Story of chalk

Like a white ribbon across the landscape, erosion along tracks such as The Ridgeway reveals chalk bedrock. A more localised indicator of chalk is the Roman Snail, found along The Ridgeway at Grangelands near Princes Risborough in Buckinghamshire, since it needs calcium carbonate to make its huge shell. Looking even more closely, chalk reveals fossils of marine creatures including coccolithophores (see banner image above). This is where the story of chalk begins many millions of years ago, when The Ridgeway area was beneath a sub-tropical sea near the equator. Read on below….

Banner image: Various microscopic coccolithophores. Credit: Monteiro, F.M., Bach, L.T., Brownlee, C., Bown, P., Rickaby, R.E., Poulton, A.J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S. and Gutowska, M.A., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

A sub-tropical sea might seem unbelievable now, but The Ridgeway area was once near the equator and underwater. This was around 145 – 65 million years ago during the Cretaceous period, when microscopic phytoplankton, including coccolithophores, lived in this sea. Millions of dead coccolithophores fell to the seabed over millions of years. Their calcium carbonate shells, or exoskeletons called coccosopheres, built up a thick, compacted bed of soft, white limestone that we now know as chalk. Other marine life was caught up in this calcareous ooze and can now be seen in the chalk as fossils of oysters, sea urchins, ammonites, tubular worms and brachiopods. Flints found in the chalk are part of this story.

The Ridgeway’s chalk bedrock is part of the largest geological feature in Britain – the chalk stretches for miles and miles into Yorkshire, Dorset and Kent. Other National Trails traversing chalk are the North Downs Way and South Downs Way. Further still, the chalk extends underneath London and the English Channel into Europe. As a result, the chalk cliffs of Dover are mirrored on the French side by the Alabaster Coast or Côte d’Albâtre.

Whilst the Alps were being formed, the same tectonic movements of the Earth’s crust formed the hills of the  Chiltern and North Wessex Downs which we know today along The Ridgeway! This took place around 66-2.6 million years BC during the Palaeogene and, since then, the chalk has suffered ongoing erosion. In the North Wessex Downs, there is around 200m thickness of chalk but it was double this thickness originally.

Erosion shaped the chalk into the landforms that Ridgeway visitors enjoy today, including features such as the The Manger and the Devil’s Punchbowl, both in Oxfordshire (see image gallery above). Starting around 2.58 million years ago, there have been periods of intense cold known as ‘glacial periods’ when ice sheets spread south from the Arctic across Britain. Ice sheets have extended as far as Aylesbury in Buckinghamshire, eroding away the chalk beneath and beside them. Beyond the ice sheets, their meltwaters also shaped the chalk under tundra conditions. Water coursed across ground which was made impermeable by freezing conditions and carved valleys into the chalk. Today these valleys are ‘dry coombes’ because today’s climate allows water to drain through the porous chalk. The five branching dry valleys in Tring Park in Hertfordshire are an example of a fossilised drainage pattern that was created by streams during the post-Anglian periglacial period.

Chalk underlies The Ridgeway in all senses; its influence being seen in the natural environment, in buildings and in cultural heritage. Human use of chalk dates back into prehistory, from the large-scale chalk figure of Uffington’s white horse to small scale objects such as children’s toys that can be seen in museums today (see image gallery above). Next time you are on The Ridgeway, don’t ignore the chalk beneath your feet!

Notes: More information about chalk streams, plants associated with chalk, buildings with chalk, chalk figures and industrial heritage linked to chalk is available in other Top 50 entries.

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