PORTH YSGO MINES EARTHCACHE EarthCache

The late Precambrian times saw considerable volcanic activity in what is now Wales but was followed by a prolonged phase of relative quiet during the succeeding Cambrian Period. At this time Wales was located on a small fragment of crust known as Avalonia which lay along the northern margin of Gondwana, a large continent that straddled the south polar region.
In Wales the Cambrian Period was dominated almost wholly by deposition of sedimentary rocks. These deposits indicate the start of the Welsh Basin, a region of subsidence that existed for about 125 million years during Lower Palaeozoic times, which was floored by a basement of Precambrian igneous and sedimentary rocks. However, there are some thin layers of bentonites in the early & mid-Cambrian sedimentary succession of the Welsh Basin, fine examples of which are exposed in the coastal cliffs of St Tudwal’s Peninsula on Llŷn and at Friog, south of the Mawddach Estuary and these indicate that there were a few distant volcanic eruptions, although the locations of these volcanoes have yet to be identified.
In contrast to the Cambrian, however, the Ordovician Period witnessed major episodes of intense volcanic activity in what is now Wales and this continued into early Silurian (Llandovery) times. The cause of this volcanism has been attributed to the onset of south-eastward subduction of the Iapetus Ocean crust beneath Wales early in the Ordovician. As well as generating the molten magma deep beneath the ground surface, the process of subduction also exerted extensional stresses in the Earth’s crust. This caused renewed movement on pre-existing faults, as well as creating new fractures. The molten rock was able to ascend to the surface along these structures, with major volcanic activity being restricted to narrow graben (blocks of the Earth’s crust that have been raised & lowered) located between the most important faults. This vulcanicity was also stimulated by the subsequent northward rifting of Avalonia away from Gondwana and towards Laurentia. The rifting caused the opening of a new seaway, the Rheic Ocean, between Avalonia and Gondwana. The subduction beneath Avalonia is believed to have ceased during the Late Ordovician.
The locations of volcanoes during Ordovician times varied both in space and time, with major centres occurring in Snowdonia, Llŷn &the Berwyns, as well in Pembrokeshire and the Welsh Borderland. In most cases this volcanism was brief. For example in Snowdonia it lasted for only about 3.5 million years, yet it accounts for nearly half of the total thickness of Ordovician rocks. The thick deposits of volcanic rocks in the Snowdon district accumulated within one of these graben, which was also associated with a series of large caldera, one of which appears to have been to the south of the Llyn Peninsula. Some of these calderas show evidence of distinct phases of collapse and resurgence , during which magma was forced upwards in ring dykes, many of which ended up containing deposits of valuable minerals.
At Porth Ysgo this was manganese, the ore of which occurs in Cambrian & Ordovician Age rocks.

The Llyn Peninsula Mines.
The mines in Lleyn fall into 2 geographic groups: the Northernmost group is situated on the SW slopes of Mynydd Rhiw and contains Rhiw, Benallt, Tŷ Canol and Tyddyn Meirion mines. About a mile to the south was Nant mine at Nant Gadwen and Porth Ysgo. These mines were the largest producer of manganese ore in the UK in the early 20th century. Nant Mine raised 45,644 tons of manganese ore prior to its closure in 1927 and 195,000 tonnes was produced during the lifetime of the Benallt mine.

During the formation of Mynydd Rhiw, the soft Arenig Ordovician mudstones, sandwiched between layers of dolerite, were subjected to immense pressures and were folded & shattered forming a scattered ore bed of manganese that was mined. It resulted in beds, the largest 30 feet thick, but more usually approx. 10 feet thick, rather than the usual veins that form with most metal ores. The main ores here were formed by mineral solution intrusion from nearby volcanic activity so that the chief ore was manganese silicate and oxides of iron and manganese.

Manganese was first discovered in Nant Gadwen, on the lands of the Nanhoron estate in 1827 and mining began in 1858. Initially the oxidised surface layer of manganese was excavated from surface from open pits, which were then enlarged following the dip of the beds downwards. When the overhanging rock became too great, the men would begin to drive levels into the bed and continue underground.
In 1903 the Liverpool based company ‘North Wales Iron & Manganese Co. Ltd’ came to the area, and set about re equipping the mines. First on the agenda was the construction of a pier in Porth Ysgo, and a 3 ft gauge tramway down to it from the mine.
1906 saw the record output of ore from the Benallt mine, which amounted to 2/3rds the UK production of manganese at that time. This didn’t last long, however, and output soon dwindled although there was a boom during WWI.
After mining ceased at Nant Gadwen manganese ore could still be found amongst the tips in commercial quantities and was sold as late as 1928. Mining continued at Benallt, however, and during WW II it produced 60,000 tonnes while being run by the Ministry of Supply.

In 1942 hopes were high of a revival in Nant Gadwen. Magnetometer surveys were conducted across Benallt and Nant, and whilst the former had revealed some 10000 tons worth of ore, it was decided that further work at Nant was not viable. The whole mine had been drained for the purpose of underground exploration, which took 2 months and was the last time the mine was unwatered.
Production at Benallt tailed off slowly and finally ceased in 1945. The closure of the mine was brought about not by lack of ore but more by it’s poor quality. After the war Britain was once again free to import higher quality & cheaper ore from overseas. When the last mine closed in this now quiet corner of Llŷn there was a workforce of 30 men below ground and 21 above who had to look for work elsewhere.

To log this earthcache .
Please send me an e mail (including your e mail address) with the answers to the following questions which will require you to do some searching on the internet and at the site:
1) Explain what is meant by subduction.
2) Bentonite is defined as a naturally occurring material that is composed predominantly of the clay mineral smectite. Do some research to explain how they are formed and why this indicates the presence of distant volcanoes.
3) What is the diameter of the largest metal wheel in the mine building at the top of the hill and how many spokes does it have?
4)Name one ore that contains manganese. See if you can find any of this in the spoil heap near the listing co-ordinates and, if you can, describe its appearance &/or post a photo.
5) Originally manganese was used as an oxidising agent in the manufacture of bleach but an important discovery in the early 1900’s that led to a new use and resulted in it becoming a strategically important mineral. What was this new use?

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