Ordovician Period
Age: 443 to 495 million years ago
Geography, environment and climate
At the beginning of the Ordovician, the part of the continental plate that is now England lay about 600 S and formed part of the supercontinent of Gondwana. Following break-up of Gondwana this part of the plate (known as Avalonia) gradually moved northwards to about 350 S by the end of the Period. England was occupied by shallow shelf seas, with the deeper water of the Iapetus Ocean being present to the north over the area of the Lake District. Arcs of volcanic islands formed on the northern edge of Avalonia as the microplate drifted northwards.
Climate was cool at the beginning of the Ordovician and gradually became warmer as Avalonia drifted northwards.
Key events
The Iapetus Ocean reached its greatest width during this period. Trilobites, brachiopods and graptolites dominated the marine fauna, while there was a significant increase in the overall diversity of marine life. Mountain building began at the end of the Ordovician, associated with the beginning of collision between the separate plates carrying what are now England and Scotland.
There is evidence of a widespread and major phase of glaciation at the end of the Ordovician which caused a major fall in sea-level and the extinction of marine organisms.
Rock types and occurrence in England
Huge thicknesses of muddy sediments accumulated on the margins of the Iapetus Ocean and these now form the Skiddaw Slates of the northern Lake District. Extensive volcanic activity in the Upper Ordovician led to the formation of islands on the edge of the Iapetus Ocean, the remnants of which now form the Borrowdale Volcanics of the High Fells of the Lake District.
