THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXII.

FRIDAY, OCTOBER 13, 1916.

No. 2911.

The Bristol and Somerset Coal Fields:

EFFECT OF MASTER FOLDS ON STRUCTURE.*

By ERNEST

The study of the faults and folds of this district is
attended with special difficulty, owing to their complex
nature and to the fact that the greater part of the car-
boniferous formation is overlain by secondary rocks. The
chief movements of the earth’s crust which gave the
coal field its peculiar form took place prior to the depo-
sition of the mesozoic rocks; the faults and folds are
only discernible at the surface where the newer strata
have been removed by denudation, and any attempt to
unravel the problem presented by them must be accom-
panied by a study of the underground conditions in the
collieries.

The flexuring of the rocks in carboniferous and post-
carboniferous times probably took place in three
principal stages and from two main directions; and, in
order to lappreciate fully the part played by each of these
great forces in building up the structure of the coal field
as it exists to-day, it is essential to ascertain which of
these lines of folding acted first upon the coal measures
after they had been laid down.

It is known from past mining experience ethat the
Parkfield series of coal measures occupy a less vertical
depth of strata than the Farrington series of Somerset,
and may be compared roughly as follows :

Parkfield series:	Yds. Farrington series:	Yds.

Rag seam to Great seam ...79 | No. 1 to No. 6 veins... 105

This thickening of the strata to the southward is
further illustrated by the circumstances of two of the
seams of the Parkfield group, namely, the Holly Bush
vein and the Great vein, which are found close together
and are worked as one seam in the northern portion of
the district, while towards the south a considerable
thickness of shale intervenes between them. Mr. J.
Anstie in his book on the Gloucestershire and Somerset-
shire coal fields states that the Pennant near Sweethouse
Farm is 600 yds. thick, whereas further to the south it
attains a thickness of 900 yds., and in writing of the
lower coal series he shows that while these attain a
thickness of 3,000 ft. and contain 25 seams of coal in
the Kingswood district and 22 seams in the Nettlebridge
Valley, they are represented by four seams of coal at
Ya to and two seams at Cromhall, and that the latter are
nearly in contact with the millstone grit. The millstone
grit at Gromhall is from 900 to 1,000 ft. thick, and its
development at this point is equal to that of the average
of the district.

These facts would indicate that the upheaval on the
eastern boundary of the coal field had its birth in the
lower coal measure period, and that during this period
the movement was not sufficient to counteract the
general sinking of 'the older rocks entirely at this point;
but it was sufficient to reduce the general depression of
the country hereabouts, and so retard the deposition of
the new strata. The lower coal measures are known to
be thin on the western side of this area, along the flank
of the western upheaval—a fact indicating that this line
of fault originated at about the same time as the eastern
boundary fold, and was probably the result of the same
pressure. A confirmation of this estimate of the period
during which the movement of the western anticline
took place is afforded by the Nailsea coal field, which is
situated on its western side. The Pennant there is only
150 yds. thick and the lower coal measures 450 yds.
thick; no correlation can be established between the coal
seams proved there and those of the Kingswood district,
as their number and quality are quite different. The
Walton Down elevation probably formed the western
boundary of this coal field : its nearness to the main
axis and the fact that it was probably raised at the
same time would account for the comparatively small
development of the coal measures in this area.

It is probable that during the upper coal measure
period these forces were quiescent, or that at least the
movement was much reduced. The upper series is
nearer the average thickness than the Pennant, and the
subsidence in the Forest of Dean area was sufficient to
admit the upper series coals being deposited there.

Nature and Direction of Folds.

The action of these folds must have been of a com-
paratively slight nature until after the deposition of the
upper series coals, after which the east-to-west pressure
flexured the beds, and raised them to the high angles at
which they are now found along the eastern and western
margins of the district.

* Paper read before the Manchester Geological and Mining
Society.

