Chalk

The chalk is an Upper Cretaceous deposit found in the south of Bedfordshire.  It is an extremely pure limestone formed predominantly from coccoliths, the skeletons of minute algae.  The lower levels of the chalk also contain fine clay particles leading to the name Chalk Marl.

In the upper chalk flint nodules can be common.  Flint is composed of silica and was probably formed from the small silica skeletons of sponges.  When the sponges died they were buried by sediment, under the mud of the sea floor conditions were right to turn the silica spicules of the sponge’s skeleton into a silica jelly.  This jelly filled the spaces in the sediment forming all manner of peculiar shapes, it also filled the hollow spaces inside sea urchins and shells.  This is how the strangely shaped flint nodules and flint fossils were formed.

At the end of the Cretaceous period a huge meteor struck what is now the Yucatan Peninsula in South America.  The resultant devastation and worldwide climate change caused the death of dinosaurs on land and also the extinction of ammonites, belemnites, ichthyosaurs, plesiosaurs and pliosaurs in the seas.  The event is responsible for the rise of the mammals that multiplied and evolved rapidly to fill the gaps in the ecosystems left by the dinosaurs.

Chalk is extremely soft and is weathered away relatively easily; it is also easy to dissolve chemically.  During the ice age the chalk was eroded by glaciers.  The fine mud formed was swept away but the hard flint nodules were left behind as gravel.  This has been quarried for building purposes.  The Lakes at Priory Country Park, Wyboston and Felmersham are former gravel pits.

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