Prior to the ice ages the River Thames followed a path through the southern part of Hertfordshire, running from the area of modern Staines up the valley of the Colne to Hatfield and then eastward across Essex originally towards the primeval Rhine but later down the valley of the modern River Lea. This path was blocked by a mass of ice near Hatfield and a lake ponded up to the west of this around St Albans. Waters eventually overflowed near Staines to cut the path of the modern Thames through central London. When the ice retreated about 400,000 years ago the river bed along the new route followed the lower path and so the river remained on its present-day course. The flow in the Colne valley reversed, now flowing south as a tributary into the modern Thames. Superficial gravel deposits from the primordial Thames, are found throughout the Vale of St. Albans.