1. Determine fold axis trend/plunge (β-axis) and axial plane orientation (π-pole) from strike and dip data using beta and pi stereonet diagrams, and explain their geometric significance in plunging antiformal folds.
2. Construct a 3D model of an undeformed stratigraphy and apply fold parameters (e.g., axis orientation, amplitude, plunge) to simulate deformation, verifying stereonet results through visual matching of outcrop patterns and rotations.
3. Analyze the effects of variations in fold parameters (e.g., plunge angle) on 3D geometry and map-view traces, interpreting implications for tectonic processes and field mapping challenges.

This lab or problem set is designed for undergraduate students in a Structural Geology course. They should understand how to describe the geometry of folds using terms like fold axis and axial plane, and they should be familiar with measurement of the orientation of planar and linear features with strike and dip and trend and plunge measurements, respectively.

This is a hybrid laboratory and workshop activity designed to teach students the quantitative analysis of plunging fold geometry. The estimated time for completion is approximately 90–120 minutes. The activity is scaffolded in three progressive steps, moving from two-dimensional graphical analysis to three-dimensional physical modeling and verification.

Activity Mechanics

Stereonet Analysis: Students use a provided set of real-world Strike and Dip (S/D) data from a plunging antiformal fold (near Dillon, MT) to perform two separate stereonet analyses:

The Beta Diagram is used to find the fold axis trend and plunge by plotting the great circles of the bedding planes and finding their common intersection point.

The Pi Diagram is used to find the axial plane orientation by plotting the poles to the bedding planes and fitting a great circle to them. The pole to this girdle is the pi-axis (normal to the axial plane).

Undeformed Model Build: Students use the Visible Geology Geology Explorer module to create a simple, undeformed (horizontal) stratigraphic column of 5-6 layers.

Reverse Engineering in 3D: Students use the calculated trend/plunge of the beta-axis from Step 1 to set the parameters (axis orientation, plunge, amplitude, wavelength) for a "Fold" event in the 3D model. They then view the resulting 3D structure and its map-view trace to visually verify their stereonet calculations against the known geometry of plunging folds and the provided satellite imagery. Finally, they "measure" the virtual limbs of the 3D fold and create a new stereonet to verify that the model reproduces the original S/D data.

Materials and Software

Activity Document/Lab Instructions: Contains the activity steps, learning goals, pre-activity reading, and the Sample Strike and Dip (S/D) Data used for the stereonet analysis.

Primary Software: Visible Geology, a free, web-based application with 3D geological modeling and stereonet modules

PSGT Textbook (Chapter 8): Required pre-activity reading on the "Anatomy of a Fold" and "Fold Classification" to provide the necessary geoscience context. Van der Pluijm, B. and Marshak, S. (2020). Processes in Structural Geology & Tectonics.

Alternative Software:

The core skill of stereonet analysis (Step 1) can be performed manually using tracing paper and a stereonet net (Wulff or Schmidt) as an alternative to the Visible Geology Stereonet module. Other digital stereonet programs may also be used. The 3D modeling component (Steps 2 & 3) is specific to the free Visible Geology web-based application, as it is uniquely designed for educational structural geology modeling.

Visible Geology teaching activity_analyzing_plunging_folds_using_stereonets.pdf (Acrobat (PDF) 651kB May15 26) - PDF Format

Visible Geology teaching activity_analyzing_plunging_folds_using_stereonets.docx (Microsoft Word 2007 (.docx) 7MB May15 26) - Word Format

• This activity is designed as a hybrid laboratory/workshop session, requiring approximately 90–120 minutes to complete. The steps are scaffolded to guide students from the 2D graphical analysis of stereonets to the visual, 3D validation of their results. This structure is intended to reinforce the spatial reasoning necessary for field methods by bridging the abstract nature of stereonet plots with the physical geometry of folds. The activity can be effectively implemented for individual work or small groups, with the goal of fostering discussion and collaborative problem-solving, particularly during the interpretation of fold parameters and the comparison of the 3D model to the original data.
• To save and share their work in Visible Geology, students are encouraged to log in with their Google Account.

Assessment is directly tied to the three Learning Goals outlined in the document's Assessment Criteria table.

The primary materials for this activity are the Visible Geology web-based application and the assigned reading.

• Visible Geology
• PSGT Textbook
• For tips, tricks and how-to videos for using Visible Geology, check out the Visible Geology Help & Resources.