Sedimentary sandwich
Igneous rocks form the high ground; conversely the sedimentary rocks underlie the low ground. Quite simply, this is because layered strata like sandstones, siltstones and mudstones are not as hard as the igneous rocks and are worn down more quickly by erosion, be it by ice, water or wind. Thus the south part of Arran is mostly lower ground and there is a marked change in height between the granite hills and the sedimentary rocks which fringe them to the west and south.
The strata which form the south part of Arran and the middle of Bute are a sort of geological sandwich that tells us a lot about the different environment in which the rocks formed. Two red layers speak of harsh, arid deserts, and are known to geologists as the Old Red Sandstone and the New Red Sandstone, with a grey filling of Carboniferous strata telling of a more pleasant, tropical, forested environment. This can best be seen along the coast at the village of Corrie on Arran, with Old Red Sandstone to the north, the Carboniferous in the village and New Red Sandstone to the south - indeed all round the southern half of Arran.
The Old Red Sandstone tells of a time when the Caledonian mountains lay to the north-west, and a vast desert plain to the south-east. Hit by sudden torrential showers, flash-floods ripped their way down from the mountains and deposited sands and gravels in the plains. Wind blew the sands into dunes. Temporary rivers flowed and lakes developed, only to dry up when the rain ceased. Thus the red sandstones, pebbly sandstones and conglomerates of the Old Red Sandstone were formed. These rocks form a belt across Arran round the south of the granite hills. On Bute they form the middle ground between the Highland hills in the north and the Carboniferous lava hills.
In the Carboniferous times as in the Midland Valley, sand from the rivers became sandstone, peat from the forests became coal, and lime from the coral reefs became limestone, which was later mined. On Arran and Bute, though, only a thin sequence is present as the islands lay on the edge of the area where these rocks were being deposited.
These rocks were squeezed to form hills. In Permian times torrential storms caused flash floods which wore down these hills and laid coarse breccias, gravels and sands in the plains. Wind picked up the sand and formed dunes which made their way across the Sahara-like desert; cross-beds from these dunes can be seen on the shore at Corrie. Under a magnifying glass individual grains can be seen to have been rounded by the wind.
Extensive plains developed, finer sands, silts and clays were deposited in lagoons and lakes which temporarily dried out leaving mudcracks, salt deposits and other features found in an arid landscape. Some life did colonise the area and left tracks preserved in the soft mud.
For at least some of the next 200 million years the region basked in the warmth of tropical seas, but the climate changed abruptly in the recent geological past with the onset of the Ice Age.