Complex sentence: A major slating operation existed in Monson, Maine during the late 19th and early 20th centuries, where the slate is usually dark purple to blackish, and many local structures are still roofed with slate tiles.
Split into simpler sentences: A major slating operation existed in Monson, Maine, during the late 19th and early 20th centuries. The slate found in Monson is usually a dark purple to blackish color, and many local structures are still roofed with slate tiles.