Planes and topography

Howell Bosbyshell, West Chester University of Pennsylvania

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This page first made public: Jun 1, 2012

Summary

This is an exercise in projecting planar contacts onto a topographic map. The students create a geologic map, cross section and topographic profile, and write a brief interpretation.

Structural Visualizations, Folds/Faults/Ductile Shear Zones | College Upper (15-16)

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Context

Audience

This activity is used in an upper level undergraduate required course in structural geology.

Skills and concepts that students must have mastered

Before beginning the activity students should know how to determine strike and dip from a contact on a topographic map and know how to determine apparent dip.

How the activity is situated in the course

This is a stand alone exercise coming after exercises on orthographic projection and solving three-point problems.

Goals

Content/concepts goals for this activity

The goal of the exercise is to have students interpret a geologic map; to increase their visualization skills by projecting planes and thinking about the appearance of horizontal, vertical, and dipping contacts on a topographic map.

Higher order thinking skills goals for this activity

The activity provides an opportunity for synthesis of a range of ideas from cross-cutting relationships through apparent dip and to think about interpretation of (hypothetical) field data.

Other skills goals for this activity

The exercise can be tedious, so it teaches patience and careful drafting. I would like to use the exercise to teach the use of Abode Illustrator or CorelDraw, but we do not presently have a site license for either product.

Description and Teaching Materials

The Word document planes_and _topography.doc details the students' assignment and includes a blank topographic map. The pdf files contain a final map, cross-section and solution for determining the orientation and thickness of the diabase dike. The PowerPoint file contains instructions for projecting planes onto topography.

Student handout for Planes and topography (Microsoft Word 2007 (.docx) 992kB May15 12)
Finished map (Acrobat (PDF) 3MB May15 12)
Final cross-section (Acrobat (PDF) 262kB May15 12)

Teaching Notes and Tips

I described a simple structure contour map of a dipping plane earlier in the semester, in the context of true vs. apparent dip. I revisit this map in the PowerPoint file projecting_planes.pptx in reference to determining the spacing of structural contours. The PowerPoint file also includes detailed instruction in creating evenly spaced lines using PowerPoint. (A student subsequently pointed out to me that this can also be accomplished by setting the row height in a Word table.) Students are instructed in the use of a light table to trace the geologic contacts by marking the intersection of structural contours and topographic contours of the same elevation. Contacts can be draw using index contours and the "rule of vee's", but it may be helpful to use 20' contours in some places.
To create the geologic map students must determine the orientation of a dike based on partially draw contacts and project those contacts across the map. In the process, they should recognize that the dike should pass through location 1, where the absence of diabase is noted. Therefore, the dike must end before it reaches that location and students have some leeway it creating a map pattern that reflects this. It can also be pointed out that the older contacts do not follow topography beneath the younger horizontal beds. Thus, the exercise is not quite as tedious it might appear initially.
The exercise is time consuming and is probably a two-week assignment.