Earth’s History in Stone

I. Fossils

A. evidence of organisms that lived long ago.

B. Fossils are found in layers of sedimentary rock. The

organism is buried in mud, sand, or clay which hardens

with time into rock.

II. Types of Fossils

A. Trace Fossils

Marks left by an organism

Ex. Footprints, snake trail…….

B. Mold Fossils

An organism is buried in sediment and decays to form an empty space

Ex. Mold of snails, clams………

C. Cast Fossils

The mold, once hardened, fills with minerals that form a replica of the original organism

Ex. Sea star, snail…….

D. Imprint Fossils

A thin organism, such as a leaf, leaves behind a Carbon imprint in the rock as it decays

Ex. Fern, leaf, pine needle…….

E. Petrified Fossils

The tissue is slowly replaced by minerals

Ex. Petrified wood…….

F. Preserved Materials Fossils

The original material, usually hard tissues, is still contained in the fossil.

Ex. Bones, teeth, insects in amber…….

III. What do Fossils Tell US?

Size of the organism

Behaviors of the organisms

Climate in the area

Age of the material

IV. Dating Fossils

A. Relative Dating –- Older layers are on the bottom/younger

layers on the top

- uses index fossils of known ages

B. Radiometric Dating –

-Uses radiometric isotopes and half lives

to determine age

A few Radioactive Isotopes and their Half-Lives