The Isle of Thanet first came into being when sea levels rose after the last glacial period, around 5000 BC. The North Sea encroached on the land which is now the estuary of the River Thames, and southwards to reach the higher land of the North Downs, leaving behind an island composed of chalk in its wake. Eventually the sea broke through river valleys in the North Downs to the south (Middle Chalk) and finally today's English Channel was opened up. The proto-River Stour then formed part of the intervening water, with a new tributary, the River Wantsum, completing it; it became known as the Wantsum Channel.