| Site Type: | Coastal Section, Foreshore | | Site Status: | ASSI | | Council area: | Ards & North Down Borough Council | | Grid Reference: | J564829 | | Bing maps: | 54.67035,-5.57568 | | Google maps: | 54.67035,-5.57568 | | Rocks |
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| Rock Age: | Ordovician, Silurian | | Rock Type: | Turbidite, Greywacke, Shale | | Interest |
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| Summary of site: |
| The Outer Ards coast is composed of Lower Palaeozoic sedimentary rocks of deep marine facies, strongly folded, faulted and weakly metamorphosed. The area is part of the Scottish Southern Uplands / Irish Down - Longford Terrane, one of a series of terranes assembled through the closure of the Iapetus Ocean at the end ofthe Silurian period, 395 Million years ago (Ma). A terrane is a geological unit, discrete in terms of structure and age, and distinct from adjoining areas. | | The terrane is primarily composed of well-bedded greywacke sandstones and shales. Characteristically, bedding dips steeply with sedimentary structures commonly demonstrating inversion. Most of the beds young or face north-westward, yet fossil evidence indicates a regional decrease in the age to the south - east. This paradox is solved by dividing the terrane into a fairly large number of elongated, parallel tracts, each between one and five kilometres across and divided by faults. | | Since 1977, the terrane has attracted international interest because of its interpretation as an accretionary prism. There is ongoing debate concerning the geographical position of the sediments which now form the rocks of this region, namely whether they were fore-arc or back-arc sediments. Irrespective of this, sediments were accreted on the north-western, Laurentian, shore of the Iapetus Ocean during plate collision and ocean closure. The unresolved argument between proponents of these rival hypotheses is critical to understanding of the British and Irish Caledonides. | | Orlock Bridge affords the most spectacular example of fault associated rock deformation in Northern Ireland. Mylonites and breccias have been generated from the country rocks by movement on the Orlock Bridge Fault. This major fracture separates the Upper Ordovician Northern Belt from the mainly Silurian Central Belt along a line traced from County Cavan to south of Dunbar. |
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