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New discoveries in the Mona Mine and some depth
measurements in Parys Mine By Oli Burrows
Ever wondered how deep the mine really is?
I recently took down an altimeter and recorded the readings shown in the table
below, relative to the entrance door. One should of course remember that the 'levels'
set
were in fact often not exactly level but slightly inclined so that water would
drain naturally - an example of a drainage channel can be seen at the side of
the 20 fathom passage running north from the ladder down on the 20 fathom level
towards the Grand Stope. Thus the 16 fathom level will not be exactly 16 fathoms
throughout. Great precision is therefore pointless, and for convenience my
measurements were all taken at the Parys footway intersections.
Nominal Depth (ft.) Measured Depth (ft.) Corrected Depth (ft.)
Mine entrance 0 0 -24
10 fathom -60 -36 -60
16 fathom -96 -69 -93
20 fathom -120 -98 -122
30 fathom -180 -157 -181
45 fathom (base of 90ft stairway) -270 -236 -260
45 fathom (Carreg y doll chamber) -270 -248 -272
Windmill +51 27
Trig point +52 28
It is immediately apparent that my readings are consistently
less than the nominal depths. This is clearly due to the mine using a different
datum level. By applying a correction of 24ft. a remarkably near match can be
obtained. I have heard it said that the mine measured depths relative to the
base of the windmill. While this may be true it is clear that the windmill was
not the primary datum point (indeed, of course, it was not there until
relatively late in the mine's history) as it is about 50 feet above the Parys
Footway entrance.
The primary datum must be somewhere roughly halfway in height between the
Footway and the windmill. Ideally it should be on a point with good lines of
sight to the rest of the mine, although this is really not essential as a number
of secondary datum points (such as the windmill) could be established in
different areas.
My first thought was that it was that it might be at the Parys mine yard, but
this proves to be slightly lower than the Parys footway entrance. The primary
datum was presumably established at an appropriate point early in the mine
history, certainly by the start of deep mining and could of course have been
covered by later tipping or removed altogether when the Great Opencast was
created with secondary datum points being triangulated from it before its
disappearance.
The Parys Footway is thought to be relatively late and there is therefore no
reason to suppose that it need be in the vicinity of this. Has anybody any
thoughts where it might have been?
Turning now to the Mona mine. They used a different datum. Dave Jenkins tells me
that this was the Mona adit entrance. Certainly their primary datum was much
lower than Parys Mine as they had a +12 fathom level as well as levels going
down. This datum would certainly make sense if early workings were centred
around the Golden Venture with the Mona adit as the principal access adit to the
underground workings.
The importance of accurate surveying becomes apparent when
considering the joint drainage level (itself pre 1820) - both mines needed to
connect with this while approaching it from opposite sides, and with both having
to survey from different external reference points.
We can now access the Mona mine from the joint drainage level, which was the
Mona 20 fathom level. Higher levels were at 10 fathoms, zero fathoms and +12
fathoms.
Measured depth
(ft) Height Relative to 45fm. level Nominal Depth relative to their datum
Carreg y Doll chamber -248 0 0
Foot of Mona ladder (20 fathom) -236 12 0
Sidney Shaft (Mona 10 fathom) -192 56 60
Mona 0 fathom - area 1 -103 145 120
Mona 0 fathom - area 2 -115 133
Henry's shaft (Mona) -56 192
Mona footway (surface)
From here we climb two ladders lashed together (the lower an original!) to reach
an intermediate level about 25ft higher which branches left to Cairns shaft at
the level where it is boarded over. This was originally one of the main pumping
shafts and continues down to submerged workings. Two pump rods remain leaning
against the walls.
To the right of the entrance passage a series of three short ladders and a stone
walled ascending passage leads to Sidney Shaft (which is open down to the water
level), from which levels run in two directions. This point is measured at 56
feet above the joint drainage level and thus corresponds to the Mona 10 fathom
level. Passage through very dubious timbering leads back to Cairns shaft some 40
feet above the boarding, where there is a balance bob (designed to counteract
the weight of the pump rods).
Nearby another passage leads on to a further ascending walled section finishing
at a short vertical ladder which emerges in a stoped area. Chambers from here
extend back down to the 10 fathom level, and also up a steep white slope to a
high platform in what turns out to be the base of another chamber.
I first reached this by climbing this unstable slope. However, Dave discovered a
convoluted but much more user friendly route to reach the same point. About 10
feet below the level of the platform this route traverses a short length of
passage which continues in both directions (partially water filled in one
direction) but is as yet unexplored. Depth measurement suggests that this is
part of the zero fathom level (labelled area 1 on the table).
Our established route now continues up spoil from the platform to reach a short
(15ft.) climb which would benefit from the insertion of a fixed handline. A few
feet from the head of this climb is Henry's Shaft. This shaft (which has not
been explored) extends upwards to surface but appears to be boarded not far
below - probably at the zero fathom level. The indicated depth at this point is
-56ft., or 192 ft. (32fm.) above the joint drainage level. This corresponds well
with the +12 fathom level marked on the 1854 mine abandonment plans.
An extensive passage with sections of poor timbering runs westward from this
point, eventually reaching a short scramble to a small ladder giving access into
a rising passage. From here a scramble up leads to a final steep stone walled
and floored passage, the head of which is timbered over. This is the top of the
Mona Footway. We are here, literally, within handshaking distance of the surface
- in the bottom of the conical depression that Dave Jenkins had already
identified as the Mona Footway entrance.
Returning to the +12fm. passageway. At one point along this another passage
leads southwards (into deep mud), and beside it a ladder way goes downwards. On
our arrival the top ladder was somehow precariously balanced on the second, but
fell as soon as DJ touched it (at least that is his story!).
However, further exploration in the area of the Mona Footway gave access to
large stopes, and at their base a lower passage, which led back to the foot of
these ladders - the point being demonstrated by having parties on both levels.
Within a few minutes members had demonstrated that despite the missing ladder it
was still possible (albeit rather risky) to climb both up and down here between
the levels.
Measurement at the foot of the ladder suggests that, as might be expected, this
is also part of the zero fathom level (area 2). Initial measurement indicates a
difference in height between here and area 1 of around 12 feet, but this may be
due to inaccuracies in measurement and possibly some slope on the passages.
From this point passages extend north, south, east and west. South soon becomes
a large passage (c. 2m. x 2m.) but end abruptly after about 50m. North continues
a considerable distance without significant workings. At present is has been
explored as far as a natural dam. Beyond it continues but with deep deposits on
the floor and limited headroom above the water. To continue would inevitably
damage formations and this has not therefore been pursued. East is about 3 feet
deep in water. Although a promising direction, it is well lined with straws and
for reasons of conservation has only been explored for about 50m.
In terms of artefacts we have made a few finds - three old bottles by the Mona
entrance ladder, the remains of a number of pump rods, at least one of which
contains some of it's internal workings, a balance bob and box , a round piece
of wood provisionally identified as part of a pulley block or sheave near Sidney
shaft and perhaps the strangest find of all a 5 foot pipe manifold made of lead.
So far two maulstones have been found just below the Mona footway entrance which
have probably come in from surface have been found ,there is no trace of in-situ
Bronze Age workings (DJ had hoped that these might be accessible just west of
the Mona Footway), however, there are several passageways and chambers with nice
formations, and obviously still plenty of scope for further exploration.
All that remains is to tie the Mona depths into the Parys
Mine figures. The measured depth of the zero fathom level as around -110ft.
relative to the Parys Footway entrance. Unfortunately surface measurement at the
Mona Adit entrance indicates that this is some 162ft. below the same point. So
therein lies another problem!
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