THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CX.

FRIDAY, SEPTEMBER 17, 1915.

No. 2855.

Some Effects of Earth Movement on the Coal Measures of the
Sheffield District.*—Part 1.

By WILLIAM GEORGE FEARNSIDES, M.A. (Cantab), F.G.S.t

In presenting this paper, the writer would first explain
that it has been prepared by request at a time when the
press of advisory work for armament firms has made it
impossible for him to devote to its preparation the
amount of attention which he could have wished. He
offers it, however, as a first instalment of a paper on
the general structure of the Yorkshire, Derbyshire, and
Nottinghamshire coal field, which will deal broadly with
the interpretation of particular items in the boring and
sinking records which were published last year by the
Midland Institute of Mining, Civil, and Mechanical
Engineers.! In the present portion of the paper the
writer will deal mostly with small details affecting the
stratification of the coal measures.

In the present portion of the paper the writer will
consider only those deformations which there is reason
to believe took place during the early period—that is,
within the period of the carboniferous.

Earth Movements Accompanying the Deposition of the
Coal Measures.

As is well known to all who have assisted at borings,
sinkings, or other of the cross-measure explorations
which always precede the winning of coal, the coal
measures of the Sheffield district present a very varied
succession of sediments. The whole thickness of the
measures in South Yorkshire cannot be less than
5,000 ft.; and since both the fossils and the chemical
composition of the sediments prove most of them to
have originated as estuarine accumulations, it is evident
that, in order to make room for them, there must have
been continual settlement of the sea bottom during the
period of their accumulation to that not inconsiderable
depth.

Interstratified over the length and breadth of the coal
field with this mass of estuarine material, both near the
base (Halifax hard bed, Ganister bed, and Alton coal),
and at several horizons high above the latest of the pro-
fitable coal seams (Mansfield marine band and Upper
marine band), are certain other beds, which, on equally
convincing evidence, are regarded as having been laid
down in the sea.

On the other hand, certain of the coal seams have
been accepted as terrestrial accumulations, and from the
occurrence of these, interbedded with the prevailing
estuarine and the occasional marine bands, it is argued
that the general level of surface of the coal field area
must, during the coal measure period, have passed
several times through the sea level of the period, and
that during the whole period it can have varied but
little above and below that datum line.

It is further inferred from the alteration of marine
with estuarine sediments that the progress of the general
downward movement of the area was discontinuous. ,
Probably it was oscillatory, and in picturing this, the
first earth movement that affected the coal field, it may
be represented as a downward sloping line with the
oscillations as a series of rhythmic pulsations super-
imposed upon it, as shown in fig. 1.

It is a notable fact that, when one seeks to make
accurate comparisons between the records from neigh-
bouring shaft sections, the datum horizons which are
most useful for comparative purposes are those which
are provided either by coals of terrestrial type, or by
the bands of shale which contain marine fossils. These
coals and marine shales are the deposits which respec-

* From a paper read before the Institution of Mining
Engineers.

f Sorby Professor of Geology, University of Sheffield.

! Sections of strata of the coal measures of Yorkshire,
together with a few Derbyshire sections, compiled from
records of borings and sinkings, by a committee of the Mid-
land Institute of Mining, Civil and Mechanical Engineers,
1902-13; and cross-country sections and map of the York-
shire coal field, 1902-13 (1914).

tively were accumulated at the times of the passing of
the arches and troughs of the pulsatory oscillations, at
those times, in fact, when the rate of change determined
by the general downward movement of the whole area
was at a minimum.

Whilst the importance of the pulsatory oscillations
which gave rise to these repeated alternations of marine,
estuarine, and coal growing conditions is emphasised,
it must not be forgotten that the outstanding charac-
teristic of the earth movements during the deposition of
the coal measures was progressive general settlement,
which, over tens of thousands of square miles of
country, just kept pace with the sedimentation through-
out the period. Never before or since in the whole
known geological history of the world have the factors
which conserve levels and topography maintained so
exact a balance against the agents of destruction and
evolution over so wide an area or for so long a time.

Settlement Due to Contraction.

In addition to the movements which are due to unrest
within the earth’s crust itself, there is, in all areas
where deposition of sediment takes place under water,
another type of movement which is due to the settling
down of particles within the new-formed sediments
themselves.

Fig. 1.

Diagram showing the movement of the pre-coal-measure
floor during the period of the deposition of the coal
measures. A.—Progressive downward earth-movement with
pulsatory oscillations superimposed upon it. B.—Accumu-
lating sediment maintained the surface of the lithosphere
at or near sea-level during the whole period.

