The Lea Valley (also spelt Lee Valley), the valley of the River Lea, has been used as a transport corridor, a source of sand and gravel, an industrial area, a water supply for London, and a recreational area. The London 2012 Summer Olympics were based in Stratford, in the Lower Lea Valley. It is important for London's water supply, as the source of the water transported by the New River aqueduct, but also as the location for the Lee Valley Reservoir Chain, stretching from Enfield through Tottenham and Walthamstow.
Geography Physical geography The catchment area of the River Lea is located in the central part of the London Basin, on that basin's northern flank.
The main underlying geological formation of the upper part of the Lea catchment, north of Hoddesdon, is Cretaceous Chalk. The main underlying geological formation of the lower part of the Lea catchment, south of Hoddesdon, is Eocene London Clay.
However, large areas of these formations are overlain by much more recent Quaternary formations, including Clay-with-Flints (on the Chalk), till and other glacial deposits (mostly in the upper part of the catchment), and fluvial sand, gravel and alluvium (in the lower parts of today's valleys, but also on some higher ground in east Hertfordshire, Middlesex and west Essex, where such deposits were laid down by the pre-glacial "proto-Thames"McGregor, DFM and Green, CP (1983), Lithostratigraphic subdivisions in the gravels of the proto-Thames between Hemel Hempstead and Watford. Proceedings of the Geologists’ Association, 94, pages 83-5. and its former tributaries).See BGS Geology Viewer (British Geological Survey) for more information on the distribution of all the formations mentioned here.British Geological Survey (BGS) maps at a scale of 1:50,000 which cover most of the Lea Valley can be viewed online at largeimages.bgs.ac.uk/ (Hertford) and largeimages.bgs.ac.uk/ (North London).
The northern boundary of the Lea catchment area rises to an altitude of almost , in hills north-east of Luton. The lowest point of the catchment area is the junction of the Lea with the Thames in east London, at an altitude of barely .
The relief of the upper part of the Lea catchment is one of gently rolling hills, which are divided by the valleys that fan out to the north and north-west from an area between Hertford and Hoddesdon.
The lower part of the Lea catchment runs from Hoddesdon southwards to east London, with the flood plain of the River Lea as its central feature. That flood plain has a width which extends to about in Edmonton. From there, the land rises on either side to an altitude of around , to gravel-capped plateaus in Hertfordshire (Northaw) and Middlesex (Southgate) to the west, and to Essex (Epping Forest) in the east.See the online topographic map at en-gb.topographic-map.com/ (London) for more details.
The upper part of the catchment area of the River Lea was formerly a group of valleys whose rivers flowed approximately north–south directly into the River Thames (the "proto-Thames"). Until the Anglian glaciation about 450,000 years ago, the Thames flowed north-eastward past Watford, through what is now the Vale of St Albans, then eastwards towards Chelmsford and the North Sea.Bridgland, DR and Gibbard, PL (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 in particular Figure 1.
The lower part of today's Lea valley was formed during the Anglian glaciation. During that period, ice from the north of England advanced at least as far south as Watford, Finchley and Chingford. As a result, the River Thames was diverted to a more southerly route, broadly along the line of its current course.
As the ice retreated, the lower part of the River Lea was formed. It flowed almost directly north–south into the newly diverted Thames (see further notes below).
Further north, the newly formed lower Lea was fed by rivers which, as mentioned above, had flowed directly into the proto-Thames prior to the Anglian glaciation. These rivers - the upper Lea, the Mimram, the Beane, the Rib, the Ash and the Stort - today follow courses which are mostly similar to those of their pre-Anglian predecessors.Brown, Joyce C. (1959), The Sub-Glacial Surface in East Hertfordshire and Its Relation to the Valley Pattern. Transactions and Papers (Institute of British Geographers), 1959, No. 26, pp. 37-50. See in particular Figure 1, Figure 4, and page 49 - "There is seen to be a general correspondence between the present and pre-glacial drainage lines".
(Note - the above summary is accurate as far as it goes, but in reality the processes of glaciation and river diversion were more complex than in this summary - for example, four separate ice advances of the Anglian glaciation in this area have been identified.Bridgland, DR and Cheshire, DA (1994), Westmill Quarry, in Bridgland, DR (1994), The Quaternary of the Thames. Chapman & Hall, London.)
Further notes on the lower Lea Valley Prior to the Anglian glaciation, a "proto-Mole-Wey" river was flowing northwards from the Weald and North Downs, through the "Finchley depression", to join the proto-Thames somewhere around Hoddesdon.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. Online at coek.info. See also Bridgland, DR and Gibbard, PL (1997), Fig. 3.
Just prior to the arrival of the Anglian ice sheet in the Thames basin, this proto-Mole-Wey river appears to have been flowing over a wide, low-gradient valley floor between Palmers Green and Hoddesdon at what is today an altitude of around .
