<P> Most underground slate mines in north Wales were closed by the 1960s although some open - cast quarries have remained open, including the Penrhyn Quarry and the untopping work at Oakeley in Blaenau Ffestiniog . Work also continues at Berwyn near Llangollen . The final large - scale underground working to close was Maenofferen Quarry (which is owned by the Llechwedd tourist mine) in 1999 although opencast quarrying continues at this location . </P> <P> Many of the mines are now in a state of considerable decay and those that are accessible should not be entered as they are on private property and contain many hidden dangers . The lower levels of many mines are now flooded and collapses are commonplace, for example the hillside above the Rhosydd workings has many pits where the roofs of the chambers below have collapsed . </P> <P> The most significant is that of Cornwall and Devon where the Delabole Quarry is thought to be the largest single quarry in the island . Many of these are no longer worked owing to lower costs of extraction in the larger British workings . The quarrying of slate in Cornwall is known to have been carried out from the late mediaeval period and there was a considerable export trade from some of the quarries near the coasts in the 19th century . </P> <P> Slate has also been quarried at Swithland in Leicestershire and there are considerable workings in Cumbria . </P>

Where does slate come from in the uk