H. STAPLES,

The North Wales-Woolhope anticline in its continu-
tion southwards crosses the line of the Malvernian ridge
at May Hill; but the direction of its axis, as seen
between Knighton and Woolhope, if projected, would
pass to the north of May Hill. The line of folding has,
therefore, been deflected slightly towards the south. A
similar deflection of the Malvernian ridge is to be
observed between the Malvern Hills and May Hill, this
fold being deflected towards the west to about the same
degree as the Woolhope anticline is deflected towards
the south.

The Woolhope anticline probably had an inter- as well
as a post-carboniferous movement in this region, and as
it was raised during the same period as the Malvernian
ridge, it had a direct influence upon it, driving it to the
west, and being itself drawn from a south-eastern line
to a line a little east of south. If a further deflection of
these ridges on the southern side of May Hill (equal in
amount to the deflection on the northern side) can be
shown to have taken place, it follows that the Malvernian
ridge bounds the coal field on the western side, while
the Woolhop‘e anticline forms the eastern boundary.

An examination of the strata near Patchway Tunnel
and between Tortworth and the River Avon at Clifton
suggests that the anticlinal ridge continues without bifur-
cation or deflection to the south of the River Avon. The
anticlinal and synclinal folds probably continued origin-
ally in uninterrupted lines beyond the Mendips; and the
mountain limestone is visible now at Cannington Park;
whilst further to the south, in the neighbourhood of
Burlescombe, culm limestone is to be seen at points
where the axis of the western ridge crossed the Mid-
Devon and Mid-Somerset synclines.

The author has made an attempt to estimate the
lateral compression caused by these folds, and to show
that, south of May Hill, their combined action would
divert their main axes towards the west. On the
southern side of the district we have the Pembroke-
Mendip anticline, which the author considers was formed
after the deposition of the coal measures had been com-
pleted, and subsequently to the two lines of folding
just described.

Continuing the study of the district southwards from
Gromhall, the first evidence of forces other than those
already mentioned is given by the Kingswood anticline.
This anticline is really a series of large overthrust faults
that have broken up the basin previously formed between
the east and west marginal anticlines, and raised the
lower coal measures above and over the Pennant series.

Anstie has already noted that in the Kingswood dis-
trict the top of the anticlinal fold rises towards the east;
it can now be shown that the the west of Kingswood the
ridge rises towards the west, thus indicating the dip of
the strata in the basin at this point before the anticline
was raised. The eastward rise is the result of a recent
movement, as will be shown later.

The faults composing the anticline on the western side
converge, and if prolonged would ultimately meet at a
point near the Great fault in the Avon gorge. Similarly,
the faults showing the limestone and millstone grit at
Wick Rocks open out towards the west, and may not be
unconnected with those just mentioned. It is a
peculiarity of the faults in the anticline that, whereas in
the centre of the basin the amount of overlap exceeds the
vertical throw, towards the western margin the vertical
throw is in excess of the amount of overlap—that is to
say, starting from the centre of the basin, the hades of
the faults increase as they are followed towards the west.
The author offers the following explanation of these
phenomena :—

Effect of Hercynian Movement.

At the time of the Hercynian movement the peculiar
formation of the basin which had been formed between
the Malvernian and Woolhope anticlines enabled the
strong beds of mountain limestone which formed the
cores of the boundary ridges to offer great resistance to
the pressure from the south. This resistance protected
the coals in the narrow basin of the Parkfield series, but
was unable to support the coal measures in the wider
part of the basin. The consequence was that, while the
limestone and millstone grit sustained one or two
fractures, the softer coal measures were fractured in
many places, and were driven forward in the centre of
the old basin in advance of a line connecting the main
faults in the boundary ridges. The northernmost over-
lap fault at Stapleton against the unmoved Pennant thus
appears in plan as an arc with its ends resting on the
limestone at the edges of the basin.

The overthrust fault to the south of the Old Easton
and Whitehall collieries is on a line almost straight
between the two faults in the limestone mentioned
above, and probably defines the southern boundary of
the Kingswood anticline.

