Fossil Hunting The Jurassic Coast's Cretaceous Rocks

Updated: Apr 21

Learn about how fossil hunting the Cretaceous rocks of the Jurassic Coast in Dorset was the inspiration for Fossil Coast's own Lime Stone London Dry Gin. Many of the fossils are found in the Cretaceous Purbeck Group a series of layers of limestone and mudstone shale sediments deposited under predominantly arid and dry conditions.

Image Anton Lammert / Unsplash Old Harrys Rocks

The Cretaceous Period (145 - 66 million years ago) was the longest and the last of the three periods of the Mesozoic Era lasting 79 million years. The Cretaceous was named in 1822 by Belgian geologist Jean-Baptiste-Julien d’ Omalius d’Halloy who studied and mapped the rocks of the Paris Basin identifying the fossil rich chalks as Terrain Crétacé. The word cretaceous translates from the Latin word “creta”, meaning chalk.

The Jurassic Coast has an almost perfect succession of Cretaceous rocks except for the absence of the youngest layers of the uppermost part of the Campanian and the Maastrichtian. These cretaceous rocks are aligned to create a concordant coastline where the same type of rock runs along the length of the coastline with alternating bands different rock running parallel to the coast inland.

The Cretaceous had two epochs known as the Early Cretaceous Epoch (145 million - 100.5 million years ago) and the Late Cretaceous Epoch (100.5 million - 66 million years ago) ending with the mass extinction of the dinosaurs, marine and flying reptiles, and many marine invertebrates known as the K–T extinction.

During most of the early Cretaceous there was a significant sea-level fall that had already begun in the late Jurassic and resulted in a 20-million-year interval of non-marine sedimentation indicated by the Purbeck Group and Wealden Group of rocks.

Many of the fossils are found in the Purbeck Group a series of layers of limestone and mudstone shales sediments deposited under predominantly arid conditions. The best outcrops are the exposed in the cliffs of Durlston Bay, Worbarrow Bay, Mupe Bay, Lulworth Cove, Durdle Door and underlie of the southern plateau of Purbeck where they have been historically quarried for building stones.

The Purbeck limestone differs from the Portland Limestone in that it is layered thinly and generally has a non-marine fossil record. The Purbeck Group limestones were created in a lagoonal and lacustrine (lakes) environment so many of the fossils are freshwater fish and land based insects (ants and butterflies, aphids and grasshoppers), lizards, turtles, crocodilians, pterosaurs, dinosaurs and mammals.

Among the Purbeck Group fossil record are remains of ancient forests of ferns, cycads, and conifers that grew along the margins of the lakes. Another indicator of the environment at the time is the Purbeck Blue Marble. This rock is not a true marble but a limestone that is almost exclusively composed of fossilised freshwater snail shells called Viviparus.

During the Cretaceous Period the break-up of Pangea continued with the creation of the continents of Laurasia and Gondwana in the northern and southern hemisphere’s respectively. They were separated by the equatorial Tethys seaway.

The Tethys was named in 1893 by the Austrian geologist Eduard Suess after the Titan Tethys, the daughter of Uranus and Gaia, and the sister and consort of Oceanus, the ancient Greek god of the ocean. The break-up of Pangea and the presence of a new sea isolated many species and an increasing number of regional differences in the evolution species of flora and fauna between the northern and southern continents prevailed.

During the early cretaceous the sea levels were relatively low and the environment of the Jurassic Coast was a warm coastal swamp with lagoons and salt flats known as “sabkas” where some of the largest and most fierce dinosaurs lived.

Dinosaurs were the dominant group of land animals, especially “duck-billed” dinosaurs called hadrosaurs and horned forms, such as Triceratops. Giant marine reptiles such as ichthyosaurs, mosasaurs, and plesiosaurs were common in the seas, and flying reptiles called pterosaurs dominated the sky. Flowering plants known as angiosperms became abundant along with conifers. The Fossil Forest to the east of Lulworth Cove shows the tree stumps of the forerunners of present-day conifers.

The late cretaceous is known for the extinction of non-avian dinosaurs and ammonites but this does not mean that new dinosaurs did not appear during this period along with many other insect groups, mammals and birds and the first flowering plants.

The late Cretaceous was a time of great productivity in the world’s oceans and represented by the huge amounts of chalk sediment.

The sea levels of the late Cretaceous were higher than today as global sea water was being displaced by tectonic movement that was forming the mid-oceanic ridges and ocean basins.

The chalk landscape is a familiar feature along the Dorset coast but its important to remember that most of the Cretaceous rocks are not chalks but conversely most chalks were deposited during the Cretaceous. Chalk was formed by the deposition of chalk forming single-celled phytoplankton or nannofossils called Coccolithophores. Coccolithophores are an important group of approximately 200 small marine phytoplankton species which cover themselves with a calcium-carbonate discs called coccoliths forming shell called a “coccosphere.” After death, most coccospheres collapse into their constituent parts and settle on the sea bed and compress to form chalk sediments.

The Cretaceous-Tertiary extinction or K–T extinction marks the end of the Cretaceous Period 65 million years ago.

This global extinction event was responsible for eliminating approximately 80% of all species of animals.

The leading ideas behind the source of this extinction include large scale volcanic activity that changed the earths climate to the Alvarez hypothesis. This hypothesis proposes a very large meteor collides with earth filling the atmosphere with gas, dust, and debris that drastically altered the earths climate.

The dinosaurs (except birds), "marine reptiles" including the ichthyosaurs, plesiosaurs, and mosasaurs; the flying pterosaurs; many species of foraminiferans or single-celled organisms plus the ammonites became extinct. However, many groups of organisms, such as flowering plants, gastropods and pelecypods (snails and clams), amphibians, lizards and snakes, crocodilians, and mammals survived the Cretaceous-Tertiary boundary.

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