 During the Anglian glaciation, approximately 450,000 years ago, an ice sheet filled much of the North Sea, with a large proglacial lake in the southern part fed by the Rhine, Scheldt and Thames river systems.The catastrophic overflow of this lake carved a channel through the anticline, leading to the formation of the Channel River, which carried the combined Rhine, Scheldt and Thames to the Atlantic.This probably created the potential for Britain to become isolated from the continent during periods of high sea level, although some scientists argue that the final break did not occur until a second ice-dammed lake overflowed during the MIS8 or MIS6 glaciations, around 340,000 or 240,000 years ago.