Enough of the Chilterns and the Cheviots. Things are even more interesting close to home. Today Nick McIlvenna invited me over to his place to take a look at his marl pits. Gosh - I have been here for 46 years and have never seen them before, although I have heard of them now and then from those who know what lurks in the depths of the woodland.
Anyway, Nick's land lies in a broad open area in the Nevern - Clydach Valley, about 4 kms to the SE of Newport and the coast. To the west is the Clydach River, with Carningli beyond. To the east is Pentre Ifan Wood. To the north is the great loop of the River Nevern, diverted northwards by glacial and glaciofluvial accumulations during the last glacial episode, namely the Late Devensian, around 24,000 years ago. To the south is Cilgwyn, with an extremely complex association of glacial sediments and rock outcrops -- surface till, mounds of sand and gravel, hummocky morainic mounds and outcrops of rhyolite, dolerite and assorted volcanics.
On the geological map we see an extensive sheet of till around Coed y Pwll, but this is very different from the till in the Cilgwyn area, which is best described as thin meltout till or flowtill mixed with patches of slope breccia and localised patches of meltwater deposits. This is genuine Irish Sea till, still exposed at the surface because there are no slopes to generate freeze-thaw breccia or colluvium that might have covered it. So on the surface there is occasional waterlogging, and exposures of sticky stony clay till. It's exposed in the sides of a recently dug pond, as heavy clay-rich lodgement till with clasts of all shapes and sizes, mostly of local dolerite, rhyolite and volcanic ash but with many small foreign erratics. There is one massive boulder which has recently been uncovered by Nick -- it's big enough to have been the capstone of a cromlech, and the rock type reminds me of some of the rocks exposed in craggy outcrops in Tycanol Wood. I did not see any sea shell fragments or lignite, but I would not be surprised to find them if we could just see a completely clean fresh face. Nick tells me (from borehole evidence) that in places the till sheet is around 20m thick, with a thin layer (about 3m) of sand and then gravelly material beneath it which rests on the igneous bedrock.
To the south the till sheet is overlain by glaciofluvial sands and gravels, and towards Fachongle Isaf the junction between the waterlogged clay surface and the drier sandy surface is quite striking. There are several hillocks of quite clean sand.
The fresh till is coloured light grey or bluish-grey, but at the surface the introduction of organic matter and pedological processes have given it a darker brownish hue, and there has been a lot of gleying here as well, giving a range of other colours from foxy red to orange to blue to white. On the coast (as at Abermawr and Mwnt) the Irish Sea till is calcareous and is composed mostly of dredged-up sea floor sediments -- but here it may be decalcified or weathered.
There is a very similar till sheet exposed in fields to the north of the A487 main road, to the east of the Temple Bar crossroads. Here again it is very sticky underfoot, and there is much surface waterlogging.
George Own of Henllys described this till in great detail in one of the earliest "geological" pamphlets, in 1599. I referred to it in this publication in the Journal of Glaciology, in 1964:
So this was the clay marl on which George Owen was so keen, back around 1600. At one time, the land around Coed y Pwll was part of Owen's Henllys estate. The marl was valued because it was calcareous and because it could be used to counteract the natural acidity of local soils --but if it was dug out, carted and spread onto rich and friable soil it must have been a nightmare to cultivate, and I suspect it did more harm than good........
Anyway, Nick has about 2 acres of clay pits in his woods, and they are magnificent. There are others within a kilometre or so, on land belonging to other farms. I had assumed in advance that they were circular waterlogged hollows, but not a bit of it -- they are elongated sloping cuttings with steep sides, and separated by ridges. They are between 5m and 8 m deep. They are remarkably dry, even in the depths of winter, because all of the pits or trenches have sloping floors, some with gullies cut into them to facilitate good drainage. There is one particularly deep trench with near-vertical sides close to Nick's house which is not a natural stream cutting -- and it must have been excavated to drain away the water from other clay diggings a couple of hundred metres to the south. So the drainage routes run broadly N or NNW, down towards the Dolbont stream. In one or two places there are signs of gently sloping "ramps" running from the outer ground surface down to the marl digging face -- these much have been used by horses and carts, by far the most efficient haulage method. There was a lot of engineering here -- pits, drainage channels and infrastructure. No buildings, as far as we know. I hope Nick will make a map of the marl cuttings, since they are impossible to see from above because of the mature tree cover.
We can assume that most of the excavated clay marl was sold to local farmers for spreading on the land, but I am intrigued by the idea that some of it might have gone to the medieval pottery kilns in Newport. It could well be that these pits were open in the Middle Ages, and we can assume they were in operation around 1600, but we do not know when the last digging occurred. Intriguingly, the clay pits are not shown on any of the OS maps.......
All in all, the evidence from this spread of lodgement till is of no relevance to the debate about the bluestones, but it tells us a lot more about the events of the last glacial episode in North Pembrokeshire. As I have said before on many occasions, I think the incoming ice of the Irish Sea Glacier overtopped Carnedd Meibion Owen and possibly Carningli too. This means that the ice which flowed across this expanse of lodgement till must have been at least 300m thick.
3 comments:
Sounds like you could make a very instructive video of your visit to Nick's marl pits! Potentially a good local history item.....
Strange how things and names from the past crop up 'out of the blue'so to speak.
For a few years now I, and my friends, have been studying large rock outcrops both for the nature of the rock and for the thickness of the beds. One such location which has only recently become a place of investigation is Tycanol Wood, a fascinating site of rich potential. However, it is also one of the creepiest places I have ever been to and it gives me the shivers just to read the name, but I fear there are several more visits to be made, but not on my own.
Tycanol Wood? I love it over there -- it's just round the corner from where we live. But don't worry, Mavis, I'll accompany you into the wood, and will hold your hand in reassurance. Most people think the wood is a place of tranquillity and beauty -- but I have found others who find it very spooky. A friend of mine camped up there once, while he was doing his dissertation on the Iron Age fortified site in the depths of the wood. He was so scared in the middle of the night that he gathered up his stuff and fled -- he remains convinced that the place is haunted by very nasty creatures.. So there we are then.
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