London - Introduction
This memoir is a review of the current knowledge on the geology of London written to describe the four 1:50 000 scale geological maps that cover the district. It presents a regional overview with some specific examples, and for more detailed accounts the reader is referred to the earlier memoirs. A concise account of the geology of the wider region is given in Sumbler (1996), which includes a bibliography of key recent papers on the principal lithostratigraphical units and a section on applied geological issues in the London district.
The district is dominated by the greater London conurbation, reaching the outskirts of Watford in the north-west, and the expanding towns of Brentwood and Billericay in the north-east. The land alongside the Thames in the east of the district from Woolwich to Gravesend, known as the Thames Gateway, has been designated for major development in the 21st century. It has a legacy of past industrial use and some of the largest brown-field sites in Britain.
The geology and topography of the district are shown on the Frontispiece and in Figure 1 respectively. The topography is dominated by Chalk downland in the south, and by a ridge that forms the edge of Epping Forest around Chigwell and Loughton in the north; rural Essex in the east is gently undulating and supports mixed farmland. The River Thames and its tributaries form the main drainage network. The river, tidal as far as Teddington Lock [168 715] in the west of the district, is associated with an alluvial tract at about 10 m above OD in the west and around sea level on the marshlands in the east. River terrace gravels rising to about 30 m OD underlie the gently sloping valley sides. Interfluves in the north-west consist of dissected London Clay, and some hills (up to 150 m OD) are capped by the Bagshot Formation and Quaternary gravels. In the north-east, the ground rises to a dissected plateau of till at about 100 m; other hills of similar height are capped by Bagshot Formation and Quaternary gravels. South of the Thames the ground rises gently across the London Clay on to the Chalk that forms the dip slope of the North Downs escarpment. The dip slope is covered, in part, by Clay-with-flints and is interrupted by subsidiary hills formed by outliers of London Clay and Thanet Sand. The crest of the escarpment reaches 200 m above OD in the south-east, the highest point in the district.