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Llechwedd Slate Caverns
Llechwedd, Blaenau Ffestiniog, Gwynedd, Wales, UK
associated engineer
John Whitehead Greaves
Charles Warren Roberts
date  1846 onwards
era  Victorian  |  category  Mining/Quarrying  |  reference  SH700471
ICE reference number  HEW 1169
Llechwedd Slate Caverns are the chambers formed by slate extraction near Blaenau Ffestiniog in Snowdonia in Wales. Slate workings in the area date from the 18th century and the vein quarried here was discovered in the mid 19th century. Opencast slate mining continues at Llechwedd Slate Quarry and its caverns are now open to the public, offering underground mine tours and other activities.
In 1775, Welsh quarryman Methusalem Jones discovered slate at Gelli Farm in Diffwys, near Blaenau Ffestiniog, about 1.6km east of Llechwedd. In 1799, Englishman William Turner (1766-1853) leased Diffwys and began commercial exploitation of the slate. John Whitehead Greaves (1807-80) began prospecting for slate around Blaenau Ffestiniog in the 19th century and his company sold slate products from 1836.
In 1846, Greaves established a quarry at Llechwedd. He knew there must be slate nearby and in 1849, he discovered the Merioneth Old Vein of blue-grey Ordovician slate that lies in five beds inclined at 30 degrees to the horizontal, sandwiched between seams of hard chert. It is around 500 million years old and of sufficiently high quality that it can be split into layers less than 1mm thick.
Llechwedd Slate Quarry soon became one of the largest and most efficient quarries in the Blaenau Ffestiniog area. Underground chambering extends down 16 floors, almost to sea level, with 32km of tunnels. The pillars between chambers are aligned through a vertical distance of over 305m below ground. The levels are drained by gravity through an adit.
To open a new level, a tunnel is constructed beneath the chert layer and inclined shafts are driven at 20m intervals perpendicular to the tunnel. The shafts are enlarged into 11m wide caverns, separated by pillars of rock some 9m thick. The caverns or chambers are 15-21m long and 18-24m high, and each one took up to 15 years to mine using hand boring and gunpowder blasting.
Mechanical innovation was required to improve the processing of slate from extraction to finished products and Greaves advanced the technology. In 1850, he designed a sawing table for cutting slate into manageable blocks and invented a slate-dressing machine to plane and trim slates to uniform sizes.
In 1851-52, mechanised mills were constructed on several levels inside the caverns. Initially water power — from water balances and direct cranking — was used extensively for driving mills, pumping and haulage. Steam power was introduced in the 1850s, for haulage and later for locomotion.
The mine had a number of powered inclines, most of them underground, and several gravity inclines at the surface. In 1854, Greaves constructed an incline to make a direct connection with the narrow gauge Ffestiniog Railway (opened 1836) in Blaenau Ffestiniog. It enabled slate to be transported to the dock at Porthmadog for onward shipment.
By 1856, the Greaves slate trimmer had become an industry standard machine. He patented "improvements in machinery or apparatus for dressing slates" on 3rd October 1860 and 18th May 1861. In 1886, his son Richard Methuen Greaves (1852-1942) upgraded the machine and took out a new patent.
From 1872, the Cambrian Railway provided rail access to the national standard gauge network at Minffordd and in 1879, the London & North Western Railway line reached Blaenau Ffestiniog.
In 1882, the quarry produced 25,119 tonnes of slate and employed 553 men, and output increased into the 1890s. Record production levels of up to 50.8 tonnes per man year were quoted, representing a 9 to 1 waste to make ratio, compared with a local average of 12 to 1.
In 1890, hydroelectric power generation was installed at Llechwedd during the tenure of Charles Warren Roberts (1852-97), civil engineer and mine manager from 1887. The quarry established its own water supply in 1904, harnessing power from two reservoirs (SH706469 and SH706468) above and to the east of the mine. Water was piped to generating stations at Maenofferen (SH709469), east of the reservoirs, and Pant yr Afon (SH697468), south west of the caverns, where it drove the Pelton wheels of Gilbert Gilkes turbines connected to a Johnson & Phillips six pole 175kW direct current dynamo running at 385rpm. Excess electricity was fed back into the national power network.
In 1972, the lower workings were opened to the public. From 1979, it has been possible to visit deeper levels using the Deep Mine Railway, a funicular with a gradient of 1 in 1.8 (30 degrees), which is said to be the steepest passenger railway in Britain. Visitors enter the quarry adit and 10 of the Llechwedd Slate Caverns in trains running on the mine’s internal narrow gauge (610mm or 2ft) railway.
A museum was set up in the buildings used originally for the processing, trimming and splitting of the slate. It contains machinery devised by Greaves, including sawing tables from 1850 and 1882 (built by De Winton & Company of Caernarfon), with trimmers from 1850 and 1886, powered by an overhead shaft driven by an overshot waterwheel. Further facilities occupy the former quarry offices, located in the house built in 1870 as the Greaves family residence Plas Weunydd (SH698472).
In 1998, hydroelectric generation ceased at Llechwedd. The generating stations were later refurbished, and in May 2010, hydroelectric power was restored to Llechwedd. Water from the reservoirs generates electricity at Maenofferen and flows into the pipeline feed to the Pant yr Afon turbines — generating electricity from the same water twice — before being discharged into the Afon Barlwyd. The 1904 turbines have been replaced with new 400kW generators capable of producing up to 2,200kWh, and supplying the National Grid.
Modern opencast quarrying at Llechwedd is reducing the ground level by removing the slate pillars from the four original uppermost floors. Some slate mining is still carried out at the eighth level down.
In 2013, new audio-visual displays were installed inside Deep Mine and its caverns. In 2014, two new attractions opened. Bounce Below is a series of netting trampolines and slides suspended over three levels inside the mine caverns. Zip World Titan is a four-person zip wire ride high above Llechwedd Slate Quarry.
Contractor: J.W. Greaves & Sons
RCAHMW_NPRN 40597, 400426, 85488
Research: ECPK
"Blaenau Ffestiniog: Understanding Urban Character", Cadw, Welsh Assembly Government, Cardiff, 201
"Obituary: Charles Warren Roberts, 1852-1897", Minutes of ICE Proceedings, Vol.129, p.396, London, 1897
reference sources   CEH Wales

Llechwedd Slate Caverns