Northern hills of Arran – pushed up from the deep
Although not the oldest rocks in this area, at a mere 60 million years old, the granite hills in the north of Arran are by far the most prominent. At that time Greenland and North America were joined to the north-west coast of Britain, and the Atlantic Ocean was just beginning to open as the Eurasian and North American continents drifted apart. Cracks related to this opening formed down the Firth of Clyde, beginning to determine the outline of Arran and the sea lochs to the north. It is worth noting that Arran has been an island longer than Britain, which was still joined to the rest of Europe after the last Ice Age.
The continental crust was stretched and thinned along the west coast of Scotland. Molten rock, or magma, exploited points of weakness at centres from Skye to Antrim, forcing columns of magma towards the surface. A dark rock called gabbro was formed at great depth, while higher up the magma cooled to form granite. In many cases the magma reached the surface and built up a massive caldera volcano of lava flows or ash falls, some of which slumped back into the crater. At least three such volcanoes are present in the Clyde region: in north Arran, in central Arran and at Ailsa Craig. Earth movements have raised each of these volcanic centres, but by varying amounts, so the erosive forces of ice, water and wind have worn them down to expose different levels of their volcanic ‘roots’.