however, t o a cer t ain degree, and appears tocoin cide approximately in direct ion with the slips, but notalwa ys ." (See also Davies, 1927, p . 95.)

Mr. C. ]. Davies, who has an intimate knowledge of thean thracit e seams of Carmart hcnshire, informed the writ er thatthere is evidence of the vert ica l cleat as well as t he slips in theseseams, alt hough it is genera lly believed that apart fro m theslips an thracite br eak s with a conchoida l fracture. Clea t isvery clea rly seen in the near-anthracit e sea ms of th e DulaisValley between t he Nea t h and Tawe Valleys, but va ries con­sidera blv in high-rank anthracites.

Kend all , howeve r (1922, p . 76) states : " I allude to theabs ence of cleat that characterises anthra cite, the world ove r ,and is the ba sis for t he broad classification of coa ls in theUnited St ates into cub ica l an d non-cubical coals." AgainRaist rick and Marshall (1939, p , 44) also point out t hat clea tis absen t in true ant hracite, th at calcite , etc., occurs on thecleat faces and th at , in the case of anthracit e free from cleat ,this fraction of the ash is missing. Calcite and other mineralswith a whitish colour are commonly seen along the cleat facesin the near-an t hrac ites of th e Dulais and Neath Valleys . andth ey occur less frequently westwards; but in one of the bestanthr acit ic seams, the Lower Pumpquart Seam of the Pontyb eremdistrict, the cleat appears to be absent, and naturall y theminerals found along the cleat faces are missing.

It is perhaps of interest t o note here that one rarely seeseve n the thinnest coal film on the plant remains found in theroofs of seams in the anthracit e area . In bituminous ar eas, forexample, in th e Warwicksh ire Coalfield. where some of theseams con tain up to 45% vola ti les, thick films of coaly material

ANTHRACITE COAL SEAMS OF SOFTH WALES. 239

can be seen on the fragments of plants, leaves, stems, etc., in theshales overlying the coal seams.

Cleat is present in the thinnest stratum of coal. Kendallstates (1922, p. 71) : " It is a vital element in the cleat problemthat it is as well developed and as definite in direction in aflake of bright coal, one-hundredth of an inch in thickness,as in a tree-trunk." The thin film of coal which represents theremains of a cuticle of a fossil plant, is seen, when minutelyexamined, to be as regularly jointed as the thickest seam.Kendall also notes the presence of well-defined and regular cleaton shiny films of probably not more than one five-hundredthof an inch in thickness. Cleat is sometimes seen at an intervalof about one-hundredth of an inch (Kendall, 1922, p. 71) ; butit may be even less, as for example, in a fragment of coal enclosedin a quartzitic siltstone boulder, found in a coal seam in SouthWales, Even so, cleat is almost always constant in direction,and in British coalfields, with comparatively few exceptions,the cleat runs within a few degrees of north-west to south-east.

As Kendall has observed (1922, p. 72) cleat is quiteindependent of the joints traversing the shales and sandstonesof the associated measures. When was cleat introduced intoour coal seams? This problem has been investigated by Kendall(1914, pp. 49-53; 1922, pp. 71-8) ; and by Kendall and Briggs(1932-3, p. 167) ; see also Raistrick and Marshall (1939, pp. 42-5).All these authors are of the opinion that the cleat was introducedsubsequent to the production of stream channels as well as thephenomena on the margins of wash-outs. Every pebble andfragment of coal in the displaced masses show a well-developedcleat, maintaining the same direction as the cleat in general.As Kendall has pointed out (1922, p. 73) cleat may be partlydependent on the original state of the material, for it appearsthat the finer the particles forming durain, the less the tendencyto cleat. He states (1922, p. 73) : " Fusain, being already greatlydecomposed, would not be as brittle and would not cleat soreadily." There is often marked variation in the cleat even inone and the same seam in a short distance; for example, in theBrass Vein at the Cefn Coed Colliery, some of the coal is closelycleated, while some of it is conchoidal.

