So, what is Chalk?
45 million years after the (Jurassic period), approximately 100 – 66 million years ago, during the late Cretaceous period what is now S.E. England was underwater and the dinosaurs of the time were the main life forms dominating the land. The oceans were filled with microscopic algae called coccoliths. When they died, they sank to the bottom of the ocean and carpeted the sea bed. Their tiny shells, which were made of calcite, formed sedimentary layers called ‘lime mud’. The sediments are created by the sea floor subsiding and as more sedimentary layers are laid down the tiny shells are compacted and subjected to heat which removes any water from the lime mud. This process gradually forms a brittle solid rock that we see today as chalk.
What else is in chalk?
There are many fossils to be found in chalk. These are marine animals preserved in the sedimentary layers on the seabed. One of my favourite questions I ask people is ‘Where do you think flint comes from?’ Most people just think it’s a rock, it is actually made of ancient sponges and other microscopic animals. These creatures had skeletal like structures made of silica and when they died these structures dissolved into a layer of silica rich water at the bottom of the sea. The pressure of the ocean above made this rich water crystallise and fossilise thus forming flint. The irregular shapes of flint are caused by the crystallised silica taking on the shape of the burrows that animals made in the soft chalky/lime mud layers, a bit like a modern day plaster cast.
We associate flint with our neolithic ancestors who managed to skillfully transform this flint into various tools from arrowheads, to scrapers and axes. Some flint was prized more than others and extensive mining operations were carried out all over the UK, one of the most impressive mining operations is a stone age flint mine in Breckland, famed for it’s jet black flint. Called Grime’s Graves it is the only Neolithic flint mine open to visitors in Britain. This grassy lunar landscape of 400 pits was first named Grim’s Graves by the Anglo-Saxons. It was not until one of them was excavated in 1870 that they were identified as flint mines dug over 5,000 years ago.
Is it all chalk?
There are many other types of rock found alongside chalk and make up the majority of south east England and contribute to the exciting environment that is chalk Downland.
Sandstone:
Sandstones are made up of pre existing minerals (clastic) rather than being made up of organic materials, like chalk. They are formed from grains that may either be fragments of a pre-existing rock or crystals. The grains are bonded together typically with calcite, clays, and silica.
Siltstone and Mudstone:
Siltstone is a clastic sedimentary rock. Siltstones differ significantly from sandstones due to their smaller pores and higher propensity for containing a significant clay fraction. Mudstone or shale are rocks that contain mud, a material that has various silt and clay. Siltstone is differentiated by having a majority of silt, not clay.
Limestone:
Limestone is mainly composed of the skeletal remains of marine organisms including coral, forams, and molluscs. It is the general name for a wide variety of sedimentary rocks which are composed primarily of calcium carbonate. Over time the layers of calcium carbonate are lithified (transformed into stone). Chalk is a variety of limestone composed primarily of the shells of single-celled, calcium carbonate secreting creatures like the coccoliths.
If chalk forms at the bottom of the ocean how does it get to the surface?
The South of England came to be above sea level when, over millions of years, multiple volcanic eruptions from the planet’s core forced the layers of chalk to rise. Eventually, at around the same time the Alps were formed, by the African tectonic plate pushing into the European plate (34-23 million years ago – Tertiary period), the collision helped raise what is now Great Britain. Simultaneously the ice caps were forming due to the earth cooling from a reduction in greenhouse gases further contributing to the reduction in the sea level.
The years went by and the continents moved apart, and as they move apart a new ocean is formed along the gap created with a new ocean floor. This new ocean floor is created from magma rising from below (see image). During the Cretaceous period there was a high volume of magma rising (known as a super-plume) this caused more magma to flow into the surrounding ocean areas raising the sea level to cover parts of the continents. Due to the increased magma the greenhouse gasses caused by this event warmed the earth melting the polar ice caps, further increasing the level of the oceans.