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During the Devonian Period the Arran land mass was at a latitude of 0oN
and the predominant environments were fluviatile and lacustrine, i.e.
rivers and lakes. The Devonian rocks of Arran belong to a sub-division of the
Devonian called the Old Red Sandstone which itself can be sub-divided into the
Lower Old Red sandstone and the Upper Old Red Sandstone. Numerous
geologists and sedimentologists have studied the ORS rocks of Arran and
the Midland Valley of Scotland and for more detail on further sub-divisions and
contention in the classification of the sediments readers are referred to
McKerrow & Atkins (1989) and
references therein.
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The Lower Old Red Sandstone of Arran outcrops in many places and is
represented by conglomerates with rounded clasts of quartzite and vein quartz,
red pebble rich grits and thin bedded sandstones and siltstones. During
the interval between the deposition of the Lower Old Red Sandstone and the
Upper Old Red Sandstone. The Lower Old Red Sandstone sediments were folded and
underwent faulting due to major crustal movements, they were also denuded and
eroded by the elements and in some locations completely removed. In Image 1
we can see an example of the Gavrock member of the Lower Old Red Sandstone, it
is exposed in Glen Rosa, here it is green coloured as it has been altered. The
iron in the coatings on the quartz grains has been reduced and has turned from
red to green.
Image 1. Lower Old Red Sandston-Gavrock member, Glen Rosa
- The Upper Old Red Sandstone is represented on Arran as red-cross bedded
sandstones (Image 2) and siltstones and conglomerates hosting angular vein quartz and
meta-sediment fragments, with some rounded quartzite fragments (Image 3).
In some of the Upper Old Red Sandstone Units reduction spots can be seen (Image
4) and in-filled mud cracks can be also be noted.
Image 2. Cross-bedded ORS on Corrie Shore.
Image 3. An Upper Old Red Sandstone Conglomerate exposed on
Corrie Shore.
Image 4. A mud cast in the centre of a reduction spot on
the surface of an Upper Old Red Sandstone Unit
exposed at Corrie Shore.
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