UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

7 

McCulloch, Charles M 

Cleat in bituminous coalbeds, by C. M. McCulloch, 

Maurice Deul, and P. W. Jeran. [washington] U.S. Bureau of 

Mines [1974] 

25 p. illus., tables. (U.S. Bureau of Mines. Report of investigations 7910) 

Includes bibliography. 

1. Bituminous coal, I. U.S. Bureau of Mines. 11. Deul, Maurice, 

jt. auth, 111. Jeran, Paul W., jt. auth. IV. Title. (Series) 

TN23.U7 no. 7910 622.06173 

U.S. Dept. of the Int. Library 

J 
CONTENTS 

Page 

Abstract ................................................................ Introduction ............................................................ Acknowledgments ......................................................... Cleats (or joints) in coal .............................................. Previous work on origin of cleats ....................................... Survey of field and laboratory relationships of cleat orientation ....... Current studies ......................................................... Pittsburgh coalbed ................................................. Lower Kittanning coalbed ..........................................a 

Lower Hartshorne coalbed ........................................... Subseam 3 of the Castlegate group .................................. Pocahontas No . 3 coalbed ........................................... Directional permeability related to coal cleat .......................... Conclusions ............................................................. Bibliography ............................................................ Appendix.--Glossary of terms ............................................ 

ILLUSTRATIONS 

Location of mines superimposed on structure of base of Pittsburgh 

coalbed .......................................................... Coal cleat orientations in seven mines operating in the Pittsburgh 

coalbed showing cleat rotation ................................... Structure of Pittsburgh coal in area of Shannopin mine and coal 

cleat readings ................................................... Generalized structural contour map of area around Cambria 

Nos . 32 and 33 mines ............................................. Strike of coal cleats, rock joints, and regional lineations ........ Structure of the Lower Hartshorne coalbed .......................... Generalized structure map in vicinity of Carbon Fuel No . 3 mine, 

Carbon County, Utah .............................................. Generalized structure of Beatrice mine showing structural contours 

and Keen Mountain fault .......................................... Stereonet analysis of cleat orientation 4, 000 feet on either side 

of the Keen Mountain fault in the Beatrice mine .................. Horizontal holes drilled into Lower Kittanning coalbed (Cambria 

No . 33 mine) subparallel to cleat direction ...................... Map of Federal No . 2 mine showing horizontal pressure measurement 

holes ............................................................ Location of bleeder holes and gas wells in multipurpose shaft ...... Initial gas flow rates from horizontal holes drilled in virgin 

coal ............................................................. 

TABLES 

1 . Surveyed mines in Pittsburgh coalbed and their major cleat 

directions ....................................................... 2 . Initial data on the seven degasification holes ..................... 
CLEAT IN BITUMINOUS COALBEDS 

C. M. McCul loch, 1 Maurice Deul,2 

and P. W. Jeran 

ABSTRACT 

The natural vertical fracture sys tem in bituminous coalbeds is called 

cleat. Cleat orientation commonly controls the direction of mining with 

major development paralleling the face cleat. 

Previous researchers have categorized the origin of cleat as endogenetic, 

relating the origin of cleat to compaction and coalification, and exogenetic, 

relating the origin of cleat to tectonic forces. In the coalbeds studied for 

this report it was found that tectonic forces were the controlling factor of 

cleat formation. Face cleats were formed as extension fractures during 

structural deformation, and butt cleats, as release fractures during erosion 

and uplift. 

The face cleat of the Pittsburgh coalbed rotates from N 80" W in northwestern West Virginia to N 57" W in southwestern Pennsylvania ; the face cleat 

maintains a perpendicular orientation to the shifting axial trend of local 

structures . Cleat orientation in other bituminous coalbeds in Virginia, 

Utah, Oklahoma, and Central Pennsylvania showed a similar relationship to 

local structure. 

Directional permeability of coal is directly related to cleat. Holes 

drilled perpendicular to the face cleat yield from 2.5 to 10 times the amount 

of gas released as compared with holes drilled perpendicular to the butt cleat. 

