clastic sedimentary rocks are rocks composed predominantly of broken pieces or "clasts" of older weathered and eroded rocks. clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material (matrix) composition, and texture. the classification factors are often useful in determining a sample's environment of deposition. an example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream.