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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.


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, for example, 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) Formation between Whitby and Saltwick Bay.


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 a 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, with the Kimmeridge Clay being the principal source rock for North Sea oil.

The above images were taken at Port Mulgrave and show the famous Jet Rock and some ammonites found there.

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