The River Thames can first be identified as a discrete drainage line as early as 58 million years ago, in the late Palaeocene Period Thanetian Stage. Until around half a million years ago, the Thames flowed on its existing course through what is now Oxfordshire, before turning to the north east through Hertfordshire and East Anglia and reaching the North Sea near Ipswich. At this time the river system headwaters lay in the English West Midlands and may, at times, have received drainage from the North Wales Berwyn Mountains. Arrival of an ice sheet in the Quaternary Ice Age, about 450,000 years ago, dammed the river in Hertfordshire and caused it to be diverted onto its present course through London. This created a new river route aligned through Berkshire and on into London after which the river rejoined its original course in southern Essex, near the present River Blackwater estuary. Here it entered a substantial freshwater lake in the southern North Sea basin. The overspill of this lake caused the formation of the Dover Straits or Pas-de-Calais gap between Britain and France. Subsequent development led to the continuation of the course which the river follows at the present day.
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