The very varied landscape of the Forest is determined by the geology and the changes that have occurred over thousands of years.
For some twelve miles, Epping Forest stretches along a ridge between the River Lea in the south and the River Roding in the north. The ridge runs north-eastwards out of the Thames Valley, rising to over 116m above sea level near Epping, forming a crescent shape.
The 2,430 hectares of the Forest vary greatly in geological terms. The different soils produced over many centuries have ensured a variety of vegetation and, along with the height of the ridge and the consequent drainage valleys, this has provided the Forest with its varied and beautiful landscape.
Blackweir Pond, also known as the Lost Pond, was originally a gravel extraction pit. Photo: 2024
London Clay and the Claygate Beds
The oldest (and ‘lowest’) deposit that occurs in the Forest, dating from long before the Ice Ages, is a thick layer called London Clay. This covers much of the southern parts of the Forest. The upper parts of the London Clay are stiffer and sandier – they are known as Claygate Beds. This upper part forms the hills of the northern half of the Forest and is noticeable when walking across Fairmead Bottom towards High Beach, with Hill Wood up ahead. This is where the Claygate hills begin and continue northwards.
The Claygate deposit, being stiff and more stable than London Clay, forms the sides of the steep valleys found in the north of the Forest. The streams here have cut down to the underlying London Clay. If you walk down some of the valleys you might notice that the stream at first runs a relatively straight course. Further downstream, as the valley broadens, the stream slows and meanders more where it has reached the London Clay. This can be seen for instance in Kate’s Cellar as you go down to Debden Slade. Another example is the main stream south of High Beach Church.
In places, the Claygate Beds are capped by a deep layer of sand, known as Bagshot Sand. This covers some of the highest parts of the Forest, such as the High Beach area and Loughton Camp. The Bagshot Sand is itself mostly covered by a thin layer of pebbly gravel. The small pits found in the north of the Forest are where this gravel has been dug out in the past. Most of the Forest’s streams begin as springs, where water drains through layers of sand and gravel and reaches the underlying layer of Claygate Beds.
Landslip on Yardley Hill, with elms on the horizon, Photo: April 1963
Chalky Boulder Clay
All these deposits were laid down before the several Ice Ages. The earliest ice sheet partially covered the Forest Ridge. Its influence is thought to have spread over the level area of Gilwell and reached Yardley Hill.
As the ice melted it left behind a mixture of stones, chalk, and clays that it had scraped up as it spread south over the chalk of the Chiltern Hills into Essex. This debris is known as Chalky Boulder Clay and, where it occurs, you’ll find plants that prefer a chalky soil, for instance Clematis vitalba (known as Traveller’s Joy or Old Man’s Beard), a member of the buttercup family, and sometimes Field Scabious (lilac flowers on long, wiry stems, adored by bees and butterflies), which can be found on Yardley Hill where there is a thin layer of chalky debris. These, and the Field Maple, can also be found decorating mid-Essex hedgerows and verges wherever the Boulder Clay occurs.
Ice Age river gravel terraces
In all the warm periods between the four main advances of the ice sheets, vast quantities of melt-water eroded the landscape and then the flooding rivers left behind further deposits of sands and gravels. The ancient Thames, many times wider than today, cut its channel deeper as the sea level fell and rose again over a period of 250,000 years with each advance and retreat of the ice. Each period of flooding left terraces of these ‘river gravels’. Three main levels can be identified with a difference of about 50 feet between them – the oldest being the highest, the Boyn Hill Terrace, which formed the Leyton Flats and the gravels around the Hollow Pond. The next has been called the Taplow Terrace, which created Wanstead Flats. The most recent, the Flood Plain Terrace has now been covered by houses in east London.
This article is based on material from Ken Hoy’s book Getting to Know Epping Forest, first published in 2002 with a new edition forthcoming edited by Judith Adams.