Candy-bar analogy: an inquiry approach to introducing relative age and the four principles of stratigraphy

The concept of relative age is established in introductory geology courses and is based on the understanding of four, basic stratigraphic principles: original horizontality, superposition, cross-cutting relationships and lateral continuity. These concepts, collectively, are fundamental to a continued, productive understanding of geology. The overall conceptual image of these four principles, however, is lacking the third dimension in introductory geology textbooks. A simple, inquiry-based, team exercise can be used to facilitate student visualization and comprehension of these principles. The candy-bar analogy exercise uses a selection of candy bars (e.g., Mars®, Snickers®, Butterfinger®, Mounds®, etc.) and a sampling tool (e.g., knife/straw). While the candy bar remains wrapped, students develop working hypotheses and sketch the candy-bar "stratigraphy" based on their pre-conceived notions. Subsequent coaching, via the inquiry method, allows development and execution of a sampling strategy that will reveal the actual "stratigraphy" for hypotheses testing. Concurrently, students can resolve data-collection errors (e.g., bisection orientation or core-extraction techniques) and resample the candy-bar for relative-age analysis. Each stratigraphic principle is easily viewed and discussed with significant variations depending on the type of candy bar. Through guidance, focus and encouragement, students may discover smaller scale sedimentological features and the spatial variability of the geological record. Students self-assess their progress by carefully sketching the sampled candy-bar “stratigraphy” and completing a side-by-side evaluation of their two sketches. Learning outcomes are an increased understanding and appreciation for relative ages and stratigraphic principles. More importantly, however, students better grasp working hypotheses as guides to critical-thinking, decision-making and problem-solving skills.