At the time when the flint which John Conyers found opposite Black Mary's was chipped into its present shape the Waveney, the Rhine, the Somme, and fifty other rivers, great and small, on both sides of the Channel, were all of them tributaries of the Thames. It may perhaps be objected that if the Thames and the Rhine were ever united, the Thames ought rather to be considered as a tributary of the Rhine than the Rhine of the Thames.
"The Scheld, the goodly Meuse, the rich and viny Rhine,
Shall come to meet the Thames in Neptune's wat'ry
plain,
And all the Belgian streams and neighbouring floods of
Gaul
Of him shall stand in awe, his tributaries all. "
During the last Ice Age (~70,000-10,000 yrs Before Present), at the end of the Pleistocene, the English and Irish Channels, the Baltic Sea and the North Sea were still dry land, mainly because sea level was approximately 120 m lower than today. At about 5000 BC, (2pm local time - according to neolithic mans wrist sun-dial recently discoverd being sold at Romford Market) flooding and erosion began to open the English Channel and create a sea.
Between about 240 MBP (million years before present) and the last Ice Age. The Rhine was all over the shop.