GeoGraph - photograph every grid square

Thick walls for storing the gunpowder and other explosives, and positioned between and well away from the work area and the dwellings; probably the manager would have held the keys and controlled the dispensing of the explosives.
"Powdwr Du (Gunpowder) was used every time since dynamite or gelignite would shatter the slate. The gunpowder was stored in the ty powdwr or cwt powdwr, (magazine) and before entering one had to wear copper soled clogs to lessen the danger of striking a spark. A bag powdwr (lit. powder bag) made of cloth or rubber was used to carry the gunpowder to the quarry face for blasting. Of course, dynamite or gelignite was used to blast away rubble to get at the slate."
See more at Slatesite Link an excellent history of slate quarrying in Wales.