INTRODUCTION 

Coal is a sedimentary rock consisting mainly of residue of degraded 

biologically , physically , and chemically altered plant tissues and plant 

tissue extract. Bituminous coalbeds are generally well layered and stratified 

para llel to the bedding planes except when strongly metamorphosed dynamica lly 

by tectonic forces. Rarely are coalbeds free of detrital materials; commonly 

silt and clay accumulations within the coalbed are thick enough to distinctly 

separate the coalbed into two or more parts (benches). 

'~eolo~ist. 

Supervisory geologist. 
The authors are especially grateful to the following reviewers who made 

helpful comments and suggest ions : Samir Khoury , Department of Earth and 

Planetary Science, University of Pittsburgh, Pittsburgh, Pa. ; Ralph Gray, U. S. 

Steel Corp. Research Laboratory, Monroevil le Pa. ; and Jack Vonfeld , Consolida - 

tion Coal Co., Library, Pa. (now retired). 

CLEATS (OR JOINTS) IN COAL 

Jointing (see appendix) is commonly observed in all rock but is perhaps 

most prominent in sedimentary rocks. Joint planes in f lat-lying or nearly 

f la t-lying strata are usually perpendicular -to the bedding plane that is, 

vertical or nearly so. In coalbeds such joints are commonly called cleats or 

cleat surfaces. 

The cleat directions, especially in bitunlinous coals, are important in 

establishing preferred directions of mine development. The coal tends to 

break along its natural fracture systems (the face cleat, also known as main 

or master cleat, and butt cleat, also knowl a< cross or board cleat). Therefore it is much easier to mine parallel to the cleat directions than at an 

angle to the cleats. 

- 

sunderlined numbers in parentheses represent items in the bibliography 

preceeding the appendix . 
Included in this report along with the results of analysis of field 

studies conducted recently is a survey of previous work on the origin of 

cleats. 

PREVIOUS WORK ON ORIGIN OF CLEATS 

Coal has been mined parallel to the directions of the cleat almost since 

the start of coal mining, but the explanations of why the cleat exists have 

always been controversial. Raistrick and Marshall in 1939 (28) quoted the 

first published report on cleat by Edward Mannett in 1834. Mannett stated, 

"The extraordinary uniformity in the direction of the slynes (cleats) and of 

the parting of the rocky strata seems to have been determined by the operation 

of some law not yet understood . 'I 

Later investigators concerned with the origin of cleats believe that the 

cleats are due to either tectonic forces such as formed the folded Appalachians or nontectonic processes such as compaction and dehydration. 

As early as 1908 Kaiser (17) - stated that cleat structure was produced by 

tectonic, forces; he pointed out that in England the strike or direction of 

the cleat has been found to be the same in a succession of parallel coalbeds 

and that the cleat strike was also parallel to the formation fissures in the 

region. However, Dran (6) in 1925 reported that in the Scottish coalfields, 

cleat trends may even vaFy between upper and lower seams in the same colliery. 

Fraiser in 1914 (8) said that the production of cleat by lateral pressure is 

plainly apparent Tn the Falkenau of northern Bohemia. Stutzer in 1940 (30) - 

stated that he felt that "cleats consist of parallel rupture planes produced 

by pressure.'I Ver Steeg in 1942 and 1944 (31-32) in Ohio also found a rela- -_. 

tionship to the cleats in the coal and the present structure. 

The nontectonic origin of cleat formation was originally championed by 

Hofer in 1915 (15)) - who believed that the origin is inherent in the coal 

itself. Hofer maintains that cleats are fissures produced by contract ion of 

carbonaceous material during progressive coalif ication of the original plant 

material to bituminous coal. From w~rk accomplished in New York and eastern 

Pennsylvania in 1932, Moore (24) felt that this idea was basically correct 

but thought tectonics might also play a role in the formation of cleat. 

Recent studies resulted in combining these two original ideas. Amrnosov 

and Eremin (1) follow both ideas to explain the two types of "fractures" 

found in coal. One type is "endogenetic," dependent on "the nature of the 

rocks ," where contraction takes place during cooling, desiccation, and 

recrystallization. The other type, "exogenetic ," is due to the effect of 

external forces during the last stage of development of the folding processes 

(tectonics). These basic ideas parallel some of DeSitterls (5) work. 

