Scottish Agates - The Geology

The Jurassic Period

206 to 144 mya

 

During the Jurassic Period Scotland lay beneath a warm shallow sea called the Tethys Ocean. This ocean was teaming with life such as shellfish, corals, ammonites belemnites and marine reptiles such as the icthyosaurs and plesiosaur. The sea level did fluctuate and there were times when the sea covered most of the land. The climate was hot and sub-tropical as Scotland lay about 40 degrees North of the equator. In these shallow seas great thicknesses of sandstone, limestones and mudstones were formed. Within these can be found fossils of the abundant life that existed at the time.

By the Early Jurassic, south-central Asia had assembled.  A wide Tethys ocean separated the northern continents from Gondwana.  Though Pangea was intact, the first rumblings of continental break up could be heard. The approximate position of Scotland is indicated by Red Arrow. [Copyright C.R. Scotese, Paleomap Project]

Sauropod, Cetiosaurus [ Mid - Late Jurassic]
Sauropod Femur [Mid Jurassic] - The three recovered sections on the right have been attached to one another.

Plant and flesh-eating dinosaurs did roam the landmass that would eventually become Scotland and fossils of these animals have been found in the Jurassic sediments on the east side of the Trotternish peninsula in the Isle of Skye. Fossils of Cetiosaurus, a plant eating land-dwelling dinosaur that stood as tall as 10m in height have been found as well as fossils of the Plesiosaur and even Stegosaurus. Footprints of a number of large meat-eating dinosaurs have been found at Staffin on the Isle of Skye but due to the fact that conditions were not ideal at the time for the fossilisation of land-dwelling animals few fossils have been found. The landscape at this time would have been very flat with numerous lagoons that would have occasionally been inundated by the sea. There are very few sites worldwide where middle Jurassic dinosaur remains are found so the dinosaur tracks in Skye are very important finds.

An excellent, well presented collection of Jurassic fossils is on display in The Staffin Museum, Staffin, Skye

 

Some fosssils from the Jurassic of Skye

 

Jurassic outcrops in Scotland occur on the Isle of Skye and Raasay with some scattered outcrops in Ardnamurchan and on Mull. On the east coast of Sutherland on the Moray Firth the famous Kimmeridge Boulder Beds occur. There was very little volcanic activity in Scotland at that time and there are therefore no Jurassic volcanic rocks in Scotland. There was some tectonic activity in the North Sea with the establishment of a rift valley and some volcanism.

On Skye and Raasay an almost complete sequence of Liassic ammonite zone
occurs within a 400 m sequence. Limestones laid down in the early Jurassic pass up into shales and sandstones with the important Raasay Ironstone deposits at the top of the sequence. This Iron ore deposit was mined on Raasay from about 1910 to 1942. The middle Jurassic comprises about 530 m of marine sandstones often channel bedded followed by 250 m of the Great Estuarine Series of sandstones with freshwater bivalves, oolites, algal beds and dolomites. On Skye the Upper Jurassic comprises 200 m of shales and sandstones, the Staffin Shales with abundant ammonite faunas ranging up into the Kimmeridge Stage.

On the Sutherland coast of the Moray Firth Liassic rocks are found at Golspie, overlying the Rhaetic. Middle Jurassic rocks are found at Brora where coal occurs within sandstones and shales. Between Brora and Helmsdale excellent exposures of Upper Jurassic strata occurs. 100m of these shales and limestones occur then followed by 700 m of the Kimmeridge Clays with bituminous marine shales containing ammonites. Within these clays are boulder beds with large blocks of Old Red Sandstone, Helmsdale granite, gneisses and eroded Jurassic sandstones. These boulder beds formed next to a rising fault escarpment on the edge of the North Sea basin. This basin subsided along the Helmsdale fault, which is a branch of the Great Glen Fault. This suggests that that there was late Jurassic fault movement on the edge of the North Sea basin in which thick Kimmeridge Clay deposits with organic matter provided a good deal of the oil found today in sandstone reservoirs.

 

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