568

THE COLLIERY GUARDIAN.

September 17, 1915.

When, two years ago, as a geologist whose chief
research work had hitherto been done on mountain
structure among the slaty rocks of Wales, the writer
came to take up work at Sheffield, his attention was
drawn to certain “ rock rolls ” and other irregularities
in the roof of the Park Gate seam at Tinsley Park
Colliery, and it occurred to him that these were effects
due to earth movement along the planes of bedding. A
visit to workings of the Haigh Moor seam at South Kirkby
Colliery convinced him that the similar rock rolls and
the tearing away first of the upper and ultimately of
the whole thickness of the coal seam, in a “ washout ”
which has stopped the development of whole districts
in that seam, are part and parcel of the same pheno-
menon. He therefore accepted “ washouts ” as
phenomena of special interest to students of earth
movement, and has taken every opportunity which has
offered to visit and investigate the details of each
“ washout ” to which his attention has been drawn.

To regard “ washouts ” as effects due to earth move-
ment is not a new idea. The old Yorkshire miners’
name of “ rock fault ” suggests a movement that has
gone so far that fracture has resulted.

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SuMtors horizontal datum-l/nb

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Fig. 2.

Section seen on north-eastern face of heading driven to prove the extent of rock-fault area in Haigh Moor seam,
West Riding and Silkstone Collieries, Altofts. In this figure and in figs. 3 to 9 coal is represented by solid black,
sandstone by dotted areas, and bind is either left blank or its more prominent divisional planes are indicated by lines.

Well-defined portions of the coal seam, or even on occasion the whole seam itself, have been duplicated, tilted up
bodily and pushed forward over many yards to rest on top of the normal undisturbed coal seam which lies beneath it.

Prof. P. F. Kendall, in the short note prepared for
the excursion of the members of the Geologists’ Asso-
ciation to Whitwood Colliery in 1910, offers the follow-
ing remarks :—

The Yorkshire coal field, like that of Durham, is
remarkable for the frequency of “ washes ” in the
seams, i.e., of areas from which the coal has been
removed and beds of sandstone, shale, and con-
glomerate substituted.*

From his experience of the coal field, he distinguishes
two kinds of washes, namely :—

1.	Narrow sinuous belts that may in a few cases be
traced from pit to pit, and that, when plotted on a
map, have the form, as they are in fact, of silted-up
channels or streams which meandered over the coal
swamps and scoured out the peaty material;

2.	The other type which takes the form of broad
tracts extending over perhaps hundreds of acres;

and he remarks : “ The fact is becoming increasingly
clear to me year by year that ‘ washes ’ are intimately
connected with tectonic movements taking place
during coal measure times. ”f

To geologists the word “ tectonics ” used in this con-
nection means the processes by which the structure of
the country has been developed on a large scale, and
implies strains or fracture set up in the rocks by stresses
which have been imposed on the country from without.

From the writings of geologists of repute, it is clear
that in recent years the current view among workers in
our district has been that “rock faults,” “washes,”
and “ wash outs ” are occurrences caused by some sort
of stream action during coal measure time. Possibly,
as Prof. Kendall suggests, two kinds (the contem-
poraneous-erosion and the tectonic) may exist side by
side, but, so far as the writer’s own experience has
extended, he has visited no single example which he
can accept as belonging to the contemporaneous-erosion
class.

He has not evidence enough to allow him to dogma-
tise, but he proposes now to state a case for the view
that the majority of the rock faults which occur in our
own district, as well as the much more frequent
examples of rock rolls which locally come down on to
the coal and displace the usual bind or “ clod ” in the
roof of seams which are subject to rock faults and wash-
outs, have been brought about by horizontal earth move-
ments due to lateral pressure at a time when the
deposition of the measures which contain the coal
seams had been already completed.

The writer will now assemble some of the outstand-
ing facts which will have to be accounted for in any
working hypothesis that can be accepted as of general
application to the rock faults of the Yorkshire, Derby-
shire, and Nottinghamshire region.

1. Rock faults affect individual coal seams, and the
existence of a rock fault in one seam is no primd facie

* “ The Geology of the Districts around Settle and Harro-
gate,” Proceedings of the Geologists’ Association, 1911,
vol. xxii., p. 27.

f Ibid., p. 43.

evidence that the other seams in the same property are
likely to be similarly affected.

2.	Rock faults are local incidents even to the parti-
cular seams in which they occur, and the abrupt changes
of thickness and character of the coal at their boundaries
bear no relation to, and do not interrupt, the general
constancy, or the continuity of the slow variation, of
quality and thickness of the seam over the whole region.

3.	Rock faults are most frequent in coal seams which
have a roof of weak clod or bind, forming a thin ripping
between the top coal and a much stronger overlying
mass of stone bind or sandstone.

4.	Rock faults generally truncate the coal seams that
they affect along oblique surfaces which make but a
small angle with the bedding planes of the beds trun-
cated. Usually, on each side of the areas affected, the
“ bottoms ” of the coal seam “ carry on ” beneath the
truncated surface of the rock fault for a considerable
distance beyond the line at which the “ top coal ” is
lost.

5.	In plan, the area over which a coal seam is ren-
dered unworkable by rock faults is generally of elongate
snake- or fish-like form. Usually its outline is like the

cross section of a convex lens, crescentric (concavo-
convex), or eye-shaped (bi-convex), as the case may be,
but always bounded by flowing curves, and with none
of the angularity which is characteristic of forms
bounded by straight lines.

