 It dammed the river in Hertfordshire, resulting in the formation of large ice lakes, which eventually burst their banks and caused the river to divert onto its present course through the area of present-day London.Repeated advances of the ice sheet progressively pushed the channel southwards to form the St Albans depression.This created a new river-course through Berkshire and on into London, after which the river rejoined its original course in southern Essex, near the present River Blackwater estuary.