The rocks – or clay – under our feet (the stratigraphy)

This page is a potted explanation of the rocks (or more like clay) that underlies Bromley, to give a bit of background to the photos of the brick-pit when it was working.

Bromley is at the southern end of the London Basin, a syncline (downward bend of the rocks) from the chalk of the Chilterns in the north, to the chalk of the South Downs. Here is an illustration of the concept from 1843:

So, working downwards, the sedimentary layers of ‘rock,’ beneath this local area, are:

London Clay	70 MA	Stiff fissured grey clay with sand partings. These were laid down in shallow Seas just offshore of a mangrove forest;the London Clay is exposed in the cliffs, to the north of Minster, at the Isle of Sheppy, and if you hunt along the foreshore you can find pyritised seeds, bits of crabs, stemless palm seeds, and old sharks teeth. The stemless palms only grows in the tropics, so the discovery of these gave the ‘Diluvian’ theory Victorians a headache – how could London have ever been in the tropics? Nowadays we understand that it was Continental Drift.
Woolwich and Reading Beds	90 MA	Local these are called the ‘Blackheath Beds’ or Blackheath facies. Typically these are pebbles and gravels, but locally it is more often silty current-bedded sands. In the exposure at Elmstead woods this layer was laid down partially tidal bars in an estuary complex and has lots of mollusc shells, some sharks teeth and fish scales.
Thanet Sands	90 MA	Fine white sand.
Upper Chalk	MA	Chalk with bands of flint.Warm tropical carbonate seas, like today’s Bahamas. Fossils mostly echinoids (sea urchins) but also dinosaur footprints etc.

“Deposition of the Thanet Formation, Lambeth Group and Harwich Formation occurred in embayments on the western margin of a deep-water marine basin of the North Sea. These marginal deposits were very sensitive to relatively minor changes in sea level. This resulted in alternating incursions and recession of the sea and migration of depositional environments, followed by erosion, changes in groundwater levels, soil formation and down-cutting by rivers that contributed to the development of complex lithologies. Rising sea level led to rapid inundation and a new phase of sedimentation. ”

“The Thanet Formation is also preserved locally in dissolution pipes and hollows in the Chalk ”

“The sediments are intensely bioturbated so that primary sedimentary structures such as lamination are generally missing.”

Engineering Geology Directorate Open Report OR/13/006 http://nora.nerc.ac.uk/503110/1/EngGeol_Lambeth_FINAL_1.02.pdf

A map of where these layers outcrop in our area:

outcrop map BR2

The layers are almost horizontal (as opposed to being tilted) but have irregularities, apparently from the extension of basement faulting and ice age features such as pingos.

3,461 total views, 1 views today