Relative dating lesson
The Law of Superposition, which states that in an undisturbed horizontal sequence of rocks, the oldest rock layers will be on the bottom, with successively younger rocks on top of these, helps geologists correlate rock layers around the world.
This also means that fossils found in the lowest levels in a sequence of layered rocks represent the oldest record of life there.
For example, most limestones represent marine environments, whereas, sandstones with ripple marks might indicate a shoreline habitat or a riverbed.
Return to top The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in the early 19th century to propose that the rock layers could be correlated from place to place.
If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata.
By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale.
However, "relative" dating or time can be an easy concept for students to learn.
This would also mean that fossils found in the deepest layer of rocks in an area would represent the oldest forms of life in that particular rock formation.
In reading earth history, these layers would be "read" from bottom to top or oldest to most recent.
In this activity, students begin a sequencing activity with familiar items letters written on cards.
Once they are able to manipulate the cards into the correct sequence, they are asked to do a similar sequencing activity using fossil pictures printed on "rock layer" cards.