When an organism dies, sediment can quickly cover it, preserving it from scavengers and physical destruction(sediment buries a dead organism). Over time, the sediment, plus pressure will fossilize the remains in a sedimentary rock. Organisms have a better chance at being fossilized if they have hard parts like bones, shells, or teeth because scavengers are less likely to eat these parts and they decay slower than soft parts.

Types of Preservation

1. Mineral Replacement – Upon death most hard parts of organisms, like teeth, bones, and shells, have tiny spaces in them from decay. Groundwater can fill these spaces and deposit minerals. Permineralized remains are fossils that have the spaces inside them filled with mineral deposits from groundwater. DNA can sometimes be recovered here. Minerals can also replace the hard parts of the fossil organisms. If a mineral inside of water dissolves a shell, it will leave the mineral in its place.

2. Carbon Films – organisms’ tissues are made of compounds that contain carbon. When sediment covers a dead organism and the result of the added pressure and heat will force gas and liquids from the organism leaving behind a thin film of carbon residue that forms a silhouette of the organism called a carbon film.

3. Coal – in swampy areas, there are large amounts of plant matter. Over millions of years this accumulation of matter carbonizes and becomes coal. We use coal for fuel. Coal is not a good source to learn about the past because the structure of the original organism is not usually found.

4. Molds & Casts – impressions are formed when hard parts of organisms fall into soft sediment and then buried by more sediment. Compaction and cementation turn the sediment into a rock (sedimentary). Open pores in the rock allow water and air into to rock and this can decay the hard part or the organism leaving behind a cavity in the rock called a mold. As time passes, more water and sediment can enter the cavity and form new rock which is a copy of the original organism. This is called a cast. It is not the original.

5. Sometimes soft parts are fossilized in tree sap, like amber. The amber protects the original material so the organism is completely preserved. (An insect in amber – Jurassic Park).

6. Trace Fossils – fossilized tracks and other evidence of the organisms’ existence. Tracks or footprints, can tell you an enormous amount about how an animal lived (how big the animal was, how fast they were walking or running at the time). Other trace fossils like trails and burrows, made by worms and other animals speak to how animals lived in the past as well.

Index Fossils – are the remains of a species that existed on Earth for a relatively short period of time. They help scientist estimate the age of rock layers containing index fossils.

Using fossils, scientists can also tell the type of climate an area once had. For example, if a tropical plant fossil was found in the US then we know that at one point, the United States must have been closer to the equator (continental drift). Also, we can tell if an area was once covered by shallow waters (maybe when the glaciers melted) because of fossils of plants and animals that only lived in shallow waters like the crinoids.

Principle of superposition basically states that when dealing with sedimentary rocks the oldest rocks are on the bottom. The bottom layer would be the first layer to form is on the bottom.

Relative Age – age in comparison to the age of other things around it. This does not tell you the actual age of the rock but it tells you which rocks are younger or older compared to what’s around it. If the rocks have been faulted or turned upside down, you may have to use other clues to help determine the relative age of the rocks. Index fossils can help determine the relative age of rocks. If a fossil is found in a rock layer that has a known life span in the Cambrian Periods, then we know that rock layer is about 488 million years old.

Matching Up Rock Layers

Rocks from different areas, thousands of miles apart to continents apart, may be apart of the same rock deposit. You can tell by looking at the fossils. If the same type of fossils were found in both rock layers it is likely that the rocks are a part of the same layer.

Unconformities

Rocks can tell a “story” of events that happened in the past. Gaps in rock sequences are known as unconformities. Unconformities develop when weathering and erosion remove rock layers. When rock is eroded and new sediment is deposited on top of the eroded surface, you have a gap in history. There are 3 main types of unconformities:

• Angular Unconformities – when horizontal layers are folded and uplifted due to plate movement. Younger sedimentary rock is deposited in top of the eroded surface.
• Disconformity – layers of rock are missing due to erosion. This leaves a gap in the rock record.
• Nonconformity – Metamorphic or igneous rock are eroded. Younger sedimentary rock is deposited on top of the eroded surface.

Igneous Intrusion / Dike

An igneous intrusion, also referred to as a dike, is when magma burns through rock layers and cools. Many rocks layers have igneous veins running through them.