Gruggandoo
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Paleogene, Silurian · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Gruggandoo Tertiary igneous The area is of special scientific interest because of its important geology. Gruggandoo exposes part of the cone sheet that almost completely surrounds the Mournes. This sheet demonstrates the relative age of the sheet with respect to the main granite in this area and the country rock. The granite and sheet rocks are igneous and date from the Palaeogene period of Earth history, some 56 million years old. The country rock is considerably older, dating from the Silurian period. The longer axis of the cone sheet bears ENE-WSW and follows the shift of intrusive activity from the Eastern to the Western Mournes. The sheet is inclined approximately 35° toward the granites, indicating a focus of approximately 3km below the surface. At Gruggandoo, outcrop extends approximately 300m north east from the summit before being truncated by the G4 granite of the Western Mournes. The cone sheet is composed of porphyritic granophyre. The granite at the site is one of the two types that make up the Western Mournes; here the granite is fine to medium grained and pale grey-brown in colour. There is also evidence that the granite chilled against the granophyre of the cone sheet which provides conclusive evidence that the cone sheet predated granite emplacement. Gruggandoo is the only locality where this contact is found at outcrop. During the early phase of igneous activity, that ultimately formed the Mourne Mountains, a series of ring fractures developed. It was into these fractures the cone sheet was intruded. The Mourne granites were then emplaced that, at Gruggandoo, truncate the cone sheet. https://www.daera-ni.gov.uk/publications/gruggandoo-assi