Granite boulder on limestone beds. Granite is an intrusive igneous rock, formed when volcanic magma solidifies under pressure deep in the Earth's crust. It consists of medium- to large-sized mineral crystals. This boulder originated far north of these limestone beds, but was transported by glaciers around 10,000 years ago. Scratches on the limestone reveal the nature of the glacial flows when the region was covered in ice. Photographed on Inishmore island, off the western coast of Ireland.