One of the three major rock types, sedimentary rock is created through the cementation of deposited sediments. Sediments are small weathered and eroded fragments of minerals and organic matter. Some sedimentary rocks have high permeability that makes them great vessels for aquifers and petroleum reserves. Of the three rock types, sedimentary rocks hold the most fossils. This is due to the lower temperature and pressure that sedimentary rocks form at in comparison to the pressure and temperature conditions during the formation of metamorphic and igneous rocks. A primary sedimentary structure is formed during deposition of sediments, while a secondary sedimentary structure is formed after deposition. An example of a primary sedimentary structures is cross bedding. Cross bedding is angled layering due to tilting during deposition, not from post-depositional deformation. Trace fossils are a type of secondary sedimentary structure that consists of impressions made by organisms or evidence of the organism in material.