Sussex: an ancient sea bed

A panoramic view of all seven sisters from the Beachy Head cliffs near Birling Gap, looking back towards the River Cuckmere and Seaford Head in the background. By Diliff – Own work, CC BY-SA 3.0,

How do we know that Sussex was covered by a warm sea? The answer lies in the familiar white cliffs of the South Coast – Beachy Head, The Seven Sisters and Dover. These cliffs are all made of a rather special rock. Chalk.

But what is chalk?

When you hold a small piece of chalk in your hand from a walk along the South Downs, you are holding the remains of millions upon millions of microscopic marine plants called algae.

Emiliania huxleyi – single-celled marine phytoplankton that produce calcium carbonate scales (coccoliths). A scanning electron micrograph of a single coccolithophore cell. Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility) CC BY 2.5

These marine plants thrived a in a warm sea that was 200 to 300 metres deep and covered the whole of the South Downs and most of the UK. The only part of the UK you would have been able to see 100 millions years ago would have been the highest peaks of the Scottish mountain range.  

The cell walls of these plants were strengthened with a skeleton made of hard plates of calcium carbonate which, after the death of the algae, slowly sank and built up on the sea floor. Layers upon layers of these skeletons built up to form what is now the South Downs and the famous chalk cliffs.  

Discover the animals that lived in the seas 100 million years ago in our fossil galleries. 

Unique chalk habitats 

The chalk has created rare habitats including chalk reefs which are home to an abundance of wildlife.T he Beachy Head West Marine Conservation Zone, which runs from Brighton Marina to Beachy Head, is a wonderful chalk reef. It is home to threatened species including blue mussel beds and short-snouted seahorses.  Read our Rockpooling guide to find out more about what you can spot in Beachy Head West rockpools.

chalk reef film from Sussex Wildlife Trust on Vimeo.