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The fossils represented by the letters on this card are "younger" than the "T" or "C" fossils on the "TC" card which represents fossils in the oldest rock layer.

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Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers.

Specific rock formations are indicative of a particular type of environment existing when the rock was being formed.

Time factors of millions and billions of years is difficult even for adults to comprehend.

However, "relative" dating or time can be an easy concept for students to learn.

INTRODUCTION Scientists have good evidence that the earth is very old, approximately four and one-half billion years old.

Scientific measurements such as radiometric dating use the natural radioactivity of certain elements found in rocks to help determine their age.

By matching partial sequences, the truly oldest layers with fossils can be worked out.

By correlating fossils from various parts of the world, scientists are able to give relative ages to particular strata. Relative dating tells scientists if a rock layer is "older" or "younger" than another.

Locally, physical characteristics of rocks can be compared and correlated.

On a larger scale, even between continents, fossil evidence can help in correlating rock layers.

*Earth and Space Science: Fossils provide important evidence of how life and environmental conditions have changed.