By Jurassic times Britain had reached the latitude of the present day Mediterranean. During this period the North York Moors area was a subsiding basin. This factor, together with a eustatic (worldwide) rise in sea-level, led to thousands of feet of sediment being deposited in this basin during Jurassic times. During late Cretaceous- Tertiary times, due to Earth movements, the basin was inverted.
The rocks are divided into Lower, Middle and Upper Jurassic. The first marine sediments (Lower Jurassic) that were deposited are known as the Lias Group. They are a thick sequence of richly fossiliferous mudstones (calcareous in part), siltstones, shallow marine sandstones and ironstones, the latter composed predominantly of an iron carbonate, siderite. Abundant ammonites, e.g. Arietites and Dactylioceras allow a very precise dating and correlation of the sequence. It is within this Group that, in addition to the ironstones, the alum shales and jet rock are found all of which were of economic importance historically.
During Middle
Jurassic times, a delta, or coastal plain spread from the landmass to
the north. This deposited a sequence of sandstones, siltstones,
shales and minor coals. There were, however, at least two marine
transgressions during Middle Jurassic times. Middle Jurassic rocks
are famous for fossil dinosaur footprints which can be seen on the
shore in fallen blocks of the Saltwick (Hayburn) Fm. between Whitby
and Saltwick Bay, and also on the shore in rocks of the Scalby Fm. at
Burniston, north of Scarborough.
(For some pictures
of dinosaur footprints, click
here.)Plant
beds are also famous such as those at Hasty Bank, Whitby, Botton
Head, Roseberry Topping and the Gristhorpe Plant Bed at the southeast
end of Cayton Bay. The disused quarry (in the Saltwick Fm.) towards
the top of the eastern end of Errington Wood east of New Marske was
once famous for fossil plants, but it is now so overgrown that it is
extremely difficult to find any.
A return to fully marine conditions occurred during Upper Jurassic times and the rocks of this age consist of marine limestones, calcareous sandstones and mudstones. The resistant calcareous sandstones and limestones in fact form a distinct north facing scarp in the southern North York Moors as well as the reef at Filey Brigg. The steep slope of Newgate Bank on the Stokesley-Helmsley road, for example, and the slope leading up to the Hole of Horcum are due to this scarp. The youngest Jurassic rocks in Yorkshire are mudstones, the Kimmeridge Clay which is the principal source rock for North Sea oil. Unfortunately it is very poorly exposed. It can be seen at the base of the cliffs near Speeton, S.E. of Filey when there is not too much sand.
A famous Lower Jurassic site can be visited by clicking here.
