Earth’s  History  

Over  time  sedimentary  rock  has  formed  and  has  embedded  plant  and  animal  remains  

and  as  a  result,  scientists  have  a  record  of  life  on  earth.    Over  a  long  period  of  time,  sediment  

accumulates  in  horizontal  layers.    The  principle  of  superposition  states  that  the  oldest  rocks  are  

found  in  the  bottom  layers  within  the  earth.    As  the  layers  progress  upward,  the  age  of  the  rock  

becomes  younger.    Within  some  of  the  layers  of  sedimentary  rock,  fossils  can  be  found  of  life  

forms  from  the  earth’s  past.  

 

Fossils  are  clues  about  prehistoric  plants  and  animals  on  earth.    If  you  have  ever  visited  a  

museum  and  witnessed  a  fossilized  skeleton  of  a  dinosaur,  you  realize  how  hard  it  probably  was  

to  remove  from  the  earth  and  put  back  together.    But,  have  you  ever  considered  the  conditions  

it  takes  for  a  fossil  to  form?    Usually,  when  an  animal  or  plant  dies,  natural  conditions  lead  the  

organism  to  rot  away  or  be  eaten  by  scavengers.      

 

One  way  that  conditions  are  set  for  a  fossil  to  form  is  by  the  organism  being  covered  

very  quickly  by  a  layer  of  sediment.    Because  the  body  of  an  animal  is  covered  quickly,  it  is  

preserved,  and  over  time  minerals,  which  are  found  within  the  sediment,  replace  the  original  

organism’s  structure.    After  minerals  replace  the  original  bone,  we  now  find  a  rock,  which  is  in  

an  identical  shape.      

 

Scientists  have  used  both  the  sedimentary  rock  found  within  the  earth’s  layers  and  the  

fossils  found  in  such  layers  to  form  a  geological  time  scale.    Scientists  named  paleontologists  

have  been  able  to  find  and  record  different  types  of  fossils,  which  are  exclusive  to  specific  

layers  of  sedimentary  rock.    As  a  result  of  the  paleontologist’s  efforts,  a  scale  spanning  4.5  

billion  years  of  different  life  forms  has  been  created.  

 

1. The principle of superposition states

o A Older rocks are found in shallow layers o B Younger rocks are found in deeper layers o C Older rocks are found in deeper layers o D Younger rocks contain more fossils

2. Fossils are usually found in what type of rock?

o A igneous o B magma o C sedimentary o D metamorphic

3. After studying the layers of rock, types of rock found in each layer and radioactive carbon dating techniques, scientist have been able to develop a

o A geological time scale o B dinosaur map o C rock cycle o D Big Bang Theory

On a vacation trip to Calvert Cliffs, Tristan and his father found several megaladon shark teeth fossils. The Park supervisor explained that the fossils come from the adjacent cliffs along the beach. Describe how the fossils were preserved and how they were dislodged from the adjacent cliffs. ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________