The elevation of the Mendips with the inverted coal
measures and limestone at Vobster, with the Radstock
slide fault, are well known. Recent exploration and
investigation tend to show that the Farmborough fault,
which has hitherto baffled all attempts to prove coal to
the north of the collieries in the neighbourhood of Tims-
bury, is a slide fault similar to the Radstock slide, but
of greater magnitude, and composed of three separate
slides which follow each other in regular order.

It seems probable that this overthrust fault at Farm-
borough is connected with the elevation of th elimestone
on Broadfield Down, and affords another example of the
overfolded structure resulting from the greater resist-
ance offered to the pressure from the south by the rocks
of the lower carboniferous ridge, as compared with the
shales of the coal measures in the valley.

The fault through Hound-street continues eastwards
to Marksbury, while in the westward direction it pro-
bably joins the Farmborough fault in the neighbourhood
of Broadfield Down. This fault is a downthrow to the
north; in the secondary rocks the throw exceeds the
thickness of the red marl, and in the coal measures it
may certainly be expected to prove greater still.

Later information shows that Mr. James McMurtrie’s
section * through this part of the coalfield may possibly
require amendment in the following particulars : The
anticlinal roll plotted there to correspond with the
Broadfield Down upheaval and to fill up the interval
between the proved positions of the coal measures at
Chewton Keynsham and Timsbury Colliery to be
replaced by an overthrust fault on the line of the Hound-
street fault, and the Farmborough fault to be shown
as a flat slide hading to the south. This part of the
district from Timsbury to Keynsham and Brislington
would then exhibit the same structure as the Kingswood
anticline, but on a greater scale. The syncline of upper
series coals worked by a chain of old pits, and described
by Messrs. Buckland and Coneybeare f as a narrow
tongue between Brislington and Burnett, is similar in
form to a minor fold of the Kingswood anticline, where
a synclinal fold, which tapers gradually to a point in the
westerly direction, has been proved immediately to the
north of and below the main overthrust fault. That a
considerable disturbance to the south of this small basin
exists there is little reason to doubt : the sandstone of
Pennant aspect which has been frequently noticed
between Compton Dando and Pensford, and to which
Mr. Anstie has assigned an horizon higher than any yet
proved in the local collieries, may possibly be genuine
Pennant upraised by the agency of an overthrust fault.
This fault should be shown on the section at a point a
little to the south of Chewton Keynsham on a line from
Broadfield Down through Winford and Burnett to
Twerton, where possibly it was the cause of the damage
to the coal seams in the old Twerton pit. The rhaetic
beds in the line of this fault are thrown downwards to
the north, and as in the case of the Hound-street fault,
it may certainly be expected to prove of considerably
greater throw in the coal measures.

Mendip Upheaval.

The thickness of the strata on the flanks of the
Mendips gives little reason to suppose that any move-
ment of the Mendip fault took place during the lower
carboniferous period; on the other hand, the evidence of
the faults shows conclusively that this upheaval took
place after the deposition of the coal measures, and
subsequently to the formation of the east and west
boundary folds. The action of this fault upon the
direction of the Malvernian ridge appears to have been
very slight : it has cut the axis by east-to-west reversed
faults, and raised the level of the ridge in two places,
namely, on Broadfield Down and at the point of inter-
section of the two ranges at Black Down. The Mendip
range appears to have almost obliterated any trace of the
older range in its immediate neighbourhood. Evidence
of the former existence of the older range, however, is
afforded by the dip of the axis of the Mendip range itself.

The limestone in the Steep Holm no doubt belongs to
the flank of the main ridge of the Mendips; while it is
open to demonstration that the limestone in the Flat
Holm is a continuation of the Broadfield Down move-
ment, which, as already suggested, caused a line of over-
folding in the coal measures at Farmborough.

The coal measures at Clapton-in-Gordano are separated
from the Nailsea coal field by a fault which is nearly on
a line drawn through the faults at Wick Rocks, in the
Kingswood anticline and Clifton Gorge. This fault and

* Trans. Inst. M. E., 1900-1901. vol. xx., pl. xiv., p. 334.

I Trans. Geol. Soc., London, 1824, vol. i., p. 261.