1. Fossils and Rock Layers

3. How are Fossils preserved?
When an animal dies, the remains are covered by sediment. This sediment hardens and forms rock layers over time.
What process
moves the rocks
and sediment?

4. How are Fossils preserved?
When an animal dies, the remains are covered by sediment. This sediment hardens and forms rock layers over time.
What process
moves the rocks
and sediment?
EROSION

5. Fossils form in Sedimentary Rocks!
Types of Rocks
Igneous Rock (formed when magma cools)
Metamorphic Rock (other rock types transformed by heat and pressure)
Sedimentary Rock (rock formed from minerals deposited near bodies of water; forms rock layers)
Fossils form in Sedimentary Rocks!

6. Why Study Fossils?
Scientists study fossils to learn what past life forms were like.
Paleontologists classify organisms in the order in which they lived.
All the information scientists have gathered is called the fossil record.

7. PETRIFIED FOSSILS
Petrified fossils form when minerals replace an organism.
Water seeps through the layers of sediment to reach the dead organism. When the water evaporates, only the hardened minerals are left behind.
PETRIFIED FOSSIL The Field Museum in Chicago displays a fossil of a Tyrannosaurus rex.

8. MOLDS AND CASTS MOLD FOSSIL
This mold, or imprint, is of an extinct mollusk called an ammonite.
A mold forms when hard parts of an organism are buried in sediment, such as sand, silt, or clay.
The hard parts completely dissolve over time, leaving behind a hollow area with the organism’s shape.
A cast forms as the result of a mold.
Water with dissolved minerals and sediment fills the mold’s empty
A cast is the opposite of its mold.
CAST FOSSIL This ammonite cast was discovered in the United Kingdom.

9. Molds and Casts
Mold
Cast

10. CARBON FILMS
All living things contain an element called carbon.
When an organism dies, eventually, only carbon remains.
The thin layer of carbon left behind can show delicate parts, like leaves on a plant.
FERN FOSSIL This carbon-film fossil of a fern is more than 300 million years old.

11. FANCY FOOTWORK This dinosaur footprint was found in Namibia, Africa.
TRACE FOSSILS
Trace fossils show the activities of organisms.
An animal makes a footprint when it steps in sand or mud.
Over time the footprint is buried in layers of sediment. Then, the sediment becomes solid rock.
FANCY FOOTWORK This dinosaur footprint was found in Namibia, Africa.

12. PRESERVED REMAINS
Some organisms get preserved in or close to their original states. Here are some ways that can happen.
Tar An organism, such as a mammoth, is trapped in a tar pit and dies. The tar soaks into its bones and stops the bones from decaying.
Ice An organism, such as a woolly mammoth, dies in a very cold region. Its body is frozen in ice, which preserves the organism—even its hair!
Amber An organism, such as an insect, is trapped in a tree’s sticky resin and dies. More resin covers it, sealing the insect inside. It hardens into amber.

13. Index Fossils
Index Fossils - Fossils that are widely distributed for a short period of time in the world and are used to tell the relative age of rocks.

14. Index Fossils

15. Relative Age vs. Absolute Age
The age based on compared position relative to other rocks – applies to both sedimentary and igneous rocks
Absolute Age
The number of years since the rock has formed based on radioactive decay of atoms in minerals in Igneous rock and some metamorphic rock

16. Law of Superposition Which layer is the oldest?
Which layer is the youngest?

17. Law of Superposition
In horizontal layers of sedimentary rocks, the oldest layer is at the bottom.
Each layer above
is younger than
the layers below it.

18. Intrusions and Faults
Sometimes an igneous intrusion or fault will cut across or into the rock layers.
The intrusion or fault will always be younger than the rocks being intruded or displaced.

19. Intrusion Fault

20. Unconformity
Unconformity – Erosion causes new rock layers to meet older rock layers. This leads to a “gap in the geologic record” (missing rocks layers and fossils).