River Brent
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.Topography, geology and evolution of the Brent catchment area
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 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, north and east, 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.
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.
Current topography is largely the result of landform evolution during the Pleistocene epoch.
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.
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.This hypothesis has since been confirmed by much subsequent research.
That "proto-Thames" river received tributaries from the south. 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 layer of sand and gravel known as Pebble Gravel.
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".
S.W. Wooldridge later suggested that it was in fact "a river of major dimensions" "entered from the south" that was responsible for transporting much of this chert to areas which are now north of the River Thames. It was clear from the broad course which Wooldridge plotted for this river that it was an ancestor of the River Mole.
In 1994, D.R. Bridgland proposed that Pebble Gravel which is located on Harrow Weald Common 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.
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.
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.
Today, the highest of those deposits rest at an altitude of around 100 metres. 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, 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 proto-Mole-Wey river which laid down the Dollis Hill Gravel thus flowed along a line broadly similar to that of today's River Brent, but in the opposite direction, from south-west to north-east. The gradient of the floor of that valley in the area now occupied by the Brent catchment was low - probably no more than 50 cm per kilometre.
Tributaries of the proto-Mole-Wey river
The number, and courses, of the tributary streams which flowed into the proto-Mole-Wey river prior to the Anglian glaciation, in the area currently covered by the Brent catchment area, are not known with any certainty. But it is known that, elsewhere, some tributaries of rivers which were themselves severely disrupted by that glaciation today still follow broadly the same lines as their pre-glaciation valleys. This is the case, for example, for certain tributaries of the upper River Lea, such as the Rivers Mimram and Stort.So it seems reasonable to suggest that parts at least of the network of tributary streams which flowed into the proto-Mole-Wey river, in the area currently covered by the Brent catchment area, proved to be equally robust. Thus, it is possible that, although the proto-Mole-Wey river itself was completely replaced by the River Brent during the Anglian glaciation, and parts of those of its tributaries which came into contact with the ice front were diverted, other sections of today's network of Brent tributaries broadly reflect parts of the pre-Anglian network of tributary streams which fed the proto-Mole-Wey. This could apply to, for example, the upper part of the Dollis Brook, the Folly Brook, the Silk Stream and the Wealdstone Brook.
In the case of the uppermost section of the Dollis Brook, which runs broadly west-east from Barnet Gate towards Barnet Playing Fields, it is possible that, immediately prior to the Anglian glaciation, that stream continued eastwards, at a today's altitude of about 90 metres, and joined a precursor of the Pymmes Brook in the vicinity of East Barnet. That precursor brook may then have flowed south-eastwards to join the proto-Mole-Wey somewhere around New Southgate.
The Anglian glaciation
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. This ice advance has since been identified as the Anglian glaciation. After reaching as far south as Ware, about 450,000 years ago, lobes of this ice sheet extended up two valleys, oriented south-west to north-east - that of the proto-Thames, and that of the proto-Mole-Wey.In the early twentieth century 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.
As noted above, the proto-Mole-Wey river was a tributary of the proto-Thames. It flowed northwards from the Weald, then passed through what is today the Brent catchment area.
As an ice sheet lobe advanced up the proto-Thames valley towards Watford, the passage of the river was blocked, and its water backed up to form a lake in front of the advancing ice. That lake eventually extended as far up the proto-Thames valley as a point south of Gerrards Cross, where lacustrine deposits have been identified.
The water in that lake eventually rose to relatively low points on the interfluve with the proto-Mole-Wey catchment area to the south-east, where that water could overflow into that catchment area. Two such points were at Carpenders Park and Uxbridge.
Water overflowed into the proto-Mole-Wey catchment in a considerable volume and with considerable force.
In the case of the overflow through the Carpenders Park gap, the Thames overspill surged through Wealdstone and Kenton. The land it crossed would have been bare of vegetation and very susceptible to fluvial erosion. The powerful overflow quickly eroded areas of higher ground. This could have included a possible ridge of higher ground running from Stanmore to Colindale, similar to and parallel with the ridge that still exists and that runs from Barnet Gate to Mill Hill. The overflow thus carved out much of the London Clay basin which today forms the north-western section of the River Brent catchment area. In particular, it probably removed much of the Dollis Hill Gravel which must have been situated in this area prior to the Anglian glaciation.
At the same time, a lake formed in front of the ice lobe which was moving south-westwards up the proto-Mole-Wey valley towards Finchley, and which was blocking the flow of the proto-Mole-Wey river. The swirling waters of that lake also caused considerable erosion of the previous topography. But isolated islands, mostly capped by Dollis Hill Gravel, were left within the lake.
The Thames eventually established a diverted course from Uxbridge to Richmond, from where it continued, in a broadly eastwards direction, towards the North Sea. This caused the lake in the proto-Mole-Wey valley to be drained, thus leaving the former islands in the lake as isolated hills in what is now the London Clay basin of the north-western section of the River Brent catchment area.