Secor (2) says that the fundamental fracture (joint) patternof a rock mass 

is established early in its history and believes pore pressures to be more 

important than overburden pressure as a causative factor. Nickelsen and Hough 

(26) show that the dominant joint sets in coal, referred to as face cleats, 

are usually nearly perpendicular to the fold axis of the local rock structure; 

they state "that nonsystematic joints indicate that a relict tectonic principal stress, related to topography and unloading are important to their genesis .I1 
More recent investigators continue to have divergent opinions on the 

origin of coal cleat. 

SURVEY OF FIELD AND LABORATORY RELATIONSHIPS OF CLEAT ORIENTATION 

Nickelsen and Hough (26) show that the dominant systematic joint sets 

mapped in the field are us~lly nearly perpendicular to the fold axis; they 

believe there is a relationship between the systematic joints and nonsystematic joints that in coal, are called face and butt cleats. According to 

Griggs and Handin (lo), - fractures are due either to shear or extension. There 

are two main types of extension fractures: Those formed parallel to a compressive force, and release fractures that form perpendicular to the greatest 

principle stress axis. 

The results of M. P. Billings' c-2) laboratory studies suggest that 

extension fractures such as the face cleat would form in a water-saturated 

state and under high confining compressive pressure. These would be parallel 

to the direction of the compressive force and formed early in the folding 

state as suggested by Hough (16) and Secor (29). Subsequently, after the load 

is released, the coalbed wouldexhibit numerzs fractures parallel to the axis 

of compression forming the butt cleat. The fractures result from expansion 

of the rock units upon removal of the load through the processes of erosion 

and uplift. Hodgson (13) noted that the nonsystematic joint (butt cleats) are 

secondary features that he felt developed after the formation of the systematic joint (face cleat); Griggs (2) felt that release fractures result 

indirectly from compression. 

Ver Steeg (31-32) -- - observed in Ohio that the change in orientation of the 

joints in the Appalachian coalbeds appear to be arranged in an arc, convex to 

the west, and that this corresponds to the arc of folded Appalachians in the 

same latitude of Pennsylvania. Ver Steeg believed the joints were created by 

deformation at the time of Appalachian folding. The work of Nickelsen and 

Hough (g) in southwestern and central Pennsylvania supported this. 

CURRENT STUDIES 

Cleat orientations were measured to detect changes due to local and 

regional structure. A Brunton compass--a combined handheld alidade and 

clinometer--was used for the measurements following procedures out lined 

by Lahee (22) . 
FIGURE 1. - Location of mines superimposed on structure of base of 

Pittsburgh coalbed. 
Pittsburgh Coalbed 

The Bureau of Mines conducted surveys in seven underground mines in 

Monongalia and Marion Counties, W. Va. , and in Greene and Washington Counties, 

Pa. These mines are in the Pittsburgh coalbed and west of the Appalachian 

front. Table 1 shows the orientations of the face and butt cleats in the 

seven mines surveyed. The locations of these mines and the structure of the 

area based on the Pittsburgh coalbed are shown in figure 1. 

TABLE 1. - Surveyed mines in Pittsburgh coalbed - - 

and their major cleat directions 

--..IceButt cleat 

N 15" E 

N 17" E 

N 18" E 

N 28" E 

N 28" E 

N 30" E 

N 30" E 

---- ine el - 

- 

Loveridge .............................. Federal No. Z.......................... 

Shannopin .............................. Gateway ................................ Marianna No. 58.. ...................... Vesta No. 5............................ 

Somerset No. 60 ........................ ti their 

-. - 

Face cleat 

N 80" W 

N 77" W 

N 73" W 

N 68" W 

N 68" W 

N 66" W 

N 57" W 

relative geographical position, with Loveridge being the 

southernmost mine and Somerset No. 60 being the northernmost mine. 

l~he mines listed in this table are arranged according 

These data show that there is a rotation in the cleat direction that 

corresponds roughly to the change in strike of the anticlines and synclines 

in this area (fig. 2). The measurements recorded are similar to those 

made by Parker (2) in western New York and eastern Pennsylvania and by 

Ver Steeg (32) directly to the west in Ohio, which relate to the axial trend 

of the Allegheny front. 

