North Arran
Erosion in the north of Arran has removed the characteristic volcano shape and worn the rocks down to the level where the molten rock cooled as granite. The granite now stands up as higher ground because it is harder and more resistant to erosion than the surrounding ancient sedimentary rocks, which were tilted when the granite was forced into place. The northern hills of Arran are in fact made up of two intrusions of granite, an earlier coarser granite which forms the higher hills and a later finer granite in the centre forming the lower ground.
Granite is a crystalline rock. As the molten magma cooled very slowly, crystals formed, and the slower it cooled the more time the crystals had to grow. Most crystals are big enough to see with the naked eye, and give the rock its rough granular surface (good grip for mountaineers). Under a polaroid microscope the different minerals can be recognised: quartz, feldspar and mica.
Like all igneous rocks granite has joints, which are cracks formed when the rock cooled and shrank. In granite the joints weather to give the typical wall-like effect called a tor; this, and the pale grey colour of the granite, makes tors distinctive features of the landscape.