Slate tiles packed in a crate. Slate, a metamorphic rock, is formed when a fine-grained sedimentary rock, such as mudstone, is subjected to high temperatures and pressures within the Earth's crust. This changes its crystal structure, and it forms a grey rock that can be split into thin, flat sheets. This property makes it a useful material for the construction industry for use as roof or wall coverings. Photographed at a quarry in north-west Wales.