Turkey Brook is a river in the northern outskirts of London. It rises in Potters Bar, Hertfordshire, and flows broadly eastwards to merge with the River Lea Navigation near Enfield Lock.
Etymology
The brook is named from the hamlet Turkey Street, which is recorded as Tokestreete (1441), Tuckhey strete (1610), Tuckey street (1615), and Turkey street (1805) (probably street of houses, i.e. hamlet, from Middle English strete, associated with a family called Toke or Tokey).
The modern form of the name Turkey, not in use before the 19th century, is no doubt due to folk etymology.
Watercourse
The brook rises near the Fir and Pond Woods nature reserve in Potters Bar, and at first flows in an easterly direction. Along its course it is joined by other streams including Hollyhill Brook, Cuffley Brook, and the Small River Lea. It flows alongside the M25 motorway and Crews Hill Golf Club, then goes past Clay Hill, Whitewebbs Park, Forty Hill and Albany Park before merging with the River Lee Navigation below Enfield Lock.
Geology, origin and evolution
The main geological formation underlying the Turkey Brook catchment area is Eocene London Clay. In some parts of the higher sections of the catchment area, the London Clay is overlain by "Pebble Gravel" and "Dollis Hill Gravel" (both Quaternary pre-glacial fluvial deposits), and by Quaternary glacial till. Downstream of where Turkey Brook goes under the M25, there are alluvium and sand and gravel deposits on the Turkey Brook valley floor. And east of Forty Hill, the brook crosses extensive Quaternary river terrace deposits laid down by the River Lea.
As a west bank tributary of the lower River Lea, Turkey Brook came into being about 400,000 years ago, after the Anglian glaciation. During that glaciation, ice from the north of England advanced at least as far south as Watford, Finchley and Chingford.
Until the Anglian glaciation, the River Thames flowed north-eastwards via Watford, through what is now the Vale of St Albans, then eastwards towards Chelmsford and the North Sea. As a result of the glaciation, the Thames was diverted to a more southerly route, broadly along the line of its current course.
Prior to the Anglian glaciation, a "proto-Mole-Wey" river was flowing northwards from the Weald and North Downs, through the "Finchley depression" and Palmers Green, to join the proto-Thames somewhere around Hoddesdon, at what is today an altitude of around 60 metres. It was this river which, during the course of the early and middle Pleistocene, deposited the "Dollis Hill Gravel" at successive altitudes.
When the Anglian ice sheet diverted the Thames southwards, the Mole-Wey was cut off at Richmond. Meltwater from the retreating Anglian ice sheet gave birth to a south-flowing lower River Lea, and that river cut into and followed in part the line of the former proto-Mole-Wey. It flowed into the newly diverted Thames, which at that time was spread over a wide flood plain extending as far north as Islington.
And, as the ice sheet retreated, west bank tributaries of the lower Lea, such as Turkey 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, cut down successively through till left by the ice sheet, then through "Dollis Hill Gravel", and then into London Clay below.
It is not known at present whether Turkey Brook, and other west bank tributaries such as Pymmes Brook and Salmons Brook, followed valleys which had been in existence before the ice sheet covered the land, or whether they fashioned a substantially different landscape after the ice retreated. But it is known that today's tributaries of the upper Lea, such as the Rivers Mimram and Stort, follow broadly the same lines as pre-glaciation valleys, so, by analogy, it is quite possible that elements at least of the pre-glaciation topography of the lower Lea basin are reflected in today's relief.
In the case of Turkey Brook immediately after the glaciation, that stream joined the River Lea somewhere around Forty Hill, where there is a deposit of "Boyn Hill Gravel". That gravel, which is on the highest of the river terraces left by the post-Anglian lower River Lea, marks the line followed by the Lea after the retreat of the ice sheet.