A good example of structural control on the orientation of cleat can 

be found in the Shannopin mine. Figure 3 shows the structure of the coalbed 

in the vicinity of the mine. The cleats are controlled by the Fayette anticline, the dominant structure in the area, because the structural contour 

lines parallel its axial trend, and the face cleats, which strike N 73" W, 

are perpendicular to it. 
Marshal 

County 

Greene County ---r-- -------. 

Marion County 

LEGEND 

Loveridge mine 

Federal No. 2 mine 

Shannopin mine 

Gateway mine 

Marianna No. 58 mine 

Vesta No.5 mine 

Somerset No. 60 mine 

Face cleat --- Butt cleat 

Monongalia County I Trend of 

At leg heny 

front 

0 5 10 

Scale. miles 

FIGURE 2. - Coal cleat orientations in seven mines operating in the Pittsburgh coalbed 

showing cleat rotation. 
KEY MAP 

Scale, miles 

FIGURE 3. - Structure of Pittsburgh coal in area of Shannopin mine and coal cleat 

readings. 

Lower Kit tanning Coa lbed 

A detailed geological study was also conducted in and around the Cambria 

Nos. 32 and 33 mines near Ebensburg, Pa., about 80 miles north and east of the 

seven Pittsburgh coalbed mines surveyed. Both mines are located in the 

Lower Kittanning coalbed. The coal face cleat strikes N 69" W and the butt 

cleat strikes N 19" E. The axial direction of the local folding ranges from 
LEGEND 

---lOOO-- Structure contour. 

contour interval-50 ft 

0 

.-i 

4000 8000 

Scale, ft 

atic rock joint direction, 

which are all nearly perpendicular to the local axial 

trends of the Ebensburg arrticline and the Wilmore syncline situated on either 

side of the mine. 

FIGURE 4. - Generalized structural contour map of area In figure 5 this relaaround Cambria Nos. 32 and 33 mines. tionship in orientation can 

be seen from the rosette 

diagrams showing the cleat orientations, the rock joint measurements, and the 

reg iona 1 1 inea t ions . 

Approximately 30 separate joints were measured in the mine roof underground. One extended upward more than 20 feet where exposed by roof falls. 

Seventy-five percent of the rock joints measured trended N 64" W to N 75" W, 

which is approximately parallel to the face cleat of the coal. 

This is not to say that all cleats and joints formed at the same time. 

The coal face cleats may have formed earlier, as suggested by Nickelsen and 

Hough (26) , "because the coals were the first to develop cohesive strength or 

because the coals are weaker than the other types of rocks." From this 
N N N Axial trends 

Butt 

coal cleat 

1 N3g0E 

Systematic Nonsys tematic 

EW EW E 

Coo I cleat measurements Rock joint measurements 3 regional lineation directions 

725 reading 300 reading 

w 

Scale, pct 

w 

Scale, pct 

FIGURE 5. - Strike of coal cleats, rock joints, and regional lineations. 

evidence, the authors conclude that the compress ion was in a northwest - 

southeast direction, and that the face cleat was formed during early folding 

of the strata, the butt cleats being release fractures formed later when the 

compressive forces were released. 

Surveys of the following two mines in the Western States show similar 

relationship of cleat orientations to geological structure. 

Lower Hartshorne Coalbed 

At the Howe Coal Co. mine in the Lower Hartshorne coalbed, located 6 

miles south of Poteau, Le Flore County, Okla. , on the southern border of 

the Arkoma coal basin, the strike of the butt cleat is N 74" E, subparallel 

to the local axial trend. The strike of the nonsystematic rock joints in 

this area is N 72" E, also subparallel to the regional trend. The face cleat 

in the coal and the systematic rock joint both strike N 15" W, which is subperpendicular to the trend and, therefore, parallel to the axis of compression. 

These details are illustrated in figure 6. 

