Complex sentence: Coal has been mined at Bradford since at least the early 17th century, when the area around the pits was largely rural; it became increasingly built-up and industrialised as nearby Manchester expanded during the 19th century.
Split into simpler sentences: Coal has been mined at Bradford since at least the early 17th century, when the area around the pits was largely rural. It became increasingly built-up and industrialised as nearby Manchester expanded during the 19th century.