Relative Age-dating -- Discovery of Important Stratigraphic Principles

Roger Steinberg
Del Mar College
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Students don't have to be passively taught the important principles geologists use to do relative age-dating of rocks and geologic events. By careful analysis and critical thinking about photos and illustrations of rock outcrops, they can discover these principles themselves, and present their discoveries to the class!

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Introductory Historical Geology course (typically includes a few majors, but primarily non-majors).

Skills and concepts that students must have mastered

Assumes practically no prior skills or knowledge. Students will have completed a 3-hour laboratory introducing (reviewing) minerals and rocks prior to this classroom exercise, and will have a handout of common lithologic symbols used for geologic maps and cross sections.

How the activity is situated in the course

Used very early in the Semester, during the development of the principles and concepts used by geologists to determine relative age relationships of rocks and geologic events.


Content/concepts goals for this activity

Students discover principles, from real and simulated examples, used for relative age-dating of rocks and geologic events.
Students create a descriptive name for the principle.
Students describe how their principle can be used as a general relative age-dating principle to the class.

Higher order thinking skills goals for this activity

1.) Knowledge--Identify rock types and other symbols.
2.) Knowledge--Observation and recall of information.
3.) Comprehension--Translate knowledge into new context.
4.) Application--Use concepts in new situations.
5.) Analysis--Pattern recognition.
6.) Synthesis--Generalize from given facts and observations.
7.) Synthesis--Creation of general principles.
8.) Evaluation--Assess value of principles.

Other skills goals for this activity

Creativity, use of language.
Working in groups.
Oral presentations (for some).

Description of the activity/assignment

When piecing together the geologic history of the Earth, geologists rely on several key relative age-dating principles that allow us to determine the relative ages of rocks and the timing of significant geologic events. In a typical Historical Geology class or textbook, instructors/authors briefly discuss the important early researchers in the geological sciences, and then give the name of the stratigraphic principle, useful for relative age-dating of rocks and events, that these 17th and 18th century scientists are credited with discovering. After the instructor/author defines these principles, students are usually shown several examples so they can see how the principle can be applied.

But why not start with the examples and let students discover these principles for themselves?

Students are split into small groups which each work to discover a different relative age-dating principle. The groups are shown photos and given handouts with drawings of rock outcrops illustrating the various principles. These handouts include worksheets for which they must answer a series of prompts that help lead them to the discovery of their relative age-dating principle. Groups must also invent a name for their principle, and select a spokesperson who will present the group's results to the rest of the class.

Determining whether students have met the goals

No objective evaluation, however, these questions should be addressed:
  1. Did each group discover a principle, and invent a name for it?
  2. Was each group able to describe the use of their principle?
  3. Did any classroom discussion ensue in agreement or disagreement with efforts of particular groups?

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