 Due to the estuary being one of the few natural harbours on the north coast of south-west England, it was important for trade and the movement of people and ideas.A hillfort, overlooks the estuary at Carnsew, and dates from the later prehistoric period.Prehistoric artifacts with Irish connections have been found around the estuary, and in the early medieval period some of the earliest post-Roman evidence for Christianity in Cornwall, which also had strong Irish influences.