Subseam 3 of the Castlegate Group A 

At the Carbon Fuel Co . No. 3 mine (near Helper, Utah) operating in the 

subseam 3 of the Castlegate Group, the face cleat strikes N 80" W and the butt 

cleat strikes N 16" E. The axial trends of the local structures are subparallel to the butt cleat. The structure dips generally to the north and 
FIGURE 6, - Structure of the Lower Hartshorne coalbed. 

and northeast as shown in figure 7; local deformation is mainly to the east of 

the mine. No studies have been conducted in other mines remote from this zone 

of deformation, but in this mine there is a demonstrable relation of cleat 

orientation and local structure . 
N 

t /~utt """ cleat 

Face cleat I I 

UTAH 

KEY MAP 

- 

Scale, ft LEGEND .....-..__..... Eroded coal 

FIGURE 7. - Generalized structure map in vicinity of Carbon Fuel No. 3 mine, Carbon County, 

Utah. 

Because the Howe Coal Co. mine and the Carbon Fuel No. 3 mine fit the 

present theory of the relationship of cleats in coal to geological structure, 

it does not necessarily follow that all other mines in Oklahoma and Utah will 

do so, but without data to the contrary, this premise is applicable to the 

areas studied. 

Pocahontas No. 3 Coalbed 

In all the areas thus far considered, folding has been the dominant 

structural feature. In Buchanan County, Va. , both folding and faulting are 

evident in the Beatrice mine. The butt cleat of the Pocahontas No. 3 coal 

strikes N 67" Egoagain nearly parallel to the axis trend of the folds (fig. 8). 

The face cleat strikes N 18" W at right angles to the axial trend of the 

folds. A strike-slip fault has been found to run across this mine. The coal 

on both sides of the fault, for about 30 feet, is badly disturbed. Figure 9 

shows the results of s teronet analysis of the cleat measurements arbitrarily 

separated into zones 0 to 2,000 feet and 2,000 to 4,000 feet on either side 

of the fault. Cleat orientation in the northeast 2,000-foot zone adjacent to 

the fault was affected by faulting, but cleat directions in the other zones 

apparently were not affected. One explanation for this is that the southwest 

block moved against the northeast block (acting as a buttress), against which 

the energy in the southwest block was dissipated. 
Scale, ft 

LEGEND 

- 100-Structure contour, 

contour interval- 100 ft 

FIGURE 8. - Generalized structure of Beatrice mine showing structural contours and 

Keen Mountain fault. 
FIGURE 9. - Stereonet analysis of cleat orientation 4,000 feet on either side of the 

Keen Mountain fault in the Beatrice mine, 

DIRECTIONAL PERMEABILITY RELATED TO COAL CLEAT 

Directional permeability in coalbeds has been observed often during the 

conduct of Bureau research. This permeability is directly related to the two 

main cleat directions. An attempt was made to quantify this relationship. 

According to Kissell @), a hole drilled into the coalbed perpendicular to 

the face cleat would be expected to yield a much higher gas flow (up to a 

factor or 10) than one perpendicular to the butt cleat. The reason for this 
is that the face cleat is a longer, more continuous joint surface. It crosses 

bedding planes in the coal and can extend for a distance of many feet. The 

butt cleat is short, often curved, and is a discontinuous feature that frequent ly terminates against the face cleat. 

From this it can be seen that holes drilled horizontally and parallel to 

the butt cleat (perpendicular to the face cleats) would intersect the more 

permeable face 

cleats and, therefore, drain a 

greater area. In 

contrast, a hole 

drilled horizontally and parallel to the face 

cleat (perpendicN 65" W ular to the butt 

Face cleat 

Butt cleo t 

7 N 25OE 

Pressures from this 

hole ~rovide face cleat I cleat) would intersect the butt 

perkeability--------A This region is cleats that are 

drained by shorter in length face cleat and less frequent. 

This region is 

drained by 

butt cleat w 

Coal bed - 

-- N25"E 

-1 - - - iPreslur.. from 

this hole provide butt 1 - cleat permeability 

0 - 100 200 

Scale, ft 

FIGURE 10. - Horizontal holes drilled into Lower Kittanning coalbed 

(Cumbria No. 33 mine) subparallel to cleat direction. 

The tests 

that support this 

observation have 

been conducted in 

several different 

mines. The 

results of some of 

this work. are discussed below. 

The Cambria 

No. 33 mine, 

located near Ebensburg, Pa. , is operating in the lower 

Kittanning coalbed. 

