1. Organically formed sedimentary rocks are the remains of pro-existing plants and animals that have been joined together by pressure e.g. petroleum, coal, lignite etc.
2. Non classic sedimentary rocks are chemically formed rocks, which are precipitated rocks. Precipitation takes place when the water in the solution has been dried off e.g. common salt (sodium chloride) NaCI.
3. Classic sedimentary rocks are mechanically formed from pre-existing rocks, which have attained a new form through mechanical means e.g. sandstone, clay, mudstone etc.