https://blog.gongquiz.com/2021/05/03/%ec%85%b0%ec%9d%bcshale%ec%9d%b4-%ec%9d%b4%ec%95%94mudstone%ea%b3%bc-%eb%8b%a4%eb%a5%b8-%ec%a0%90%ec%9d%80-%ec%9d%91%ec%9a%a9%ec%a7%80%ec%a7%88%ea%b8%b0%ec%82%ac-2010%eb%85%84-7%ec%9b%94-25/