When the Anglian ice sheet diverted the Thames southwards, the Mole-Wey was cut off at Richmond.Gibbard, PL (1979), Middle Pleistocene drainage in the Thames Valley, Geological Magazine, Volume 116, Issue 1, January 1979. As meltwater from the retreating Anglian ice sheet gave birth to the south-flowing lower River Lea, that river cut into and followed the line of the former proto-Mole-Wey, between Hoddesdon and Palmers Green. It flowed into the newly diverted Thames, which at that time was spread over a wide flood plain extending as far north as Islington.Ellison, RA (2004), Geology of London, British Geological Survey, Fig. 29, p54.
And, as the ice sheet retreated, west bank tributaries of the lower Lea, such as Pymmes Brook, Salmons Brook and Cuffley Brook, flowed eastwards and south-eastwards from higher ground running roughly south–north through Potters Bar, down towards the newly formed lower River Lea. They (and their own tributaries such as Bounds Green Brook) cut down successively through till left by the ice sheet, then through "Dollis Hill Gravel", and then into Eocene London Clay below.
The earliest line of the lower River Lea is indicated by what appear on the BGS 1:50,000 map as deposits of "Boyn Hill gravel", notably at Forty Hill, Bush Hill and Palmers Green (Broomfield Park). These deposits lie at an altitude of approximately , just to the east of, and slightly lower than, the lowest gravel deposits left by the proto-Mole-Wey (shown on BGS maps as "Dollis Hill Gravel"British Geological Survey - Dollis Hill Gravel Member. See also Gibbard PL (1979).).
But the River Lea has clearly moved eastwards since the Boyn Hill terraces were laid down. In fact, the lower Lea Valley has been noted for the striking width of its valley floor, especially the section from Wormley down to Tottenham, as well as for the relative steepness of parts of its eastern slope.
For example, in the London Borough of Enfield, the ground slopes from an altitude of only about at the Ridge Avenue library to about at the foot of Kings Head Hill, some to the east. It then rises to an altitude of on the summit of Pole Hill, barely a further one kilometre to the east. Across that of valley floor, the ground is mostly covered by river terrace deposits of decreasing altitude and age, thus demonstrating that the lower River Lea has migrated eastwards since it was formed some 400,000 years ago.The celebrated "Lea Valley Arctic Beds" have been found among the youngest and easternmost of these deposits. These beds, carbon-dated to approximately 28,000 years ago, contained "a distinctive vegetation which has no precise modern equivalent. Known as ‘tundra steppe’ this grassy vegetation covered much of Europe, northern Asia and North America during the Devensian stage (100,000 to 10,000 years ago) and was the main habitat and diet of the woolly mammoth." The Essex Field Club, Pickets Lock and Angel Road Pits (site of) (Lee Valley Arctic Bed). Online at www.essexfieldclub.org.uk
Some authors have proposed that the notable width of the lower Lea Valley indicates that it was once occupied by a larger and more powerful river, namely the proto-Thames.For example, Ellison RA (2004), page 74.See references dating from 1912 cited in Bridgland, DR and Cheshire, DA (1994), "Interpretation".
However, this hypothesis is not universally supported. In any case, even if all or some of the water being conveyed by the proto-Thames as it flowed north-east through the Vale of St Albans was diverted southwards from around Hoddesdon by an advancing Anglian ice sheet, this could only have been for a brief (geologically speaking) period, because the ice then progressed further to the south and caused the above-mentioned complete diversion of the Thames to its more southerly course of today.
In addition, it is clear that the River Lea alone has been powerful enough to cause significant erosion below the surface across which the Anglian ice sheet flowed and on which it left till and other deposits. A cross-section across the Lea Valley at Enfield shows that the Lea has cut down by as much as , over a width of more than , since the Anglian glaciation.Wells, AK and Wooldridge SW, Notes on the Geology of Epping Forest, Proceedings of the Geologists' Association, Volume 34, Issue 3, 1923, page 251.This downcutting by the River Lea as it moved eastwards lowered the base levels of its tributary streams. This in turn enabled, for example, Pymmes Brook, Salmons Brook and Cuffley Brook, at times of "high discharge, under cold climatic conditions" (Bridgland, DR (1994), The Quaternary of the Thames. Chapman & Hall, London, chapter 1, "Terrace Formation"), to become deeply incised into slopes to the west of the Lea flood plain. This is apparent, for example, in the steep slopes of Pymmes Brook between Cat Hill (Barnet) and Arnos Park, of Salmons Brook between Slades Hill (Enfield) and Grange Park, and of the headwaters of Cuffley Brook in the Cuffley - Newgate Street area.
But why did the River Lea move eastwards? It has been suggested that the River Lea has been "tilted...into its eastern bank" by "a north-south monocline" which was established "at least as early as the beginning of Eocene times".Ellison RA (2004), Fig. 45, page 81. Pleistocene isostatic adjustment of the London Basin (with uplift in the west caused by erosion and subsidence in the east caused by accumulation of deposits in the North Sea) may also have played a role.Bridgland, DR (1994), The Quaternary of the Thames. Chapman & Hall, London, chapter 1, "Terrace Formation".
Human geography The northern section of the valley, although including several towns (Luton, Harpenden, Hertford and Ware), is mainly rural. Below Hertford the Lea flows on a wide floodplain, which becomes an increasingly urban transport corridor as it enters Greater London. Many of the upper sections have been exploited for sand, gravel or brickearth, and are now part of the Lee Valley Park.