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Rainbow Basin, CA Mapping Project

Joan E. Fryxell, California State University, San Bernardino

Intended Audience: This is an introductory geological mapping course that is required of all BA and BS majors.


Rainbow Basin, which is just north of Barstow, CA.

This is a 3-day mapping project during which students camp nearby.
Students return to Rainbow Basin during their structural geology course to collect fracture data, so this may be incorporated in this project depending on student skill level.


Basic field mapping of a faulted syncline, with a couple of alluvial unconformities. Exposures are excellent, and the fold is of a scale that is easily seen from many vantage points. This exercise forms the basis for follow-on projects.


Rainbow Basin is just north of Barstow, CA, and is an excellent badlands-style exposure of the Barstow syncline in the Miocene Barstow Formation. In the first mapping class, their assignment is to map the basic geology of the basin, using a couple of distinctive marker beds within the Barstow Formation. Several faults postdate the fold, and three different ages of alluvial deposits occur within the basin. We camp in the nearby Owl Canyon campground, and spend three days in the field. More time could be spent with an introductory class, but three suffices to get most of the basin on the map. They are charged with writing up descriptions of the rock units they encounter. From the field map, students transfer information to an office copy, add a map explanation, and draw a cross-section through the map area. Mapping is done on a topographic map, specifically developed for the basin, with 10-foot contour intervals. This is a proprietary map, so permission is needed for its use.


This field exercise should improve student skills in 3D visualization, recognizeably mapping the structures and units, and understanding what they've seen so they can draw a plausible cross section. Students also integrate individual attitude measurements into a coherent version of the overall structure. Finally, students must plan field traverses to cover the area in the allotted time, learn basic mapping skills, practice describing rock units, hone their drafting skills, and work with field partner and camp in a group setting.

Assessment and Evaluation:

The field map, office map, and cross section are evaluated for geologic plausibility (e.g., does the modern alluvium stay in the streambeds, or does it wander around the hillsides?) and internal congruence (e.g., field map vs. office map, or map vs. cross section). In addition, drafting quality is evaluated. I have developed a grading rubric that is handed back with points earned as well as points possible, and comments, to help students focus on specific ideas or techniques that they did well, or that need improvement.

Materials and Handouts:

Activity Description/Handout (Acrobat (PDF) 75kB Nov19 04)

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