The River Brent is a river in west and northwest London, England, and a tributary of the River Thames. in length, it rises in the Borough of Barnet and flows in a generally south-west direction before joining the Tideway stretch of the Thames at Brentford.
Hydronymy and etymology A letter from the Bishop of London in 705 suggesting a meeting at Breġuntford, now Brentford, is the earliest record of this place and probably therefore that of the river, suggesting that the name may be related to the Celtic *brigant- meaning "high" or "elevated", perhaps linked to the goddess Brigantia.Canham, Roy; Glanville G H (1978). A London Museum Archaeological Report: 2000 years of Brentford. Ch 2; pg 3. Her Majesty's Stationery Office.
Topography, geology and evolution of the Brent catchment area
(For the purposes of this section, the Brent catchment area is taken to include the catchment areas of all its tributaries.)
The catchment area varies in altitude from just over 150 metres above sea level at Bushey Heath, on its northern watershed, to barely 10 metres at the confluence of the Brent with the Thames at Brentford.
Broadly speaking, the catchment area can be divided into three topographical zones:
- a southern zone, lying south of a line from about Harlesden to Northolt, which is of low relief and which (apart from the hill at Hanger Lane) lies below an altitude of 40 metres;
- a basin-like north-western zone which is surrounded by areas of higher ground which rise fairly steeply to the west (Harrow on the Hill), north (Bushy Heath) and east (Mill Hill), and where several isolated hills such as Horsenden Hill, Barn Hill and Dollis Hill are located; and
- a north-eastern zone consisting of the relatively narrow and steep-sided valleys of the Dollis Brook, Folly Brook and Mutton Brook; and of the high ground which surrounds those valleys (at Hampstead Heath, Whetstone, Totteridge, Chipping Barnet, etc).
The oldest and most extensive geological formation in the Brent catchment area, as in much of the London Basin, is Eocene London Clay. This is mostly a stiff blue-brown clay, over 100 metres thick. In some higher parts of the area, a relatively thin, upper part of the London Clay formation, sandier in content and known as the Claygate Beds, is also found.
In some 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, principally fluvial deposits and glacial deposits. The oldest Pleistocene deposit, Pebble Gravel, is found across the upper, northern margin of the catchment area, from Bushey Heath to Chipping Barnet. The most recent Pleistocene deposits include the post-Anglian river terrace deposits of the Thames and Brent rivers in Ealing and other southern parts of the catchment area. Glacial till is found in the north-eastern sector of the catchment area, around Finchley.See Geology of Britain Viewer (British Geological Survey) for more information on the distribution of all the geological formations mentioned in this article.
Current topography is largely the result of landform evolution during the Pleistocene epoch (that is, during the last two million years or so).
Although a tributary of the River Thames, the Brent is much younger in age. An Ancestral Thames is thought to have come into being over 60 million years ago, during the post-Cretaceous uplift of Britain (an uplift which was tilted to the south-east). Gibbard, P.L. & Lewin, J., 2003 ("Gibbard & Lewin 2003"), The history of the major rivers of southern Britain during the Tertiary, Journal of the Geological Society, 160, pages 829-845. Tectonic map and Palaeocene sections. Online at www.qpg.geog.cam.ac.uk. The Brent, as a southward-flowing, left-bank tributary of the Thames, was formed as a result of the Anglian glaciation, which occurred about 450,000 years ago.
The area prior to the Anglian glaciation In the early twentieth century, it was suggested that the River Thames, after descending through Oxfordshire, entering the London Basin near the Goring Gap 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. This hypothesis has since been confirmed by much subsequent research.As summarized in, for example, 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).
That "proto-Thames" river received tributaries from the south.Bridgland, D.R. and Gibbard, P.L. (1997), Quaternary River Diversions in the London Basin and the Eastern English Channel, Géographie physique et Quaternaire, vol. 51, n° 3, 1997, pp. 337-346. Online at www.erudit.org/fr/revues/. See Figure 1. At least one of those tributaries traversed what is today a dissected plateau which lies to the south-east of the Vale of St Albans. This plateau stretches from Bushey Heath to Northaw and beyond, and is capped over wide areas, at altitudes ranging from about 150 metres to 130 metres, by a fairly thin (average 3 metres) layer of sand and gravel known as Pebble Gravel (or, in places, Stanmore Gravel).Stanmore Gravel Formation, British Geological Survey Lexicon of Names Rock Units.
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. It was suggested early on that this "pointed to the former existence of streams from that area".Humphreys, G. (1905), Excursion to Hampstead, Proceedings of the Geologists’ Association, Volume 19, Issue 5, pages 243-245 (remark by A.E. Salter, page 243). Patches of Pebble Gravel also occur on the summit of Hampstead Heath, at an altitude of about 135 metres, where they overlie Eocene Bagshot Sand. (The more extensive deposits of Pebble Gravel which are found on the Bushey Heath plateau lie directly on London Clay and Claygate Beds.)
