Slate splitters practiced a highly skilled trade that turned slate into thin roofing layers by exploiting its natural cleavage, a property formed when ancient mud settled in layers on the bottom of an inland sea four hundred to six hundred million years ago and later hardened under extreme pressure; Peach Bottom slate formed during the Ordovician geological period about five hundred million years ago. Splitting slate was long considered an art that looked easy but was actually difficult, and no machine ever replaced the splitter, so the work continued for hundreds of years with virtually the same tools. Splitters were the highest paid workers and the aristocrats of the quarry, usually working in two-man teams called a block, with one worker splitting and the other trimming or dressing the slate to proper size. In 1880 their wages were $1.50-$2.00 a day, and the craft had become a dying art with very few slate splitters still working in the United States.