Complex sentence: Ferryhill grew dramatically as a mining town in the early 20th Century but before the Reformation, Ferryhill belonged to the Priory of Durham and was a thriving agricultural concern.
Split into simpler sentences: Ferryhill grew dramatically as a mining town in the early 20th Century but before that it was an agricultural village. Before the Reformation, Ferryhill belonged to the Priory of Durham and was a thriving agricultural concern.