page_title: Shimna River
section_title: Geology
hierarchical_section_title: Shimna River / Geology
caption_attribution_description: English: Shimna River, Tollymore Forest Park, Bryansford, County Down, Northern Ireland, September 2010
context_page_description: The Shimna River is a river in County Down, Northern Ireland. It rises on the slopes of Ott Mountain, in the Mourne Mountains, and enters the Irish Sea at Newcastle, on Dundrum Bay. It is acidic and nutrient-poor, as a result of which its most common flora are mosses and liverworts, including the rare Portuguese feather-moss and Holt's mouse-tail moss. Its principal fish are salmon and sea trout, and it is managed by the Shimna Angling Club. The river is an Area of Special Scientific Interest.
  The Shimna has a history of flooding; the most serious recent flooding was in 2008. There have also been incidents of pollution, and there were serious fish kills in 2004, 2006 and 2009.
context_section_description: The Shimna is a short spate river, meaning it becomes deep and fast-flowing after periods of rain and is slow, with low water levels in drier weather. In sections of the river where the gradient is low, there are riffles and pools, sandy banks, and pebbles and scattered boulders on the river bed. Along steeper parts there is a faster flow, rapids and cascades, and the bed is made up of bedrock and boulders.
  The bedrock of the upper part of the river, located in the Mourne Mountains, is composed of granite formed approximately 50 million years ago (mya). The bedrock of the river as it flows through Tollymore is shale from the Silurian age, more than 400 mya, and it forms step gorges along some sections of the river banks. Dark bands of basalt are found were molten basalt lava was forced through cracks in the bedrock about 60 million years ago. The river bed contains stones and gravel from as far away as Antrim, Tyrone and Scotland, which were carried there by glacial ice sheets during the last ice age.
caption: Shale rock with dark basalt bands along the edge of the river as it flows through Tollymore Forest Park
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