It has been suggested above that there may be s.-meconnection between cleat, slips and the normal faulting, and itis likely that original differences in the characters of the coalsare of minor importance compared with the stresses which weresubsequently introduced and which gave rise to these structures.Kendall and Briggs, however, came to the conclusion that jointsor cleat were formed in a bed not long after the bed wasdeposited. They suggest (1932-3, p. 187) that joints or cleatwere formed as follows :-" A bed or series of beds is laid down

E . DIX ,

on stra ta th at ar c already joint ed . At first , soft enough 0 1'

incoheren t enough easily to adjust itself to the tid al wave withoutbreaking, it is sooner or lat er covered up and gradua lly becomesconsolidated by pressure, loss of moisture, and inflow ofcementing material, and, with. coal, by chemi cal changes.Losing it s initial plasti cit y and gaining brittleness, it t hen find sitself called upon to flex as a continuous elastic sheet un derthe alte rnating tidal tor que. Event ua lly it fail s throughfatigue along th e planes of maximum shear, and t he jointst hus come into existence."

WASH-OUTS.The majority of "wash-outs " or barren areas (as defined

by Raistrick and Marshall, 1939, p. 80) present in the coal s ofthe western part of the South Wales Coalfield ar e du e to tectonicph enomena (see pp. 230, 23I). Nevertheless, wash-outs occurwhich undoubtedly owe their origin to the effects of erosion of thepeat , for instances are known where the coal has been replaced bya black , but not coally sha le, containing lenticles of cannel andcast s of L epidostrobus spp. Th e " wash-outs" or " barr en ar eas "or " rock faults" are aligned in vari ous directions, .bu t usuallythey are parallel to the norm al faults, north-south, ' or north­north-west to sout h-sout h-east , a lt hough some of them hav e atrend which is almost eas t-west and ot hers trend north-eastto sout h-west. Th ey va ry in extent , and they affect one seamand the associate d measures ; sometimes one seam shows th esedisturbances and sometimes an other , alt hough certain seamsare particularly prone to t hese phenomena , for example, th eBrass and Nine Feet Seams (see Davi es, 1927, p . lIO).

Fearnsides (I9 I6, p. 573) has described in det ail the charac te rsof a number of barren areas or rock faults in the YorkshireCoalfield , and he points out t hat such st ructures affect indi vidu alseams , and only local a reas of these seams, and th at they aremost frequent in coa l seams which have a roof of weak clod orbind (shale), forming a thin rip ping between the top coa l anda mu ch st ronge r overly ing mass of 'stone' or sandsto ne. Heattributes these st ruc t ures to the effects of horizontal movem entsdu e to pressure at a time when the deposition of the measureswhi ch contain the seam s had been completed.

Kendall also investi gat ed t hese "wash-outs" in seams inthe Yorkshire Coalfield , and he discusses (1922, pp. 68-71) theorigin of these peculiar features which affect particular seamsand which are aligned in th at coalfield in directions north-south,east-west , north-east to south-west , and north-west to south­eas t. H e interpreted these peculiar barren ar eas, which charac ­t erise th e Haigh Moor Seam in particular, as well as certain

ANTHRACITE COAL SEAMS OF SOUTH WALES. 241

other scams in the Yorkshire Coalfield, as the result of the effectsof extensive arid repeated earthquake activity, the main partof the movement taking place after most of the sedimentsbetween the Haigh Moor Seam and the next above had beendeposited, but before the succeeding seam, which has notsuffered the effects of these movements, was formed.

In South Wales, it appears that, as a result of horizontalmovements affecting competent and incompetent beds, oneseam has yielded sometimes at one place, and sometimes atanother, in a not distant area. While it is possible that seismicphenomena, as described by Kendall (1922, pp. 69-70) may beresponsible for some of the structures met with in the Nine FeetSeam (for example, the sandstone lenticles in the seam, and thelocal rapid changes in the character of the top coal, which is some­times replaced by shale and at other places by good coal), yet thewriter believes that the barren areas or rock faults noted aboveare due to tectonic movements. It is interesting to note here"in view of Kendall's investigations, that the Nine Feet Seamat the Cefn Coed Collierv, which is about the horizon of theHaigh Moor Seam of Yorkshire, often exhibits both barren areasor belts of disturbed measures and other structures which may beseismic in origin.

