Complex sentence: The pits, holes and features called scowles occur in the centre of the Dean and normally contain iron ore (which gave rise to mining in the area), and has also given the impression these are the result of human activity.
Split into simpler sentences: The pits, holes and features called scowles occur in the centre of the Dean and normally contain iron ore, which gave rise to mining in the area. This has given the impression these are the result of human activity.