Finchley Gap

As a topographical feature approximately eight kilometres wide, lying between higher ground to the north-west (Mill Hill) and to the south-east (Hampstead Heath), it has probably existed for the last one million years or more.

[3] From 1938 until the 1960s, it was supposed that the River Thames once flowed through the Gap, along the line of a "Middlesex loopway" running from somewhere around Harefield to the Hoddesdon-Ware area.

Prior to the Anglian glaciation, the "proto-Mole-Wey"river, a then south bank tributary of the "proto-Thames", flowed northwards from the Weald, through Richmond and the Finchley Gap, to the Hoddesdon-Ware area.

About 450,000 years ago, a lobe of the Anglian ice sheet advanced up the valley of the proto-Mole-Wey at least as far south as the Finchley Gap.

The Finchley Gap now lies on the watershed of the catchment areas of the Rivers Brent and Lea, both south-flowing tributaries of the Thames.

[5] In the early twentieth century, it was suggested that the River Thames, after descending through Oxfordshire, entering the London Basin near Goring and running north-east from there, continued in that direction prior to the ice advance, past Watford and along the line of the Vale of St Albans.

By the Middle Pleistocene, the Thames was flowing through the Vale of St Albans itself and was heading to the North Sea further south, in the Colchester area.

[11] In the first stage, it was assumed that "local or Chiltern ice" advanced from the north-west across the line of the Thames "in the neighbourhood of St. Albans", thus blocking the river and forcing it to find a more southerly course.

It was suggested that the Thames thus turned roughly eastwards from around Harefield, passed north of Harrow on the Hill, went through the Finchley depression, and then continued north-eastwards "to rejoin its former line near Ware".

It was later suggested that the section of the supposed "Middlesex loopway" going north-east beyond the Finchley depression "may well have originated as a ... south-bank tributary of an earlier axial line of drainage even though it later served for the diverted Thames".

The almost complete absence of any gravels which might have been deposited by the diverted Thames in the western section of the "Middlesex loopway" (between Harefield and the Finchley depression) was judged by Wooldridge to be a "slight" consideration, bearing in mind "how extensively the London Clay surface has been dissected and lowered here".

In the 1970s, detailed study of deposits in the Vale of St Albans led researchers to the conclusion that the River Thames never flowed through the Finchley depression.

Instead, it was established that the river stayed on a course north-eastward through the Vale of St Albans until it was diverted, by the Anglian ice advance, approximately 450,000 years ago.

[16][17] Furthermore, by the 1980s, evidence for a "Chiltern Drift" glaciation, which Wooldridge believed had diverted the Thames from the Vale of St Albans into its supposed intermediate route through the Finchley depression, had been seriously challenged.

[20] Although flint is the main component of this gravel, it has been known since the late nineteenth century that it also contains in places a notable quantity of chert derived from Lower Greensand Beds in the Weald.

He also suggested that similar gravel, located further north-east near Northaw at a slightly lower altitude, was also deposited by an ancestral Mole-Wey, but at a later date (which could have been around 1.75 million years ago).

[16] And meltwater which poured from the front and sides of the Finchley Gap ice lobe carved routes southwards, west of Hampstead Heath, towards the newly-diverted Thames.

A similar process led to the creation of the south-flowing lower section of the River Lea, which was able to establish a link to the diverted Thames to the east of Hampstead Heath.