That coal measure binds were deposited under water,
and must therefore have been laid down as muds and
slimes containing their full quota of entangled water, has
long been recognised by all students of the coal
measures. Nevertheless, it is only in the writings of
Sorby, the eminent founder of the chair of geology at
Sheffield, that the writer finds allusion to the circum-
stance that by reason of the removals of that water there
must have been movements and readjustments within
the pile of sediments, which, in total effects, were con-
parable to results produced by earth movements on a
grand scale.

Sorby devoted considerable attention to this matter,*
and as the mean of many observations he arrived at
13| per cent, by volume as an average value for the
porosity remaining in unweathered coal measure binds
like those from which his garden clays had been derived.
Sorby argued that since the downward pressure “would
increase with the thickness of the deposit lying above,
the amount of included water would decrease as the
depth became greater,’’! and “ that the amount of
interspace varies in some way inversely as the
pressure.’’!

The total thickness of the coal measures in South
Yorkshire exceeds 5,000 ft. The change of condition
from 86 per cent, of interspaces, as deposited, to the
final stage with only 13| per cent, of porosity, demands
a contraction_of about 6 Jo 1. In the 5,000 ft. of sedi-

* See “ On the Application of Quantitative Methods to the
Study of the Structure and History of Bocks,’’ by Henry
Clifton Sorby, Quarterly Journal of the Geological Society of
London, 1908, vol. Ixiv., p. 171.

t Ibid., p. 190.

tlbid., p. 229.

ment as deposited, one may reckon 5 x 1,500 ft. for the
contraction of the shales, etc., plus, say, 1,500 ft. for
the stone binds, equivalent in effect to a downward
earth movement of about 9,000 ft.

It is to weak layers which were deposited as particu-
larly liquid slimes that one must turn in order to observe
contraction effects in their most notable development,
and if, as it often happens, there are shales inter-
stratified between hard coals or sandstones which
suffered no appreciable contraction, the contrast which
can be observed is very marked indeed.

Roofs of coal seams not infrequently show well 'the
phenomena of crushing due to overloading. The most
satisfactory roof from the point of view of the coal miner
is one which parts cleanly from the coal, and in which
any splitting that takes place is accurately parallel to
the bounding surface of the coal. Water-logged swamps
overlying vegetable accumulations are hardly localities
where one would expect fine true stratification due to
regular deposition of successive layers of sediment, and
generally in such places the chemical conditions are
favourable to the granulation by clotting of .any mud
which may be settling towards the bottom. Neverthe-
less, because these muds contained abundant minute
plates of mica, etc., and because the water has been
expressed from them by the slowly-increasing weight of
the later deposits, they are in mining not infrequently
found as well-laminated rocks. The lamination, how-
ever, is not due to bedding, but is an effect like slaty
cleavage, exhibited by reason of the position of polarity
adopted by the flat-sided particles, which, during the
process of compression, have rotated and set their flat
faces at right angles to the (vertical) onset of the
pressure (see Sorby, loo. cit.f p. 213).

[The author then considered the influence of chemical
changes in the sediments resulting in processes known
generally as “segregation,” examples of which occur in
the form of clay ironstones, coal balls, iron pyrites, and
other concretions.]

As has been pointed out, an apparently homogeneous
mud rich in flat-sided mica particles, when compressed
by later sediments, loses its water and becomes a pseudo-
laminated shale. In so doing, the deposit diminishes in
thickness in the ratio of about 6 to 1. Rigid masses,
however, cannot undergo such contraction, and if there
be any nodules in which by segregation the interspaces
between the particles have been filled with solid, these
too will have mechanical properties quite different from .
the watery muds in which they lie.

Generally, the change of rigidity across the margins
of the segregated nodules is abrupt, and accordingly it
is not unusual to find that, in the course of contraction,
the normal shale has been forced past the edges of the
nodule, on which it has slipped, producing a burnished
and “ slickensided ” surface. Small burnished nodules
of the type indicated are often taken from the roof
shales in pits known to the writer, and handed to visitors
as interesting curiosities. Large ones, which by their
very size have caused the surrounding shales to slide
over them for a considerable distance, are treated with
greater respect : being only scantily attached to their
surroundings along the middle of their girth, they are
apt to come down unexpectedly. Other incidental rigid
masses, such as tree trunks, which, growing up through
the mud and later becoming hollow, were filled up when
a stiffer sort of sediment was being deposited, behaved
similarly, and the contracting mud was squeezed past
them, leaving them standing to form that scourge of
the careless collier’s existence known as “ pot holes in
the roof.”

Rock Faults or Washouts.

Deep-seated intercrustal movements belonging to the
period of deposition, and movements associated with the
process of expulsion of water from deposits as they con-
solidate, have it in common that both give rise to move-
ments in the vertical sense. The subsequent move-
ments which have left their mark upon the coal field
have been movements which had at least some hori-
zontal component.