The Lea Valley, London

Geological setting

The area is mainly underlain by silty clay of the London Clay Formation of Eocene age. Outcrops of the silty Claygate Member of the London Clay capped by the sandy Bagshot Formation occur near Loughton and Chigwell. To the north and north-west the London Clay has been eroded to expose the clay, silt and sand of the Lambeth Group and, below these, outcrops of the Lewes Chalk of Cretaceous age.

Much of the bedrock is overlain by much younger (Quaternary) geological deposits. Along the valley of the River Lea alluvium, comprising varying amounts of sand silt clay and gravel, predominates while towards the valley sides sand and gravel terrace deposits of former river levels are found together with deposits of silty clay river deposits. Further from the river valley, deposits of glacial till are present together with deposits of sands and gravels left by rivers and streams that flowed as the ice melted at the end of the last ice age.

Potentially significant geological hazards

Compressible material is restricted to the alluvial deposits, along the River Lea and streams, which might contain layers of peat or interbedded clay, silt and sand.

Deposits containing sand which may become mobilised by flowing water (running sand) may be present in sandy alluvial deposits associated with the River Lea and streams within the area. Potentially running sand deposits may also be present in deposits left by rivers and streams that were active when the ice melted at the end of the last glaciation (Fluvio-glacial deposits). Since these deposits are not usually associated with current drainage networks they may not be saturated with water and are less likely to have the conditions for running sand to occur.

The London Clay Formation contains significant amounts of high plasticity clay that can result in shrink/swell clay subsidence due to volume changes associated with drying out at times of drought or trees with a high moisture demand. Where there is a sufficient covering of superficial deposits that do not contain clay, the potential for shrink/swell subsidence is much reduced.

The course of the River Lea is associated with deposits of river silts (the Enfield silts) that may have a very open structure with a potential for collapse if it becomes wetted under load or is subjected to an excessive load. However, it is likely that these silts have been reworked by the river and the open structure has been destroyed but the potential for collapse should be considered in decisions about landuse and construction associated with these deposits.

In general there is little potential for natural landslides in this area except where steep slopes are formed by the London Clay Formation particularly where there is a spring at the base of the overlying Claygate Member that promotes saturation and weakening of the underlying London Clay. Areas where this may be significant include to the north of Chingford and to the North of Waltham Abbey.

The outcrop of the Lewes Chalk on the north and Northwest margins of the area may have a significant potential for dissolution hazards especially near to the outcrop of the less permeable strata to the south. Water draining off the clays may be concentrated along the junction with the Chalk and cause solution of the chalk to form dolines or swallow holes. These are sites of weakness that may cause subsidence of structures built over them as they continue to dissolve, compress or as overlying fine material is washed into them.

Open cracking in the ground due to active landsliding at Fernhill, Chingford.

Hummocky ground due to ancient, shallow landslides at Barn Hill, Chingford