Triassic Period
Age : 205 to 248 million years ago
Geography, environment and climate
During the Triassic, the 'British Isles' formed part of the supercontinent known as Pangea. The area that now constitutes the British Isles drifted northwards as Pangea rotated, to a latitude of ~10o - 20o N, equivalent to the latitude of the present day Saharan desert.
Sediments (dune sands and shallow lake mudstones) accumulated in a number of large, shallow basins. Large braided rivers crossed the desert plains which were punctuated by more mountainous areas formed by the older rocks of Dartmoor, the Mendips and the Malverns.
Towards the end of the Triassic, sea-level started to rise and a warm, shallow sea developed over what is now southern England.
Key events
The Triassic marked a new beginning for life on Earth following the mass extinction at the end of the Permian. Forests of conifers and cycads dominated the land flora replacing earlier plant forms, such as ferns, and the reptiles started to attain dominance, with the first dinosaurs evolving in the late Triassic. Many new species of marine life evolved to fill the vacant space left by the demise of the groups that had dominated the Palaeozoic seas.
Rock types and occurrence in England
Triassic rocks are widespread throughout England outcropping either side of the Pennines, the Midlands, Gloucestershire, Somerset and east Devon. The rocks are relatively soft and have been weathered and eroded to form the lower lying ground of much of Worcestershire, Cheshire and Nottinghamshire. Triassic river deposits occur along the south coast between Exmouth and Sidmouth. Further north, Triassic sandstones form the red sandstone cliffs at St. Bees Head in Cumbria.
The rocks of this period are characteristically red in colour due to the oxidation of iron-rich minerals in the sediments under the arid conditions. The arid conditions led to extensive evaporation and thick deposits of salt accumulated in some of the desert basins, notably in the area that is now Cheshire.
