Rainbow Basin, CA mapping project

Joan E. Fryxell
California State University, San Bernardino
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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.

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Introductory geological mapping course. Required of all BA and BS majors.

Skills and concepts that students must have mastered

Using a brunton compass for attitudes, locating oneself on a topographic map, consistently recognizing units.

How the activity is situated in the course

The 2nd of 3 mapping exercises. Three days of field work, camping nearby. Followed by a brief report describing the units, and drawing a cross section across the fold. The students return to Rainbow Basin in the Structural Geology course to collect fracture data for analysis.


Content/concepts goals for this activity

3D visualization; recognizeably map the structures and units; understand what they've seen, so they can draw a plausible cross section.

Higher order thinking skills goals for this activity

Integrate individual attitude measurements into a coherent version of the overall structure.

Other skills goals for this activity

Planning field traverses to cover the area in the allotted time; basic mapping skills; practice describing rock units; drafting skills, camping logistics; working with field partner.

Description of the activity/assignment

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.

Determining whether students have met the goals

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.

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