Tieveshilly
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Geology & landscape
Geological Period: Ordovician, Silurian
Description
Tieveshilly Silurian stratigraphy The rocks at Tieveshilly contain a rich graptolite fauna within shales of Ordovician and Silurian age. The graptolitic shales are historically important for their contribution to resolving Upper Ordovician and Lower Silurian stratigraphy in this area. The site is of international importance as it contributes significantly to an understanding of the processes involved in the formation of the British and Irish Caledonide rock series, and their timing. The overlying succession of Tara Sandstones and Kearney Siltstones is also apparently unique. Graptolites, the only fossil remains found here, are extinct colonial marine animals and have been collected from Tieveshilly for at least 150 years. The site has made a significant contribution to the study of this fossil group. Although the rock sequence is complicated by frequent faulting and overturning of beds, the succession is reasonably clear. The rocks at Tieveshilly comprise fossiliferous shales, together with siltstones and mudstones. The range of graptolites in successive beds show that sediment deposition continued from the Upper Ordovician, some 440 million years ago, through to Lower Silurian times, some 430 million years ago. Such dating is important in understanding the staged closure of the Iapetus Ocean. The shales are interpreted as deep-water sediments deposited in the closing Iapetus Ocean. This ocean disappeared some 410 million years ago. What is now Tieveshilly was then on the ocean's northern margin, part of the Laurentian or American crustal plate. The sediments were accreted on the northwestern, Laurentian, shore of the Iapetus Ocean during plate collision and ocean closure. https://www.daera-ni.gov.uk/publications/tieveshilly-assi