The Finchley Gap (or "Finchley depression") is a location centred on Church End, Finchley, in north London, England. 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.The term "Finchley Gap" has been used in archeological literature - for example, in Collins, D. (1976), The Archaeology of the London area: current knowledge and problems, Issue 1 of Special paper, Volume 1 of Occasional Publications, London and Middlesex Archaeological Society; and in Roe, D.A. (2014), The Lower and Middle Palaeolithic Periods in Britain, Volume 46, Routledge, , page 140. The term "Finchley depression" has been used in geomorphological literature, as in various references cited in this article, notably Wooldridge (1938).
The Finchley Gap has lower ground to the north-east (the catchment area of the River Lea), and the south-west (the catchment area of the River Brent). The North Circular Road (A406) passes over the Gap, from one catchment area to the other.See the online topographic map at topographic-map.com (London) for a clear picture of the current topography of the region around the Finchley Gap.
The geological history of the Finchley Gap, and its place in the history of academic research, makes it one of the most interesting topographical features of the London area.
The principal geological formation in the Finchley Gap region, as in much of the London Basin, is Eocene London Clay. This is mostly a stiff blue-brown clay, over 100 metres thick. In parts of this region, a relatively thin, upper part of the London Clay formation, sandier in content and known as the Claygate Beds, is also found. In certain areas of relatively limited extent, such as on the higher parts of Harrow on the Hill, Hampstead and Highgate, the London Clay and Claygate Beds are overlain by sandy Eocene Bagshot Beds. All these formations are overlain in several areas by much younger Pleistocene formations, as explained below.See BGS Geology Viewer (British Geological Survey) for more information on the distribution of all the geological formations mentioned in this article.
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.The term "Finchley Depression" has sometimes been used to refer to a more extensive area extending to the north-east of Church End, along the eastern section of the "Middlesex loopway". For the purpose of this article, the terms "Finchley Gap" and "Finchley Depression" are given the more limited definition cited above. This supposition was later shown to be erroneous.
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. There, it joined the proto-Thames (which at that time was flowing north-eastwards, through the Vale of St Albans).
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. There, the ice left glacial drift which today is up to 18 metres in thickness.
Another lobe of the Anglian ice sheet moved up the Vale of St Albans, thus diverting the Thames southwards. When this happened, the proto-Mole-Wey was cut off at Richmond.
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.
The "Middlesex loopway" hypothesis - Context It has been known since the nineteenth century that an ice sheet once descended from the north of England as far as north London and left behind extensive spreads of till and other glacial deposits. Lobes of this ice sheet extended up two valleys, oriented south-west to north-east, reaching as far south as Watford and Finchley.Nineteenth century studies of glacial deposits in the Vale of St Albans and near the Finchley Gap include: Prestwich, Joseph, On the Occurrence of the Boulder Clay, or Northern Clay Drift, at Bricket Wood, Near Watford, The Geologist, June 1858, page 241; Walker, H. (1871), On the glacial drifts of North London, Proceedings of the Geologists’ Association, 2, page 289; and Goodchild, J.G. and Woodward H.B. (1887), Excursion to Whetstone and Finchley, Proceedings of the Geologists’ Association, 10, page 145.
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.Salter, A.E. (1905), On the superficial deposits of central and parts of southern England, Proceedings of the Geologists’ Association, 19, pages 1–56. It was concluded that the Thames must have been diverted to its more southerly course of today by the ice advance up the Vale of St Albans to Watford.Sherlock, R.L. and Noble, A.H. (1912), On the glacial origin of the clay-with-flints of Buckinghamshire, and on the former course of the Thames. Quarterly Journal of the Geological Society of London, 68, pages 199–212.
These hypotheses have since been confirmed by much subsequent research, some of which is cited in this article. In fact, in the Early Pleistocene, the Thames flowed somewhat north-west of, but parallel to, today's Vale of St Albans, then continued towards North Norfolk on its journey to the North Sea. 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.Maddy, D. Bridgland, D. and Westaway R. Uplift-driven valley incision and climate-controlled river terrace development in the Thames Valley, UK. Quaternary International, 79, 1, 2001, Pages 23-36.Bridgland, D.R. (1994, 2012), Quaternary of the Thames, Geological Conservation Review Series, Springer Science & Business Media, ISBN 978-94-010-4303-8, ISBN 978-94-011-0705-1 (eBook) (originally published by Chapman and Hall); especially chapters 1 and 5.
The "loopway" hypothesis In 1938, it was suggested by the British geomorphologist S.W. Wooldridge that the River Thames had been diverted from its course through the Vale of St Albans to roughly its present, more southerly, course, in two stages.Wooldridge, S.W. (1938), The glaciation of the London Basin, and the evolution of the Lower Thames drainage system. Quarterly Journal of the Geological Society of London, 94, pages 627–64. This two-stage hypothesis was presented again over the subsequent three decades in several publications.In, for example, Wooldridge, S.W. and Linton, D.L. (1955), Structure, surface and drainage in South-East England (first published in 1939), Philip, London (text online at archive.org); and in Holmes, A. (1965), Principles of physical geology, Nelson, page 709. In archaeological literature, the notion that the Thames once flowed through the Finchley Gap persisted for even longer - as in Roe (2014).
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".Wooldridge (1938) page 658. (As explained below, this hypothesis of a "Chiltern" ice advance is not now accepted.)
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".Wooldridge and Linton (1955), page 133. This "south-bank tributary" would have been the proto-Mole-Wey. In his 1938 paper, Wooldridge suggested that deposits which do exist in this eastern section of the "Middlesex loopway" (known today as Dollis Hill Gravel) were "possible" gravels laid down by the Thames.Wooldridge (1938) page 653. (This too has since been shown to be a false assumption.)
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".
The second stage in the diversion of the Thames supposedly took place "a long period of time" after the first stage.Holmes (1965) went so far as to suggest that the Thames flowed through the Finchley depression "for perhaps half a million years", from about 1.25m years ago, to about 750,000 years ago. This has since been shown not to be the case. During an ice advance referred to by Wooldridge (1938) as that of the "Eastern Drift" (and today known as that of the Anglian stage), lobes of the ice sheet advanced from the north-east along the Vale of St Albans towards Watford, and towards the Finchley depression. The Thames was diverted to a more southerly course, through Richmond, beyond the furthest reach of the Anglian ice sheet.
The "loopway" hypothesis dispelled No deposits which were unambiguously of Thames origin were ever found along the line of the "Middlesex loopway".
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. The river was diverted to a more southerly route, roughly along the line of that followed by today's river.Gibbard, P.L. (1979), Middle Pleistocene drainage in the Thames Valley, Geological Magazine Volume 116, Issue 1 January 1979.Bridgland, D.R. (1994, 2012), Quaternary of the Thames.
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. And, although the pre-Anglian history of glaciations in Britain is known to be complex, and is still the subject of varying interpretations, the hypothesis of a pre-Anglian "Chiltern" glaciation is today nowhere supported.Lee, J.R., Rose J. and others (2011), The Glacial History of the British Isles during the Early and Middle Pleistocene: Implications for the long-term development of the British Ice Sheet, pages 59-74 in Quaternary Glaciations - Extent and Chronology, a Closer Look. Developments in Quaternary Science, 15, (Amsterdam: Elsevier).
Gravels in the eastern section of the "Middlesex loopway", shown on current geological maps as "Dollis Hill Gravel", have been demonstrated to be deposits of the proto-Mole-Wey river, not of the River Thames.