This smaller image complementing Geology shows a fossilized trilobite, an ancient crustacean that lived in the early oceans of the earth but is now extinct. Many millions of these creatures were trapped in accumulating sand and silt, fossilized, and the layers compressed over eons of time. Uplifted by great forces within the earth, these fossils of sea creatures eventually were discovered high upon mountains and other formations. The locations of these and many more species within various layers or strata were used to date meticulously the various formations and their extent, and to compile the basic chronology of the earth.