Slieve Croob ASSI

Protected area type: Areas of Special Scientific Interest
Feature type: 
  • Earth Science
County: 
  • Down
Council: Newry, Mourne and Down
Guidance and literature: Slieve Croob ASSI

Slieve Croob is a special place because of its earth science interest.  The area provides access to exposures of a granite-like rock called granodiorite and Silurian sedimentary rocks that together with a number of other sites, describe the Newry Igneous Complex.

The granodiorite was formed some 410 million years ago.  This is an igneous rock type, that is, it was injected as magma (molten rock) into pre-existing older rocks – Silurian sedimentary rocks.  These older rocks are known as ‘host’ rocks.  The magma then cooled slowly eventually forming huge masses of solid rock deep beneath the surface.  Subsequent erosion has now exposed parts of these enormous rock units.  

Each of these igneous rock units is referred to as a pluton.  Three of these are present which collectively make up the Newry Igneous Complex.  The complex extends over an area of about 45km2 from Slieve Croob in the northeast to Forkhill in south Armagh.  

The rocks at Slieve Croob form part of the northeast pluton and are of great importance as they display the complex relationships between the granodiorite and other igneous rock types, and the Silurian sedimentary ‘host’ rock.  

The older Silurian sedimentary rocks that formed in this ocean are exposed at Slieve Croob.  These sedimentary rocks have been altered by the heat of the igneous intrusion; a process known as contact metamorphism.  This has caused the rocks to change composition resulting in a different appearance from the rest of the Silurian sedimentary rocks that are widespread across Co.  Down.  In some cases, the intrusion of the granodiorite has caused the ‘host’ rock to mobilise and this can be seen as contorted or twisted layers.  The Silurian rocks at Sleve Croob contain the mineral cordierite that is common in metamorphosed sedimentary rocks of this type.  Where this mineral has been weathered out the rock takes on a pitted appearance.  

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