The Finchley Gap is a topographical feature in North London and describes a band of relatively low-lying land linking the valley of the River Colne, which at one time carried the primordial River Thames, in the north in Hertfordshire, to the valley of the River Brent to the west of London. The gap is a hypothetical glacial drainage channel that carried melt waters south during the Quaternary ice age when the valley of the Colne/Thames was blocked by ice and moraines.
The Finchley gap is itself considered to have been blocked by later advances of the ice sheets known as the Anglian glaciation and a subsequent overflow channel then developed in the region of Staines which remains the course of the River Thames today.
The feature was first detected and described by Professor Sidney Wooldridge (1938 and 1960), who dated it to the Anglian Stage glaciation around 450,000 BP, when ice and boulder clays advanced up the Thames valley as far as Bricket Wood and Finchley in the west and Hornchurch to the south. Later work (by Moffat and Catt, 1982; Avery and Catt, 1983; Green and McGregor, 1983), which matched gravel deposits in the Vale of St Albans with those in the modern lower Thames valley suggests that the Finchley overflow route is unlikely.and that once ponded back by the ice the primordial Thames overflowed directly into its current route around Staines. 
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