Prior to the last Ice Age, the River Thames followed a more northerly route to the North Sea, from Reading via Marlow, Chorleywood, St Albans, Hertford and along the present Suffolk–Essex border. During this period, the Mole is thought to have merged with the River Wey near Byfleet and then flowed in a north-easterly direction via Richmond to meet the proto-Thames near Ware in Hertfordshire. Today the Mole and Wey are less than apart at their closest point near Painshill Park. During the Middle Pleistocene period, a large ice sheet built up across much of the East of England, reaching as far south as St Albans and Chelmsford, blocking the path of the proto-Thames. Glacial meltwater from the Anglian ice sheet caused the Thames to divert southwards and flood the valley of the Mole–Wey river, thus adopting its present route through London.