It is difficult to ascertain the time when the movementswhich caused these disturbed tracts in coal seams of this areaoccurred, but it is probable that they took place some time afterthe Eighteen Feet Seam and the associated strata were con­solidated, and it may be that these structures were introducedinto the seams of the Lower Coal Series in pre-Pennant times,and that they were further complicated by the stresses whichaffected the rocks of this region at the end of the Carboniferous.It is a well-known fact that the higher seams of the Lower CoalSeries are comparatively undisturbed (see Davies, 1927, p. no).However, occasional disturbances are found in these higher seams,for example in the Red Vein (Davies, op. cii., p. 107), andmonoclinal folds are sometimes encountered in that seam, andtherefore the absence of the great development of these rockfaults or barren areas in the higher seams may be due in partto the cover of the competent Llynfi and Pennant Sandstones.

The fact that these belts of disturbance are not confined to aparticular seam in a comparatively small area, and the prevalenceof slickensides and horizontal grooving appear to refute thetheory that the structures are due to seismic phenomena shortlyafter the seam was deposited. Considerable movements havetaken place along bedding planes, and these have been influencedby the competency of the beds, the softer shales and coalsyielding more easily.

E. DIX,

CONE-IN-CONE STRUCTURE.Cone-in-cone structure is often well developed in seams in

the Pennant Series, especially where the seam is overlain by asandstone roof. This structure is more often seen in bituminouscoals. It occurs rarely in anthracite, but generally in anthracitesof low rank (near anthracites). Robertson (1932, p. 220)points out that, in seams of the Merthyr district, the powderycoal and soft shale in the Backs frequently pass into, or areassociated with, an irregular type of cone-in-cone structure,that is very prevalent in semi-anthracitic coals. He believesthat these structures seem to be connected with the brittlenessof the transitional coals (for example, in the Aberdare district).]. H. Davies (1927, p. 95) records the occurrence of cone-in-conestructure in the anthracitic coals of the Ystalyfera andGwauncaegurwen districts, and he suggests that the structureis due to stresses which caused internal deformation of the coal.Cone-in-cone structure also occurs in the Brass, Nine Feet andDulais Seams at the Cefn Coed Colliery, in the Dulais Valley,near Neath, where the coals are anthracitic, although not of highrank. In some of the specimens obtained from these seams itappears that this peculiar structure is related to stresses whichcaused slickensiding and deformation of the coal. Evidenceof shear can be seen on the sides of the imperfect cones, whichdo not pass through the thickness of the seam, but appear tobe terminated by a band of hard coal or iron pyrites, the undersurface of which is polished or slickensided. Radial striationscan be seen on the sides of the imperfect cones. It is probablethat this structure was introduced into the coal at the abovecolliery, when all the strata were subjected to stresses resultingin the folds, faults, etc.

The cone-in-cone structure found in the bituminous seamsappears to have a different origin. In such cases the apicesof the cones point away from the roof and floor, and it is possiblethat their origin may be related to the stresses of the superin­cumbent load of sandy material, and to the escape of gases fromthe coal through the porous sandstone.

The author wishes to record her sincere thanks to Mr. C. ].Davies and Mr. \V. D. Ware for information and discussion onthis subject, to Mr. Lewis, the manager of the Cefn Coed Colliery,and to Mr. Ware for facilities for examining sections underground,and to Dr. H. D. Thomas for reading through this paper inmanuscript.

Seventeen members and friends attended the meeting.The lecture was illustrated by lantern slides and exhibits,including numerous specimens of the varied folds found in

1897. The Geology of the South WalesThe Country around Merthyr Tydfil. Mem,

ANTHRACITE COAL SEAMS OF SOUTH WALES. 243

anthracite at the Cefn Coed Colliery, and other structures, e.gcone-in-cone, found in seams of the South Wales CoalfieldAfter a discussion on various aspects of Coal Measure geology,tea was served in the Library. A vote of thanks was proposedby Mr. Bell to Miss Dix for her lecture and hospitality andto Mr. S. Ware for his services as lanternist.

