Clues to Earth Past

Fossil
are the remains, imprints, or traces of prehistoric organism.

Permineralized remains
are fossils in wich the spaces inside are filled with minerals from groundwater.

Carbon film
A thin film of carbon residue is left forming a silhouette of the ogirginal organism. 

Mold
a cavity in the rock.

Cast
a copy of the original object.

Index fossils
are the remains of species that existed on Earth for relativly short perios of time. 



Relative age
is the age of comparrison to other things

 Unconformity
 gasp in rock sequences.  

Absolute age   
is the age in years of a rock or other object.

Radioctive decay
the process of breaking down.

Half life
of an isotope is the time it takes for half atoms to decay

Radiometric dating
is the total of calculating the ratio of the absolute age of the rock.

Uniformitarianism
 scottish scientist James Houston used it