Fossilized leaves are exposed in the sediment that preserved them.
Sedimentary rock deposited over much of Earth's existence contains a fossil record of plants, animals, and micro-organisms dating back hundreds of millions and even billions of years. Only sedimentary rocks or preserved materials such as amber, tar or ice can preserve remains, traces, tracks, or imprints of once-living things. Igneous rock, once molten, was far too hot before it took its final cooled form to preserve fossils. Metamorphic rocks have been transformed by heat and pressure, destroying any fossils the original sediments might have contained.
Types of Fossils
A petrified ammonite is revealed by weathering of the rock that preserved it.
Fossilized evidence includes traces such as tracks or footprints, mummified or calcified remains, or rock-encased remains and imprints. Fossils can, in rare cases, be entire remains of large mammals, encased in sand, peat bogs, and ice under specific conditions. In most cases, fossils show evidence of smaller flora, partial skeletal remains, marine or insect imprints, or small calcified birds. Rocks that house fossilized remains must withstand heat and pressure for over 10,000 years, providing a optimal tablet for evidence of prehistoric life.
Sandstone Fossils
Sandstone is composed of sand particles bound together over time by a mineral matrix deposited between the sand grains by groundwater. Sandstone is evidence of an ancient environment where sand accumulated, such as a beach, streambed, lake, desert, flood plain or delta -- the same kinds of places where sand accumulates now. When you look at layers of rock exposed in a road cut, you are looking at a record of processes and changes that took place over time millions of years ago, right where you're standing. Fossilized fauna remains such as insect exoskeletons or mammal imprints or shells of marine animals are often found in sandstone. Dinosaurs, mammals and other vertebrates often have left their marks in sandstone. Whatever the traces are, they are part of the key to understanding Earth's geologic history and how life forms have changed over that history in distribution, form, and function.
Shale Fossils
Shale is a rock formed of very fine, evenly sorted sediments deposited as mud, and then solidified. It is made up of clay, mica, and fine sedimentary particles. Shales form very fine layers that can be separated, and sometimes fossils such as fish scales can be found between the layers. Because of the fineness of the particles, some shales have trapped large quantities of natural gas between their layers, which can be extracted for use by modern techniques. Shales also are great sedimentary rocks in which to hunt fossilized marine life such as trilobites, an extinct paleozoic marine arthropod. Trilobite fossils are found in large numbers in the Burgess Shale on the U.S.- Canada border in the Rocky Mountains, and in Utah's Spence Shale. Shales often also contain the fossils of ancient foraminifera, tiny protozoa that still inhabit modern oceans and that have been so abundant and widespread throughout Earth history that their evolutionary changes allow them to serve as specific markers of points in geologic time.
Limestone Fossils
Limestone is sedimentary rock that is formed by collected particles of CaCO3, or Calcium Carbonate. Limestone is made up of plankton shell fragments, coral, algae, as well as fecal debris found in calm, marine waters. The calcification process that creates limestone can also be found in stalactites hanging from the cave ceilings formed during evaporation. Although many fossils are discovered in shale deposits near the ocean floor, many types of fossilized marine life, including both flora and fauna, are most commonly found in limestone.
Tags: fossilized marine, fossilized marine life, have been, heat pressure, marine life, over time
