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Chalk ammonites

• The Chalk is exposed in all six counties of the Region; it is at its thickest in Norfolk (460m was recorded in the Trunch borehole!);

• The Chalk of the region has the widest stratigraphic range in Britain, ranging from the oldest strata, belonging to the Cenomanian Stage (with its base at the Chalk Marl horizon) to the youngest, of the Maastrichtian Stage, found only in north-east Norfolk;

• The Chalk is dipping gently towards the south-east, though the angle of dip varies across the region, e.g. about 6.6m per km in Norfolk, but about half this figure in Suffolk.

The Chalk has many variations in rock type. It includes clay-rich horizons, known as marl-bands, and hardgrounds, which are horizons of harder Chalk which developed during pauses of sedimentation on the sea floor; it also includes bands of flint nodules, which formed from a silica-rich ooze on the sea floor.

The Lower Chalk is typically grey in colour due to a high proportion of clay, and has many marl-bands, whereas the Upper Chalk is typically white and has a higher proportion of flint. The most significant bands of harder rock are named, for example the Totternhoe Stone and Melbourn Rock. Alternating layers of softer and harder chalk have influenced the shape of the region's landscape.

The Chalk has been divided up into a number of time horizons or ‘zones’ according to the characteristic fossils which occur in them. Using these zonal fossils it is possible to follow the continuity of Chalk strata as they outcrop across the country. Hardgrounds, marl bands and flint bands are also useful for correlating strata across wide areas.

In Cretaceous times the Earth’s climate was very different from that of today. It was a ‘greenhouse’ world, with high temperatures (on average over 4°c higher) and high concentrations of carbon dioxide (c. 150% higher). Sea-levels were over 200m higher than today!

Studying the Chalk may yield insights into future conditions on Earth in a ‘global warming’ scenario induced by human activity.
 

1. Ventriculites sponges
2. The crinoids (sea-lilies) Bourgeticrinus
3. The belemnite Belemnitella
4. An ammonite Mantelliceras
5. Carneithyris brachiopods
6. Inoceramus, a bivalve
7. Spondylus, a bivalve
8. Stereocidaris, an urchin with club-shaped
    spines
9. Echinocorys, an urchin
10. Micraster, a heart urchin
11. A lobster Enoploclytia and its burrow
12. The shark Cretalamna
13. A ray Ptychodus

The Chalk has undergone many changes since it was laid down horizontally on the Cretaceous seafloor. In early Tertiary times (45 to 60 million years ago), in places many of its uppermost layers were eroded and then covered by a succession of marine sands and clays, including the Reading Beds and London Clay. In the mid-Tertiary (about 30 million years ago) it was uplifted and folded as part of the same Earth movements which formed the Alps; later it was tilted towards the deepening basin of the North Sea.

In the later Tertiary, Britain was part of a continental land-mass, and underwent steady erosion which uncovered the Chalk from beneath later sediments, and in places removed further layers. The Earth’s climate progressively cooled at the end of this time, ushering in the ice ages of the Pleistocene period during which the Chalk was directly eroded by ice sheets in most parts of the region, and its surface layer was disturbed by frost action. Today the Chalk is mantled by a veneer of superficial 'drift' deposits such as glacial boulder clay and erosional residues such as clay-with-flints.