Gritstone or grit is a hard, coarse-grained, siliceous sandstone. This term is especially applied to such sandstones that are  quarried for building material. British gritstone was used for millstones to mill flour, to grind wood into pulp for paper and for grindstones to sharpen blades. "Grit" is often applied to sandstones composed of angular sand grains. It may commonly contain small pebbles. "Millstone Grit" is an informal term for a succession of gritstones which are to be found in the Peak District and Pennines of northern England. These sediments were laid down in the late (upper) Paleozoic era, in the Carboniferous period, in deltaic conditions. The Millstone Grit Group is a formal stratigraphic term for this sequence of rocks.