Complex sentence: Salt panning was a very important industry in the early history of Prestonpans, indeed by the beginning of the 15th century there were ten salt works belonging to the town capable of producing between 800 and 900 bushels of salt per week.
Split into simpler sentences: Salt panning was a very important industry in the early history of Prestonpans. By the beginning of the 15th century there were ten salt works belonging to the town capable of producing between 800 and 900 bushels of salt per week.