Splitting of the slate blocks with and chisel to produce roofing slates requires great skill. This process was not mechanized until the second half of the 20th century, and some slate is still produced in this way. These quarrymen are working at the Dinorwic Quarry, Wales, about 1910.
Please note that Welsh did this on both sides of the Atlantic as slate was used as weights on ships and then broken into slates in America. My home church St. John’s Episcopal Church still has Welsh Slate roof in North Haven Cr. (1756)