Complex sentence: Whilst of considerable historical significance, the origin and nature of the brook is still unclear; it is probable that some parts of the channel are natural, whilst other parts were created in mediaeval times to supply water to water mills and fish ponds.
Split into simpler sentences: Whilst of considerable historical significance, the origin and nature of the brook is still unclear. It is probable that some parts of the channel are natural, whilst other parts were created in mediaeval times to supply water to water mills and fish ponds.