1. Fossils
Fossil-remains, imprints or traces of prehistoric organisms. Only the hard parts of a plant or animal become fossils.
Types of Preservation:
--Mineral replacement: permineralized remains are fossils in which the spaces originally containing live cells are filled with minerals from groundwater.
--Carbon films: a carbon silhouette of the original organism.
--Coal: large amounts of plant matter accumulate and eventually become coal. Known more for fuel source than for preserving organisms.

2. Fossils
--Molds and Cast: molds are created when mud covers seashells or other hard parts of organisms. Through compaction, cementation and mineral replacement the sediment particles turn to rock. A cast is created when particles enter the mold and form a replica of the original organism.
--original remains: sometimes the soft tissue of an organism is preserved in amber (sap from a tree), frozen ground, or tar pits (La Brea tar pits CA)

3. Fossils
--Trails and burrows: made by worms and other small animals, can help discover how the animals lived.
Index Fossils—remains of species that existed for relatively short periods of time. Makes rock age estimation easier.
Fossils and ancient environments—scientist can use fossils to determine what the environments was like during that time.

4. Relative Age
Principle of superposition- in undisturbed layers of rocks the oldest rocks are on the
bottom and the rocks become progressively younger toward the top.
Relative age- the age of something (fossils) compared with other things by looking at the layers of rocks.

5. Relative Age
Unconformities-gap in the rock layer that is due to erosion or periods without any deposition.
Angular unconformities-horizontal layers of sedimentary rock often are titled and uplifted.
Disconformity—horizontal rock layers are exposed and eroded, and new horizontal layers of rock are deposited on the eroded surface
Nonconformity-horizontal layers of sedimentary rock overlie older igneous or metamorphic rocks.

6. Absolute Age
Absolute age—age, in years, of rock can be determined by using properties of the atoms that make it up.
Radioactive decay—process in which some isotopes break down into other isotopes and particles.
Half-life—time it takes for half the atoms of an isotope to decay
Radiometric dating—process to calculate the absolute age of a rock by measure the ratio of the isotopes and half-life.
Uniformitarianism—principle stating that Earth processes today are similar to those that occurred in the past.