Law of Superposition


Abel Olivencia

Irwin Altman Middle School, Queens

Summer Research Program for Science Teachers

August 2007


Subject: Earth Science


Grade Level: 8th and 9th


Unit: Earth and Space, Geology, Dating techniques


Objective: Students will be able to:



Prior Knowledge: Students should have an understanding of the geologic effects of Earthquakes on Earth’s strata, “The Geologic History of New York State” table in the “Earth Science Reference Tables”, electrophoresis, function and structure of the DNA molecule.



Introduction: The geological events in Earth's history are a series of processes that have changed the structure of Earth’s crust. Geologists use scales to date these geological events and to measure the age of the Earth. One scale records relative time and helps to describe the geologic record. Relative time places events in a sequence but does not tell us the actual date of occurrence. It indicates age in comparison with other events. The relative time scale is based on the sequence of layering of the rocks and the evolution of life. Being able to determine when a particular organism existed, for example by comparing samples of DNA through electrophoresis, within particular geologic strata,  layers of rock can be correlated to reveal the sequence of changes the Earth’s crust experienced. The correctly ordered layers of strata often reveal an untold story of our planet’s hidden history and perhaps a look at its future. This lesson serves to allow for a preliminary integration of current laboratory assays and Earth Science content. If all materials necessary to have students actually run their own gels is available, this lesson may effectively be modified to accommodate such inquiry.




Copies of the Earth Science Reference Tables (Pages 8 and 9)

Worksheets entitled “DNA Match up”, “Build It Up”, and “The Order Mat” (PDF)


Transparent tape




Overhead projector

Internet connected laptop


Time: 45 minutes




Show students a 5 second clip of a geologic disturbance located at, be sure to increase screen size. Explore with students what they saw. Explore with students how the rock layer that fell into the Earth could be arranged when the Earth closes up and other layers of rock form over it.


1. Cooperatively grouped students should be given the handout “Build It Up”. Student responses about the type of information on the handout should be elicited, followed by an explanation.


2. Student’s should be asked to cut out each rock layer, and attempt to arrange them into chronological order, on the worksheet entitled “The Order Mat”, based on the information on the worksheet “Build It Up”. Students should be asked what difficulties in doing a correct horizontal correlation exist. Explore what information would be helpful in putting the rock layers in the correct horizontal order, from youngest to oldest.


3. Students should then be given the worksheet “DNA Match Up” and pages 8 and 9 from the Earth Science Reference Tables. Elicited student responses to the information on the handout should be followed by an explanation. They should then be asked to find the names of the pictured organisms and their approximate age based on the information in their “Earth Science Reference Tables” on “The Geologic History of New York State”. Next they should determine which creatures’ DNA is most closely related to the known organisms’ DNA by examining the analytical gels. They should then be asked to use the results from the analytical gels to determine the age of their creatures as they relate to the known organisms’ DNA. As you facilitate group inquiry you should ensure the understanding that the DNA matches, between the unknown creatures’ DNA samples and the known organisms’ DNA samples, indicate that they most likely existed about the same time, and therefore within the same rock layer. Finally they should apply what they have gathered in order to correctly arrange the rock layers on their worksheet entitled “The Order Mat”.


4. A summary of students’ findings should be explored, and steps they took to find the correct order of the rock layers, placed on the chalkboard in a flow chart.

Related to their experiences with the inquiry based activity, a definition for relative dating and the Law of Superposition should be arrived at, and the official definition of horizontal stratigraphy, or the Law of Superposition, compared. The important use of biotechnology in Earth Science should be elicited.


Additionally, students may be given, as a follow-up assignment, a web adventure that supports the methods of relative dating techniques. Students may be asked to hand in a summary of their experiences at the following link:



Science Standards:


Standard 2

All students will understand and apply scientific concepts, principles, and theories pertaining to the physical setting


Standard 5.1

All students will understand the interrelationships between science and technology and develop a conceptual understanding of nature and process of technology.


Standard 5.4

All students will develop problem-solving, decision making, and inquiry skills reflected by formulating usable questions and hypotheses, planning experiments, conducting systematic observations, interpreting and analyzing data, drawing conclusions and communicating results.