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) where the chalk of the Chilterns in the north, curves down under London, to come back up as 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. Laid down in warm tropical carbonate seas, like today’s Bahamas. Fossils mostly echinoids (sea urchins / sea potatoes) but also dinosaur footprints etc.

There are a number of boreholes that have been drilled, in the local area, and the geology they’ve gone through has been recorded; these can be examined on the British Geological Survey site. Some of the boreholes date from Victorian times, and one of the interesting features of looking at the older ones, is that the names for the different horizons (ie layers) have changed. Another is that the older ones are measured in feet and inches, so any correlation of the different layers, being done by hand, means converting them.

Below are some descriptions of the kinds of environments that this area experienced at the times that these layers of sand & clay were laid down, over 70 million years ago. Back then, the earth’s climate was much warmer than at present (it’s before all the Ice Age fluctuations came in, in the 2.58 million years) and the UK’s position was further south (a latitude of 43º to 47ºN, instead of the current 53ºN). Most rocks are laid down under water, usually the sea, so the London Clay, which has all the debris from an estuary and mangrove forest washed out to sea, has an unusual amount of terrestrial remains (mostly plants).

Thanet Sands:

“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

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.

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