Two holes were 

drilled horizontally into the 

coalbed parallel 

to the N 65" W and 

N 25" E direction. 

The gas pressure, 

after the hole was 

sealed along the 

N 65" W direction, 

was 12 psi, 

whereas the gas 
LEGEND 

/-4 Measurement holes 

FIGURE 11. - Map of Federal No. 2 mine showing horizontal pressure measurement holes, 
fourth parallel to it. These holes were all drilled to different depths into 

the coal and at different time intervals following mining. Figure 11 shows 

the relative position of these holes to the cleat directions. 

Hole 1 was drilled perpendicular to the face cleat 75 days after the 

entry was mined; pressures of 16 psi at the 40-foot horizontal depth and 19.4 

psi at the 78-foot depth were measured. 

Hole 2, also drilled perpendicular to the face cleat 18 days after the 

entry was mined, had a pressure of 62 psi at the 49-foot depth and 74 psi at 

the 69-foot depth. 

Hole 3, drilled parallel to the face cleat 15 days after the entry was 

mined, showed a pressure of 9.5 psi at 43 feet and a pressure of 27 psi at 

113 feet. 

Hole 4 was drilled perpendicular to the face cleat 60 days after the 

entry was mined. Pressures of 11.6 psi at the 40-foot depth and 21.1 psi at 

the 100-foot depth were measured. 

Considering the age of the entries it is evident that hole 3, drilled 

parallel to the face cleat, developed the lowest pressure gradient as a consequence of a higher gas flow rate. The pressures measured in hole 3 were comparable with those measured in hole 4, which was 45 days older and had a 

longer time to flow and to desorb the coal (11). This again is a good example 

of the effects of directional permeability. 

Near the Federal No. 2 mine, a multipurpose borehole has been driven 

vertically to the Pittsburgh coalbed. Holes were drilled radially into the 

coalbed at the bottom of the hole. Again the same correlation was found, in 

that the holes drilled parallel to the face cleat showed the lowest permeability, while those drilled parallel to the butt cleat had the highest permeabilityandgas flow. Figure 12 shows adrawing of the multipurpose borehole and the 

radiating holes; also shown are the face and butt cleats directions. Table 2 

shows the individual data on the horizontal holes as taken from a report by 

Fields (7). - 

TABLE 2. - - Initial - data on the seven degasification - - - holes 

In situ gas pressure at I II I I I 

, - - 

- 7 J 

608 

79 

166 

Angle of hole with respect to 

face cleat .................. deg.. 

Distance from gas well ........ ft.. 

Average water discharge. ..... gpm. . 

199-ft depth.. .......... ....psi.. I - I - I -- 1 - - a 

IFiaure 12. 

5 

500 

171 

435 

- _1C 

2ole I............................. 1 

83.5 

- 

6.5 

2 

850 

257 

385 

6 - 

616 

104 

215 

- 

Length ........................ ft.. 

Gas emission ........... 1,000 cfd.. 

Gas flow per square foot of 

coal surface in holes ....... cfd.. 

- 4 

549 

159 

369 

646 

201 

396 

28 

180 

6.8 

35 

160-400 

5.6 

61 

130-370 

8.0 

12 

300 

5.0 

28.5 

- 

5.0 
we1 l 

No 2 hole 

050 ft deep 

-- -- \\ 

LOCATIO

WITH H

FIGURE 12. - Location of bleed
Face Butt Face Butt Face 

0.4 I I Figure 13 shows the I I I 1 "calculated trend curve of 

the butt cle-a-t--1 and 5, 

0 go 90 180 270 for example-have much 

HOLE ANGLE, deq higher gas flow. 

'U, 

; .3 

E 

.b0 

r' 

W 

z 

a 

I 

I- .I I" 

FIGURE 13. - Initial gas flow rates from horizontal holes Figure 13 shows the 

drilled in virgin coal. effects of directional 

permeability on the flow 

(25) where the maximum flow is from holes drilled along the butt cleats(butt) 

and perpendicular to the face cleats (face). Also it is pointed out that gas 

wells in the area have an effect on the gas flow; quoting from reference 35, 

"however, since boreholes that pass near wells may have flow rates above the 

predicted values, it is not surprising to find that the initial flow rates 

from holes 2 and 4 were approximately twice as high as those to be expected. 

