Geology Site Account

Historical site only. However any future significant excavations in the motorway cutting should be recorded.

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Site description

During construction of the M25 motorway a continuous section from the Thanet Sand to the London Clay was exposed in the cutting between the north abutment of the Mar Dyke via-duct and Stifford Road Bridge in 1979 (Ward 1987, Ellison 2004 p.32). A layer of sand nearly a metre thick, belonging to the Oldhaven Beds (now part of the Harwich Formation), contained shell seams which yielded at least 60 species of fossils and in places the sand was cemented into blocks of sandstone that were packed with fossil shells.

Of particular interest at this site was a thick seam of lignite (a type of coal) which indicates emergence and growth of vegetation. This lignite was formed 55 million years ago at the boundary of the Palaeocene and Eocene epochs, precisely at a time of widespread global warming. Known as the Palaeocene-Eocene Thermal Maximum (PETM) it was one of the most rapid and extreme global warming events of all time and had profound effects on life on land and in the sea. The warming is thought to have been due to the natural but catastrophic release of methane into the atmosphere by marine gas hydrates and it lasted about 100,000 years. Recent studies of this lignite seam in Kent have revealed the presence of charcoal from ancient wildfires that periodically destroyed the vegetation, possibly on a regional scale (Collinson et.al. 2007).

This period of the Earths history has attracted a lot of interest as the PETM may provide us with an analogue for the present day accelerated emission of man-made greenhouse gases.

A block of Oldhaven sandstone found in 1979 during excavations for the M25 Mar Dyke interchange containing a large number of fossil shells. Photo © G.R.Ward