The Nantlle Railway was built in 1828 and was operated using horse-power to carry slate from several slate quarries in the Nantlle Valley to the harbour at Caernarfon.
The same year, Lord Penrhyn opened a new quarry at Caebraichycafn near Bethesda, which as Penrhyn Quarry would become the largest slate quarry in the world.
Wales was by now producing more than half the United Kingdom's output of slate, 26,000 tons out of a total UK production of 45,000 tons in 1793.
Methusalem Jones, previously a quarryman at Cilgwyn, began to work the Diffwys quarry at Blaenau Ffestiniog in the 1760s, which became the first large quarry in the area.
Alun Richards comments on the importance of the slate industry: The prosperity of the slate industry led to the growth of a number of other associated industries.
There was an increase in demand for slate in the 1980s, and although this came too late for many quarries there was still some production in the Blaenau Ffestiniog area at the Oakeley, Llechwedd and Cwt-y-Bugail quarries, though the bulk of roofing slate production was at the Penrhyn Quarry.
The Mining Journal estimated in 1859 that the Penrhyn quarries produced an annual net profit of GB£100,000, and the Dinorwig Quarry £70,000 a year.
The quarrymen proper, who made up just over 50% of the workforce, worked the slate in partnerships of three, four, six or eight, known as "bargain gangs".
The Ffestiniog Railway line was constructed between 1833 and 1836 to transport slate from Blaenau Ffestiniog to the coastal town of Porthmadog, where it was loaded onto ships.
The Cilgwyn quarries were taken over by a company in 1800, and the scattered workings at all three locations were amalgamated into a single quarry.
By 1972, the number of men employed in the North Wales slate industry was down to under 1,000.
The earliest confirmed date of operating dates from the early 16th century when the local house Plas Aberllefenni was roofed in slates from this quarry.