REFERENCES.DAVIES, D. FARR. 1932. Presidential Address. Proc. S. Wales Inst.

Eng., vol. x lviii , p. 73.DAVIES, J. H. 1927. The Geological Structure of the Strata of the

Ystalyfera and Gwauncaegurwen. Proc. S. IVales l nst, Eng.,vol. xliii, p. 73.

DAVIES, R. 1923. Some Effects of Intra Coal Measure Movements: AFurther Studv of Thickness Variations in the Lower Coal Seriesof East Glamorganshirs. Proc. S. Wales Inst. Eng" vol. xxxviii,p.6Il.

DAVIES, R, and A. H. Cox. 1922. On Thickness Variations in the LowerCoal Measures of East Glamorganshire and Monrnouthshire.Proc. S. Wales Tnst, Eng., vol. xxxviii, p. 41.

FEARNSlDES, W. G. 1916. Some Effects of Earth-Movement on theCoal Measures of the Sheffield District (South Yorkshire) andthe neighbouring parts of 'Vest Yorkshire, Derbyshire anaNottinghamshire, Part 1. Trans. Lnst, Mining Eng., vol. i,Part iii, p. 573.

HOWELL, A. 1925. Minor Folds in the Lower Coal Measures of CertainPortions of the East Glamorgan and Monmouthsire Coalfieldand their relation to the Valleys of the North Crop. Proc: S.Wales Inst. Eng., vol. xli, P.243.

JORDAN, H. K. 1910. The South \Vales Coal-field: Sections andNotes. Proc. S. Wales In.st, Eng., vol. xxvi, p. 1.

KENDALL, P. F. 1914. On' Cleat' in Coal-Seams. Geol, Mag., vol. i,P·49·

---- 1922. The Physiography of the Coal Swamps. Report Brit.Assoc. for Advancement of Science (Hull), p. 49.

---- and H. BRIGGS. 193=-3. The Formation of Rock Joints andthe Cleat of Coal. Proc, Roy. Soc. Edin., Part II, vol. liii, p. 1b4.

---- and H. E. \VROOT. 192+ The Geology of Yorkshire, vol. i.NELSON, A. 1937. Major and Minor Disturbances in the Anthracite

Area of the South Wales Coalfield. Colliery Guardian, vol. cliv,No. 3980, P: 675·

NORTH, F. J. 1931. Coal and Coalfields in Wales. znd Edit. Nat.Mus. Wales.

PRINGLE, J., and T. N. GEORGE. 1937. British Regional Geology SouthWales. Ceol. Surv.

RA1STR1CK, A., and C. E. MARSHALL. 1939. The Nature and Origin ofCoal Seams. English. Uniu. Press Ltd.

ROBERTSON, T. 1933. The Geology of the South \Vales. Coalfield,Part V, The Country around Merthyr Tydfil. Mem, Ceoi, Suru,2nd Edit.

STRAHAN, A., and others.Coalfield, Part V.Ceol. Surv.

STRAHAN, A., and POLLARD, W., 1915. Tl.e Ceals of South Wales withSpecial Reference to the Origin of Anthracite. Mem. Geol; Suru,and Edit.

244 AN TH RAC ITE COA L SEAMS OF SO UTH WALES .

F ig . 1.

Fi g . 2.

Fi g . 3·

EXPLANATION OF PLATES 16 AND 17.

P L ATE 16.Sy m me t rica l fold in a nthr aci t e fro m the Ni ne Feet Sea m, Cefn

Coed Colliery, So uth Wal es, x I (approx .). Note t he slicken ­sided bedding planes a nd the banding of the coa l.

R ecumbent fold in a n t hra cite from the Nine Feet Seam. Cef nCoed Colliery, So ut h ' Va les, x I (approx.). Note t he ba ndsof bright a nd d ull coal.

':-'Ionoclinal folding in a n thracite fro m the N ine Feet Seam,Cefn Coed Colliery, South ' Vales, x I (approx .) . Note thebands of bright a nd dull coal.

P LAT E 17.

Folding in a nt hracit e from the Ni ne Feet Scam, Cefn Coed Collie ry ,Sout h Wales, x ! (a pprox) .