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the remains, imprints, or traces of prehistoric organisms


where there is gap in the rock layers

Principle of uniformitarianism

Earth processes occurring today are similar to those that occurred in the past

relative age

the age of something in comparison to other things

Law of Superposition

in undisturbed rock layers, the oldest rocks are on the bottom and the youngest rocks are on the top

index fossil

a type of fossil that indicates the geologic age of the rocks in which it is found


the amount oftime it takes for half of the atoms in an isotope to decay

absolute age

the age, in years, of a rock or other object, such as a fossil

Age of Mammals

The Cenozoic Era is know as:

Age of Invertebrates

The Paleozoic Era is know as:

Age of Reptiles

The mesozoic Era is know as:

Recent Life

Cenozoic means:

Middle Life

Mesozoic means:


The plants characteristic of a geological period.


the animals characteristic of a geological period.


The seas covered the large areas of the continents two times during this era.

Mold fossil

a space in a rock that has the shape of the remains of a living thing that once occupied that space.

Cast fossil

a model in the shape of a living thing or its remains, it forms when minerals or rock particles fill the space

Trace fossil

includes the footprints, tracks, trails, and burrows made by living things.


Atoms of the same element with different atomic masses

Carbon dating

is a method using radioisotope carbon-14 to determine the age of objects such as fossils or rocks


the process in which organic material is changed into inorganic material such as a fossil

geologic time scale

the division of Earth history into blocks of time--eons, eras, periods, and epochs. The time scale was created using relative dating principles.


a geologic time unit; eons are divided into these, and inturn are divide into periods

Precambrian time

Pre-Cambrian time covers from 4.6 billion to 544 million years ago. During this time life developed as bacteria and softbodied, multicelled organisms. First mass extinction probably occurred near the end of the Precambrian

Cenozoic era

the latest of the four eras into which geologic time is subdivided ; 65 million years ago to the present

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