Views of Earth’s Past

Big Idea:  Rocks, fossils, and other types of natural evidence tell Earth’s story.

Key Concept: Earth’s past is revealed in rocks and fossils.

Activity:  How Do You Know What Happened?

·     Observe and think:  Write down evidence that suggests what happened in these photos.  Is the evidence direct or indirect?

Photo 1       Photo 2       Photo 3      



4.1 Earth’s past is revealed in rocks and fossils.

I.  Fossils, rocks and original remains give clues to the Earth’s past.

      A.  Fossils- traces or remains of living things from long ago

      B.  Most are hardened animal remains

            1.  Shells, bones, teeth

            2.  Minerals replace remains, forming a fossil of the skeleton parts

      C.  Others are impressions preserved in rock

II.  Original Remains     

A.  Definition:  fossils that are the actual bodies or parts of organisms

            1.  Soft parts usually disappear

            2.  Soft parts can be preserved if airtight

      B.  Original remains give direct evidence

III.  Substances that preserve remains

      A.  Ice

            1.  One of the best preservers of prehistoric life

            2.  Siberian and Alaskan ice fields contain 10,000 year old mammoths

      B.  Amber

            1.  Forms from resin-  a sticky, syrup-like substance that trees produce

            2.  Protects trees from insects

            3.  Tree dies, gets buried, resin hardens, amber

            4.  Only preserves small insects

      C.  Tar

            1.  Thick, oily liquid

            2.  Sabre-toothed cats and other animals have been found in tar

            3.  La Brea Tar Pits, Los Angeles, CA

III.  Fossil Formation (hard parts only)

      A.  Form in sedimentary rock

      B.  Heat in igneous and metamorphic rock destroys fossils

      C.  Formation process:  Sediment builds up around dead organisms (sand, mud)

IV.  Fossils in rocks

      A.  Some fossils are not original remains

      B.  They can be impressions or traces made of rock

      C.  These provide indirect evidence

      D.  They include

            1.  Molds and Casts (illustration page 115)

                  a.  Organism dies and falls into soft sediment

                  b.  The sediment becomes rock and the organism decays, leaving a mold

                  c.  Minerals fill the mold to make a cast

            2.  Petrified Wood

                  a.  Stone fossil of a tree

b.  Forms when:  A fallen tree gets covered with sediment, water passes through and into the trees cells, minerals in the water are deposited and take the place of tree cells

            3.  Carbon Films                 

a.  Carbon- an element found in all living things

                  b.  Sometimes a dead organism leaves a layer of carbon behind as it decays

                  c.  Can show details of soft parts not seen in other fossils

            4.  Trace Fossils

                  a.  Not parts or impressions of animals

                  b.  They are evidence of an animal presence in a given location

                  c.  They include:  Preserved footprints, trails, holes, scat

d.  Scientists compare these clues with modern animals to show how prehistoric animals lived, ate, and behaved

e.  Example:  Dinosaur tracks can tell how fast they ran (by pairing trace fossils with others)

V.  Fossils and other natural evidence show changes in life and the environment

      A.  South Pole ice contains fossils of crocs, dinosaurs, and palm trees

      B.  Tree Rings

            1.  Width-  shows wet or dry years

            2.  Help to study past weather patterns

      C.  Ice Cores

            1.  Tubular sample showing layers of snow and ice built up over as much as 500,000 years

            2.  Layers are a vertical timeline

            3.  With these, scientists can analyze:

                  a.  Trapped air to analyze atmospheric conditions of the time period (temperature, climate)

                  b.  Ash layers showing when volcanic eruptions occurred