Furthermore, comparing the flows from holes 4 and 7 (two diametrically opposed 

holes), it can be seen that the flow from hole 4, which passes near a gas well, 

was about 2.5 times higher than that from hole 7, which does not pass near a 

well." 

CONCLUSIONS 

initial gas emission rates 

Cleat in the bituminous coalbeds studied is formed by local structural 

forces rather than by internal forces within the coal such as result during 

diagenesis and compaction. The face cleats tend to be perpendicular to the 

axial trend of the folds and probably formed as extension fractures. The butt 

cleats tend to be parallel to the axial trend and formed after compressive 

forces were released. 

- 05 

I 

o4 0T0\; 02 

.2- 

- 

o6 - 07 

- - 

The authors' research supports the view that in the Pittsburgh coalbed the 

orientation of the cleat structure shows an axial rotation perpendicular to 

the Allegheny front. There are some anomalies but these can readily be 

explained by showing relationships to local structural variations. 

from the degasif ication 

holes .ll Holes 6 and 7 

which are more nearly parallel to the face cleat 

(therefore intercepting butt 

cleats), have the lowest 

methane flow, whereas the 

holes drilled subparallel to 

Studies of cleat orientations in mines other than in the Pittsburgh coalbed as far away as central Pennsylvania, Oklahoma, Virginia, and Utah show 

the importance of local structural control on the orientation of cleats with 

the same general dependence of face cleat and butt cleats to the axial trend 

of folds. 
The directional permeability of coal is due to the orientation and magnitude of the cleat planes, which control the flow of water and gas through the 

coalbed. A hole drilled perpendicular to the face cleat produces 2.5 to 10 

times as much gas as one drilled perpendicular to the butt cleat. 
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for Sci. Transl., 1963, 112 pp. 

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York, rev. 2d ed., 1964, 370 pp. 

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APPENDIX.--GLOSSARY OF TERMS 

1. Bedding plane--A planar or nearly planar bedding surface that 

visibly separates each successive layer of stratified rock (of the same or 

different lithology) f r an its preceding or following layer. 

2. Butt cleat--A short, poorly defined cleavage plane in a coal seam 

usually at right angles with the face cleat and sometime terminating against 

i t. 

3. Cleat--In coal seams, a joint or system of joints along which the 

coal fractures. There are usually two cleat systems developed perpendicular 

to each other. 

4. Desiccation--A complete or nearly complete drying out or drying up, 

or a deprivation of moisture or of water not chemically combined. 

5. Dip--The angle at which a structure or any planar feature is 

inclined from the horizontal. The dip is at a right angle to the strike. 

6. Extension fracture--Fracture that forms parallel to a compressive 

force. In a sense this is a tension fracture. 

7. Face cleat--A well-defined joint or cleavage plane in a coal seam, 

The major joint in a coal seam. 

8. Fissure--An extensive crack, break, or fracture in rocks. 

9. Joints--Fractures in rock, generally more or less vertical or transverse to bedding, along which no appreciable movement has occurred. 

10. Pore pressure--The hydrostatic pressure of the water in the pore 

space of a soil or rock. 

11. Rank of coal--A generalized classification of coals according to 

their degree of metamorphism, or progressive alterat ion, in the natural series 

from lignite to anthracite. 

12. Release fractures--Fractures that form perpendicular to the greatest 

principle stress axis. The assumption is that these fractures form when the 

load is removed, hence their name. 

13. Rosette diagram--Circular or semicircular diagram for plotting 

strikes (or dips) of planar features, such as joints and cleats. 

14. Shear fracture--A fracture that results from opposed stresses that 

tend to displace one part of a rock mass past the adjacent part. 

15. .- Stereographic analysis--(Schmidt Net) A coordinate system used in 

structural geology for statistical analysis of planar orientation data. 
16. Strike--The direction of the trace of an inclined bed of planar 

structure projected to a horizontal plane. It is perpendicular to the 

direction of the dip. 

17. Tectonic forces- -The forces involved in deformation of the earth's 

crust resulting in folding and faulting. 

18. Tension fracture--A fracture that is the result of stresses that 

tend to pull material apart.