Pymmes Brook

At this point it is known as Monken Mead Brook before flowing in a south-easterly direction to merge with the River Lee Navigation at Tottenham.

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.

[7] 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.

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.

[11] In the case of Pymmes Brook immediately after the glaciation, that stream joined the River Lea somewhere around Broomfield Park, where there is a deposit of "Boyn Hill Gravel".

During the course of the following 400,000 years, the lower Lea moved steadily eastwards, leaving river terrace deposits of decreasing age and altitude as it did so, as well as a relatively steep eastern slope.

[12]) Pymmes Brook thus extended its course eastwards from Broomfield Park, across the valley floor of the lower Lea, through Upper Edmonton, to Meridian Water.

From Cockfosters down to Arnos Park, Pymmes Brook, like the lower River Lea, has a notably steep eastern slope.

In addition, that section of the Pymmes Brook valley today seems disproportionately deep for such a small stream – for example, at Cat Hill.

But we are currently in an interglacial period, and the stream would have been flowing more strongly than today at times of "high discharge, under cold climatic conditions".

Furthermore, as the River Lea itself cut down as it moved eastwards, it lowered the base level of its tributary stream, Pymmes Brook.