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Newsletter - March 2017
The full, illustrated newsletter is available as a pdf for download.
Text extracts are given below.
Halkyn Mountain
On the 14th of August Members of the MGA joined OUGS Members at Rhes-y-cae for the annual joint field trip. This year we went to Halkyn Mountain Common SSSI not far from Mold and the A55. The day was in two parts; in the morning we looked at the ecology and industrial history of the area (with a little geology) and after lunch it was geology all the way (well with the occasional rare plant thrown in for good measure).
Our leaders for the morning were Rachel, a Wildlife Warden for the SSSI, and John Watson a Geological and Environmental Consultant who had undertaken a review and risk assessment of the 4000 mineshafts for estate owners (Grosvenor Estates). The area is on the earliest Carboniferous Limestone (Dinantian), a competent packstone, lies directly over older Silurian Shales. The shallow water marine conditions allowed the deposition of a thick succession of limestones - the Cefn Mawr and Loggerheads. The east to west trending faults were mineralised and have vertical movements of up to 36.5m. There are also open fissures that were formed by tension stress. The lodes generally dip between 70° and 80°. Other north-west trending faults formed by compression are not mineralised.
The mineralisation is mainly lead (galena) and zinc (sphalerite); the gangue minerals include calcite, pyrite, chalcopyrite and fluorspar. After Derbyshire the common land became the second largest lead producer in the UK; 10% of UK production at one stage. Mining ceased in the 1960s.
The Halkyn estate covers 1800 acres, has 4000 mine shafts/deep mines, three active quarries, disused limekilns, Moel y Gyr Iron Age Fort, six villages and 200 residents with grazing rights (although only 12 actively graze sheep). John showed us the history of one shaft-collapse that was very close to the car park.
To remedy the collapse large plugs of PFA/cement (9:1) were put in the hole; 687 tons of rock and 227 tons of grout were used! Locally sourced topsoil was used to cover this then the area was fenced. It is hoped that the fence will be removed shortly.
After looking round this area Rachel took us on a walk that encompassed most of the Common. In particular we were able to see at first hand the various types of mine caps in place. One in particular, the Clywd Cap, was developed locally and won a Prince of Wales Award. These are also used in Cornwall and Derbyshire. We looked down into the Chwarel Pant-y-pwll-dwr quarry where one of the tunnels dug to drain the mines could be seen.
This was mainly in the Loggerheads formation. We climbed to the Trig Point at 294m where we had excellent views of the Dee Estuary, Blackpool Tower (just), Liverpool Anglican Cathedral, the Clywdians and Snowdonia. After passing by the Knocking Stone, where the ore was broken up, looking at rare Stemless Thistles and the lead ore washing areas, we visited the lime kilns where we found crinoid fossils and the occasional productid. The limekilns have been restored with small holes left in the brickwork for bats.
John and Rachel then left us having been thanked by Pam Norris for such an interesting morning. We drove to Halkyn village where we had lunch at the local community pub - the Bluebell. Our afternoon leader, Tony Kirkham, took us on a tour of various quarries to investigate the limestones and finally the Pentre Chert formation. He had produced an excellent handout with colour photographs of various aspects of the quarries and, as these had been taken over a number of years, it was very interesting to see how the sites had changed through time.
We recommenced at the Waenbrodlas Quarry that is still working.
It is set in the Brigantian age Cefn Mawr Limestone formation that is generally dark carbonates. It is easy to tell them apart from the Loggerheads formation, as the latter are much paler. As with everything, just because a limestone was light coloured, it did not mean that it was the Loggerheads formation as some of the Cefn Mawr formation limestones are also pale; this has been interpreted as them having been re-sedimented. There is a very dark band of shale, up to 3m thick, that crosses the quarry and acts as a good marker bed. This is the Waenbrodlas Shale.
Tony said that the depositional environment of these limestones was considered to be a carbonate ramp, distally steepening, eventually developing into a rimmed platform with a very steep frontal edge. Mud mounds seem to have formed on this front and later slumped. We saw several examples of slumping within the beds. Whilst in the past, faults could be seen in the quarry, we could no longer see the rollover anticlines in the hanging wall so took his word that the faults were listric. He informed us that there were two major oblique faults that appeared to have operated as a pair and this had probably been the cause of the slumping. A thrust fault could be made out above the shale, but it petered out in the quarry in a decollement.
We moved on, not far, just around the corner to a disused part of the quarry. This exposed the Pentre Chert formation (Namurian), which is the thickest chert formation in the UK - there certainly was a lot (as we were to see near the end of the trip). As Bryn Mawr Quarry (now subsumed into Waenbrodlas) chert had been quarried rather than limestone.
The source of the silica for the chert was discussed, but no firm conclusions were reached. Siliceous skeletons have been found in the chert, but not in the abundance required for the volume of chert produced, but it is likely to be of biological origin. We also saw what Tony considers is a Neptunian dyke (where a crack in the sea floor is in-filled with sediment). He postulated that there is a tendency for rimmed carbonate platforms to lose their edges resulting in debris flows. This debris can fill cracks in the sea floor. Certainly there was an area where the rock was definitely mashed up (although now heavily vegetated this could just about be seen).
The final stop was at the disused Pen-yr-Henblas Quarry that showed the extent and thickness of the chert. Bedding was seen easily and there were many examples of fine lamination indicating quiet water. There had been two phases of chert formation. Again we saw slumping down slope to the northeast indicating instability in the sloping area of the platform. Looking at the bigger picture (by way of a long rock face probably 100m long) we could see what appeared to be a river channel, possibly with others nested in it especially near the edge. There seemed to be slumping into the main channel. It was suggested that perhaps turbidites flowed down the platform slope had resulted in channels with further slumping into these at a later date. Nothing was straightforward!
The trip finished at this point (although a few people moved on to look at the view over Pant Quarry) and Pam thanked Tony on behalf of everyone for a thought-provoking and interesting afternoon.
Jane Michael
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