Geology Site Account

Site name: Hainault Forest

Grid reference: TQ 478940

Brief description of site:

Hainault Forest is partly in Essex (Epping Forest District) and partly in London (London Borough of Redbridge). On the Essex side of the border the forest is managed by the Woodland Trust on behalf of Essex County Council.

Hainault Forest is situated on high ground which is a miniature version of the Epping Forest ridge with a very varied geology. This has an effect on the diversity of trees plants present, creating a number of different habitats. The geology is also similar to that of the high point of Havering-atte-Bower to the east (London Borough of Havering).

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Summary of geological interest

The foundation of the area is London Clay and this passes up into sandy clay called the Claygate Beds, a result of the shallowing of the London Clay Sea some 50 million years ago. Dog Kennel Hill to the south of the forest is capped by Claygate Beds but on the highest ground “ Cabin Hill €“ the Claygate Beds are overlain by Bagshot Sand, a fine yellow sand laid down as the sea became even shallower.

During the earliest part of the Ice Age, perhaps as much as a million years ago, it is thought that a river was flowing north across this area to join the early Thames which was then flowing across north Essex and through East Anglia. We have almost no idea what the landscape of Essex was like then but geologists think that some of the gravel deposited by these ancient Thames tributaries has survived. A patch of this gravel, often called pebble gravel, exists on the summit of Cabin Hill and if you come across rounded pebbles of flint on the paths here or in the roots of fallen trees, give a thought to the fact that this spot may once have been the floor of an ancient river valley! On the top of Shooters Hill in Kent, visible from Cabin Hill, is another patch of this gravel which could have been deposited by the same river. Other outcrops of pebble gravel, which may be from other northward¬flowing tributaries of the Thames, exist on other high points such as Havering-atte-Bower, High Beach and the Langdon Hills.

Later in the Ice Age, about 450,000 years ago, the Anglian ice sheet spread into Essex and laid down a distinctive clay known as boulder clay or till. Remnants of this material is present on the north-west slopes of Cabin Hill and results in the ground here remaining wet and sticky, even in summer. Beneath the boulder clay there is glacial sand which may have been deposited by melt water in front of the advancing ice sheet. A small area of this sand occurs near Chigwell Row Church where it supports a heathland flora.

There are several springs in the area, most of them the result of groundwater flowing from the Claygate Beds where it meets the impervious London Clay beneath. The most famous of these is Lambourne Well, the water from which drains into the River Rom.