 At this time the river-system headwaters lay in the English West Midlands and may, at times, have received drainage from the Berwyn Mountains in North Wales.Brooks and rivers like the River Brent, Colne Brook and Bollo Brook either flowed into the then River Thames or went out to sea on the course of the present-day River Thames.About 450,000 years ago, in the most extreme Ice Age of the Pleistocene, the Anglian, the furthest southern extent of the ice sheet reached Hornchurch in east London.