Sedimentary rocks form when sediments become pressed or cemented together, or when minerals come out of mineral rich solutions, or are left behind by evaporation, These sediments are loose materials such as rock fragments, mineral grains, and bits of plants and animal remains that have moved by wind, water, ice, or gravity. Geologists place sedimentary rocks into three broad categories:

(1) clastic rocks, which form from clasts, or broken fragments, of pre-existing rocks and minerals; (2) chemical rocks, which form when minerals precipitate, or solidify, from a solution, usually sea water or lake water; and (3) organic rocks, which form from accumulations of animal and plant remains. It is common for sedimentary rocks to contain all three types of sediment. Most fossils are found in sedimentary rocks because the processes that form igneous and metamorphic rocks prevent fossilization or would likely destroy fossils.
 
 

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