Sills make cliffs
Much of the molten rock in a volcano is never in fact erupted as lava flows. Instead of reaching the surface vertically through pipes and cracks, the magma finds an easier route sideways between layers of rock. Eventually it cools as a sandwich of hard crystalline igneous rock, called a sill, between softer layers such as sandstone and mudstone. Many such sills were formed in the red sandstones in the south part of Arran. Some consist of a dark rock called dolerite; others of a light coloured rock, felsite.
As the rocks of Arran have been tilted by earth movements, so the sills are normally inclined rather than horizontal. The effect of erosion on any landscape formed from alternating hard and soft rocks is that the harder layers resist and stand out. Hence most of the hills in the south part of Arran are formed of the harder sill rocks, with the softer rocks in between forming hollows. This leads to a stepped landscape called ‘trap featuring’, from ‘trap’, the old word for lava.
Round the coast of Arran the sills form many of the fine cliffs, such as Holy Island, Pladda and Drumadoon. Indeed, the very shape of Arran has been dictated by sills, which form most of the headlands, such as King's Cross Point and Brown Head. Inland, rivers flowing across the stepped landscape produce waterfalls where the sills form resistant layers. Falls such as Glen Ashdale and Auchenhew are a feature of the south Arran landscape.