6.	Rock faults which affect broad areas are of less
simple shape. Their outlines appear as if built up by
the assembling of many fish-like forms, all in sub-
parallel position. Some groups appear as if their units
had been laid side by side, others as if end to end; but
in the more usual cases, areas of several sizes are
entangled together or strung out en echelon along a
curving line.

7.	The under-surface of a rock fault, if well defined,
is generally somewhat slickensided, and, if the rock be
suitable and the truncation not too oblique, is provided
with a ‘ ‘ leather bed. ’ ’ Often the bounding surface
continues along each successive parting in the coal seam
for a considerable distance before it breaks across to the
next bed. In places where below the bounding surface
layers of fine-grained bind, “ clod,” or “ jacks ” are
interstratified between the layers of coal, the partings
are often emphasised as if by slipping, and the surfaces
along which the several rock types adjoin are striated
and burnished to a gloss.

8.	On one or other side of a rock-fault strip, the
general thickness of the coal is increased, it may be,
over an area which is quite comparable in extent with
the area over which the coal seam is lost. (Fig. &). •

9.	Associated with the flanking strips of “ proud ” or
thickened coal, irregular lens-shaped, spindle-shaped, or
meniscus-like masses of sandstone, bind, or clod appear
in the midst of the coal. These are of various sizes,
and most of the rock types which can be recognised in
the rock-fault “ filling ” are generally represented.
Generally these “ foreign bodies ” are found lying with
their long axes parallel to the margin of the rock fault,
in surfaces which transgress the bedding planes of the
coal. In some cases these “ foreign bodies ” seem to
be joined by an irregular tongue of rock to a somewhat
abnormal roof, or if nearer the rock fault, to the
“ filling ” of the rock fault itself. (Figs. 3 to 6.)

10.	Close up to the margin of the rock fault, and
especially in the strips where the coal seam is partly or
completely duplicated, shreds or tongues of the rock-
fault filling wedge in between the normal and the added
seam. Often, also, in the termination of the seam,
the beds and partings are wedged open by bind or stony
material, and the coal comes to an end in a sheaf of
divergent wedges, the transverse sections of which
recall fluttering pennons or the fin-spines of a swimming
fish. (Figs. 7, 8, and 9.)

11.	The “ filling ” or material which replaces the coal
displaced in a rock fault is in general a motley complex
of a very limited number of types of sedimentary rocks.
Hard sandstone, sandy bind, stone bind, blue bind, iron-
stone, shale, clod, coal, and perhaps conglomerate, are
the usual components, and just so many of these rocks
are involved and appear in the “ jumble ” as form the
adjoining ground between the lowest and the highest
measures which are interrupted by the rock fault at the

particular locality. Individual rock masses in the
jumble are evidently pieces which have been detached
from an already compacted parent rock. In shape the
pieces are irregular. In size, the association of pieces
follows no apparent law. In composition and texture,
the individual rock fragments are not in any way
different from the rock types which compose the
measures in the undisturbed adjacent ground. Some
of the pieces may vary in character across or along their
length, but in these the variation is of the normal type
of sequent beds, and was produced during the episode
of sedimentation before the consolidation of the parent
rock. In condition of surface, the different rocks in
the jumble vary considerably from clean fractures to
glassy slickensides, according to their position with
respect to the bounding surface of the rock fault and
the rocks which they adjoin. On no fragment large
or small has the writer been able to find any evidence of
a chemically-altered skin, and all the curious burnish-
ings and adherent films of glaze that have been observed
seem due to mechanical attrition of one rock where it
has ground against another.

12.	Of ordered segregation of materials within the
rock-fault filling, the writer notices but little trace.
Sometimes cakes of particular beds of rock lie compara-
tively undisturbed in the jumble, and form the roof or
floor of the workings, over scores or hundreds of square
yards. Often two or three sorts of rock are concentrated
to the exclusion of other types, and are jumbled together
to form a lens-like patch or large lenticle. Such
lenticular masses, of diverse constitution, wedge in and
out, over and under, other aggregates of similar or
dissimilar constitution, and between them rock frag-
ments and shreds of beds behave as do the foreign bodies
among -the coal, on the flanks of the rock fault area.
Generally, each lenticle is separated from its neighbours
by a surface which shows in section as a sweeping curve,
and along these surfaces there is generally some evidence
of crush. Where the change of lithology between
adjacent lenticles is abrupt, the evidence of shearing
movement is at its greatest, and polished surfaces and
slickensides are the rule. Cracks and displacements also

Figs. 3 to 6.

Diagrams showing details of the Bizarre shapes charac-
teristic of large masses of rock “ foreign bodies ” included
in the Haigh Moor coal in the neighbourhood of a rock-
fault area. Foreign bodies of various sizes, irregular, lens-
shaped, spindle-shaped, or meniscus-like masses of sand-
stone, bind or clod, in some cases joined by an irregular
tongue of rock to a somewhat abnormal rcof.

are not infrequent in other of the sandstone masses in
the rock fault aggregate, and the way in which tongues
of various materials break across the sandstone, or are
wedged in along open bedding planes that have been
made to gape, gives the impression that under the
mechanical conditions which prevailed at the time of the
making of the jumble the sandstone behaved as a brittle
substance, and was incapable of accepting any bend.

13.	So far, in considering the rock fault filling, no
mention has been made of the fate of the displaced
coal. Coal in some proportion is always present among
the jumbled material, and close down upon the bound-
ing surface of rock faults, where the bottoms of the
coal seam remain in place, shreds and masses of con-
siderable bulk are never absent for many yards together.