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.
