Dykes, when the islands were stretched and stretched
Another effect of the opening of the Atlantic Ocean was that the land, including the Clyde Islands, was stretched from east to west. The result was cracks running north-south; and molten rock flowed up from the depth into these cracks. They may indeed have erupted as fissure volcanoes such as those which occur in Iceland at present. The crust stretched by as much as 7 per cent. The magma in the cracks solidified underground in vertical sheets, or dykes, of black dolerite. These have been worn down by erosion and are particularly well seen where the black dykes cut red sandstones along the south coast of Arran.
The width of the dykes varies from a few centimetres to as much as 30 metres. The dolerite is well jointed and can show spheroidal or onion-skin weathering. On the beach rock platform, the dykes commonly stand a little above the sandstone because they are harder. Sometimes, though, wave action has plucked out the dyke rock. The heat of the magma baked the rock on either side, making it harder than the dyke rock so that in some cases the baked sandstone stands proud.
In earlier times the sea acted in just the same way. Thus dykes were eroded into stacks on what are now the raised beaches. On the Great Cumbrae some of the large dykes form very spectacular wall-like features, such as the Lion Rock and the Diel's Dyke.