Special category: you can see them forming, it happens all the time. Constitute only 7% of the earth"s crust by volume. But covers approximately 75% of the earth"s surface. Acts like a history book of what happened on the surface like the environmental record like biological. They have an enormous economic significance because of valuable metals composites like gold, silver. They are also building stone that have fossils in them. Aggregates like sand, road, gravel, they have to have a really deep base so when it freezes it doesn"t deform. Fossil fuels, gas, and water is also in sedimentary rocks. Some are large enough you can see with naked eye or hand-lens. Carbonate are turned biochemical, (something was living to produce that material)(created shells) Most sedimentary rocks are deposited in the ocean, underwater. Some are deposited in fresh water (lakes and rivers) Some deposited by glaciers, land slides , and wind.