Chalk Geology

The Cretaceous seascape where the chalk was born...

Image: Winter Wind by Martin Brady courtesy of SDNPA

The rocks of Sussex and surrounding areas are from the Cretaceous era 145-66 million years ago. The Cretaceous name is actually derived from the French and Latin word for chalk which is Creta.

During the Cretaceous dinosaurs were walking on the land, the area that is now southeast England was underwater at various depths. It was located on the edge of the giant supercontinent, Pangea, around 40°N, which is equivalent to the latitude of the Mediterranean Sea today.

Look carefully at the image below to find a Tyrannosaurus Rex footprint from the upper Cretaceous period (Image: Inferred Tyrannosaurus rex footprint (Laramie Formation, Upper Cretaceous; Parfet Prehistoric Preserve, Golden, Colorado, USA) 1" by James St. John is licensed under CC BY 2.0.)

Global sea levels during these times were often over 200 meters higher than they are today. As the sea moved in and out at different depths, different sedimentary rocks were deposited all over the area. Sandy shores and sandstones and were deposited near the coast, clay was deposited in shallower waters, and chalk was deposited in deeper waters.

The purity of the chalk indicates that it formed far from land, virtually free of terrestrial sands and silts that would otherwise have coloured it. Chalk is largely composed of the skeletal remains of planktonic algae known as coccolithophores. These tiny organisms accumulated to form a white ooze on the seafloor. This soft sediment was later compacted and hardened to form chalk, a relatively soft rock. Coccolithophores are so tiny that around 200,000 of them could fit onto a single pinhead.

They are fascinating and beautiful. The image below shows them under a microscope.

Image: Emiliania huxleyi coccolithophore (PLoS) by Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility) is licensed under CC BY 2.5.

Among the chalk we can also find nodules and layers of flint. Flint is composed of silica, which was originally derived from the dissolved remains of sea sponges and siliceous planktonic micro-organisms. As the chalk was buried, the flint filled burrows and cavities and enveloped the remains of marine creatures. The flint then dehydrated and hardened into microscopic quartz crystals.

Image: Horizontal flint beds (silicious sponge spicules). Brown flows - Miocene/Pleistocene Seaford Head, Sussex, 02/07/62 by Mary Gillham Archive Project is licensed under CC BY 2.0.

Wealden Anticline & Erosion

Over the last 65 million, years the African plate has been colliding with Europe and as a biproduct it has caused the Alps mountain range with ripples creating the local Wealden anticline. This was once a vast dome of chalk laid over alternating clay and sandstone, covering much of Sussex, Hampshire and Kent.

Over time and once clear above the ocean the centre of the dome was eroded leaving just the edges as a range of hills to the north and south which we know as the North and South Downs.

Rivers carved their way through the chalk downs at Seaford/ Newhaven and Cuckmere etc. and  along with ice age periglacial conditions created shallower undulating valleys in the soft chalk which today can be seen as The Seven Sisters.

Image: Elaine Higgins courtesy of SDNPA

The Chalk Aquifer of the South Downs

Chalk is an aquifer, which means that it is able to soak up and hold water – a bit like a sponge. Water can also move through the chalk in cracks called fissures. 

The Chalk aquifer of the South Downs is vast, it runs roughly parallel to the south coast of England between Eastbourne in the east and Poole harbour in the west. The Chalk is highest in the north of the area and dips south towards the English Channel. It is divided into five areas, or ‘blocks’ defined by a series of roughly north-south trending river valleys. This is one of the best studied aquifers in the world and water levels in Sussex have been recorded for nearly 200 years. 

Most of our drinking water in areas of Chalk across Southern England  comes from the chalk Aquifers (100% in many places) which supplying around 1.2m people. The clear cool water that emerges from the chalk as springs, streams and rivers is also home to a rare and unique ecosystem present only in parts of England and Northern France and Denmark.  The chalk acts as a temporary reservoir by regulating the amount of water supplied to the springs as levels of groundwater rise and fall with the seasons giving rise to seasonal springs like the Winterbourne in Lewes.

"Drink more water" by katerha is licensed under CC BY 2.0.

Text kindly supplied by Chris Cook