Selwicks Bay, Flamborough

The Cretaceous period is marked by a significant increase in world sea-level, the main cause being an increase in the number of mid-ocean ridges due to the break-up of Laurasia and Gondwanaland. During  Lower Cretaceous times sea-level began to rise. Over the Cleveland Basin the rise in sea-level  led first to the deposition of richly fossiliferous mudstones, the Speeton Clay Formation named after the locality at which it is best exposed, about 6km S.E. of Filey.

By the start of Upper Cretaceous times a world-wide rise in sea-level resulted in nearly the whole of Britain and western Europe being covered by the sea. Deepening seas led to the deposition of a thin limestone stained red by iron oxides, the "Red Chalk". Subsequent lack of sediment supply due to lack of nearby land areas led to the deposition of a very pure limestone, the Chalk, which consists almost wholly of the tiny platelets (called coccoliths) of single celled algae which flourished in the warm sea of that time. Other fossils, however, are also found.
Within the Chalk, nodules of flint, sometimes in distinct bands, are found. These make very good marker horizons for correlation. The flint, a variety of cryptocrystalline quartz, is thought to be derived from silica bearing organisms such as sponge spicules.

Red Chalk, Speeton

The chalk in East Yorkshire reaches a  thickness of about 550m. It is estimated, however, that a maximum of a 1000m. of chalk was deposited altogether, and that deposition was at the rate of about 1mm. per 30 years,  i.e.  it took 30 million years for the chalk to be deposited.

The Chalk is best exposed along the coast from the Speeton area, where the "Red Chalk" can be seen, to Sewerby just north of Bridlington. Inland, apart from quarries, it is mostly covered by boulder-clay.

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