S.W. Wooldridge later suggested that it was in fact "a river of major dimensions" (which) "entered from the south" that was responsible for transporting much of this chert to areas which are now north of the River Thames.Wooldridge, S.W. (1927), The Pliocene Period in western Essex and the preglacial topography of the district, Essex Naturalist, 21, page 247; online at www.essexfieldclub.org.uk. It was clear from the broad course which Wooldridge plotted for this river that it was an ancestor of the River Mole (and/or River Wey).
In 1994, D.R. Bridgland proposed that Pebble Gravel (or Stanmore Gravel) which is located on Harrow Weald Common (near Bushey Heath) was deposited by an ancestral Mole-Wey, and that that river was a tributary of the River Thames at a time when the latter river was flowing to the north-west of the Vale of St Albans. That could have been nearly two million years ago. 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).Bridgland, D.R. (1994, 2012), Quaternary of the Thames, Chapter 3, Part1, Harrow Weald Common. Bridgland suggested that the Stanmore Gravel could have been contemporary with the Stoke Row Gravel of the proto-Thames. In 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), it was suggested that the Stoke Row terrace was of Marine Isotope Stage 68 age, about 1.9 million years ago. Lee, etc also suggested that the Westland Green terrace of the proto-Thames (which Bridgland suggested was of the same age as the Northaw gravel) was of MIS 62-54 age, around 1.8-1.6 Mya.
At those times, the topography of the country in what is today the Brent catchment area would have been very different from today's topography, because the Pebble Gravel was laid down on a valley floor, whereas today it occupies the highest ground in the area. The relief has thus been inverted.
But, in 1979, P.L. Gibbard mapped younger deposits, known as Dollis Hill Gravel and named after one of the locations where this deposit is found, which were also laid down by an ancestral Mole-Wey river.Gibbard, P.L. (1979), Middle Pleistocene drainage in the Thames Valley, Geological Magazine Volume 116, Issue 1 January 1979.Dollis Hill Gravel Member, British Geological Survey Lexicon of Names Rock Units. (This river is also referred to in places as the "proto-Mole-Wey".)Murton, Della K. and Murton, Julian B. (2012), Middle and Late Pleistocene glacial lakes of lowland Britain and the southern North Sea Basin. Quaternary International, Volume 260, 18 May 2012, Pages 115-142, Fig. 7A.When the Dollis Hill Gravel was laid down, the River Wey would have been flowing across the North Downs at Guildford, and the River Mole would have crossed at Dorking - as now in both cases. Gibbard (1979) suggested that the two rivers were "confluent near Weybridge and from there flowed north-eastwards to Finchley going on to join the Thames east of Ware". Today, of course, the two rivers stay apart right up to their respective junctions with the River Thames at Weybridge and Hampton Court. These deposits are found at what is today the so-called Finchley Gap, and to the north-east and south-west of it. Dollis Hill Gravel is found, for example, south-west of the Gap at Hendon and Horsenden Hill, and north-east of the Gap over wide areas from Southgate to Goff's Oak.Gibbard (1979) found deposits of Dollis Hill Gravel up to six metres thick in the vicinity of the Finchley Gap. According to the British Geological Survey (Lexicon of Names Rock Units), Dollis Hill Gravel generally can be up to 15 metres thick.
Today, the highest of those deposits rest at an altitude of around 100 metres (for example at Muswell Hill). So the proto-Mole-Wey valley around Finchley, in the sense of being an area of lower ground lying between higher ground on either side (for example, at Mill Hill and Hampstead Heath, both at altitudes of over 120 metres today), must have existed by the time those highest deposits of Dollis Hill Gravel were laid down. That could have been around one million years ago.The highest Dollis Hill Gravel deposits at 100 metres have not been dated precisely. But deposits of Gerrards Cross Gravel (which were laid down by the proto-Thames) have been tentatively estimated to be nearly one million years old (see Lee, J.R., Rose J. and others (2011), where Gerrards Cross Gravel is dated at Marine Isotope Stage 22, about 0.9 Ma). Such deposits have been mapped, at a similar altitude (about 100 metres), to the north-west, near Radlett. There is no certainty that the oldest Dollis Hill Gravel was laid down at exactly the same time as the Gerrards Cross Gravel, but the ages are probably comparable, because, in any given section of the London Basin, the altitude of river-borne Pleistocene deposits is normally closely correlated with their age. (This age-altitude correlation principle is illustrated in, for example, Figure 3.2, Bridgland (1994, 2012), Chapter 3, Idealized transverse section through the classic Middle Thames sequence of the Slough-Beaconsfield area, with the oldest Thames gravels at an altitude of over 150 metres, and the youngest at almost sea level.)