Poleta Bedrock Geologic Mapping

Sarah R. Hall, College of the Atlantic

Calla Schmidt, University of San Francisco

Becca Walker, Mt. San Antonio College

Summary

Students produce a bedrock geologic map of the Little Poleta field area from which they write a short interpretation of the paleoenvironment and geologic history of the region.

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Learning Goals

Content/Concept Goals:

Students will...

  • Identify and describe sedimentary rock units to establish the major lithologies in the field area.
  • Take strike and dip measurements of geologic outcrops to determine the orientation of and stratigraphic relationships between the sedimentary rock units in the field area.
  • Illustrate the spatial relationships between rock units in the field area by creating a geologic map.

Higher Order Thinking Skills Goals:

Students will...

  • Use geologic (lithologies and sedimentary structures) and paleontologic (trace fossil) data to interpret the depositional environment of the mapping area.
  • Use geologic data (strike and dip measurements, spatial relationships between rock units) to interpret geologic structures in the field area.

Other Skills Goals for this Activity:

Students will...

  • Use a Brunton compass to measure the strike and dip of rocks in the mapping area.
  • Locate oneself on a topographic map.
  • Identify the locations of contacts between lithologic units, and mark contacts' inferred locations on a topographic map.
  • Distinguish between 5 sedimentary rock units in the mapping area.
  • Create a geologic map.

Context for Use

Audience:

This activity was completed as part of a 2-part field activity in a 2-week summer E-STEM Field Course with ~20 undergraduate students interested in environmental science. This activity could be modified to be more advanced. However, the area that was selected for this activity makes for an intermediate-level exercise, plus, in this implementation, the activity was heavily scaffolded.

See the other part: the Poleta: Stratigraphy activity.

Prerequisite Skills and Concepts:

Students completed the following work during the field course prior to visiting Little Poleta:

  • Students had some familiarity (from a pre-field activity) with geologic maps--namely, what a geologic map looks like, use of different colors to represent different stratigraphic units, how to use an accompanying stratigraphic column to interpret stratigraphic relationships between units, symbols for geologic contacts and geologic structures, and strike and dip symbols.
  • Practice with reading a Brunton compass, using a Brunton compass to measure the attitude of bedding, and how to record these data on a base map.
  • Description and identification of the rocks that they would see in the mapping area (units 5, 6, 7, 8 and 9.)

Some students had experience with geologic maps, Brunton compasses, and identifying sedimentary rocks prior to the field course, but the prerequisite skills and concepts above were done as a pre-field day exercise at the Sierra Nevada Aquatic Research Lab (SNARL) facility classroom. During this exercise, students rotated in small groups between 4 stations:

  • station 1: students learned to assemble a Jacob staff and practiced using it on the bocce ball court at SNARL. A bocce ball was placed at one end of the court, students were given a hypothetical dip angle, and with a partner, they determined how many Jacob staffs they measured between their start point and the bocce ball.
  • station 2: students answered some basic questions about the Geologic Map of Mammoth Mountain and its Mafic Periphery.
  • station 3: students described and identified the 5 sedimentary rock units that they would see in the mapping area.
  • station 4: students completed a short written exercise as an introduction to reading Brunton compasses and took some practice strike and dip measurements around the classroom using some hypothetical outcrops (several lids of plastic tubs had been taped to the wall in various places in the classroom and tilted at various angles to simulate beds. Students moved from lid to lid with their Bruntons, measuring and recording strike and dip of each "bed".)

How the Activity is Situated in the Course:

View the E-STEM field course timeline for more information about how this activity is situated in the course. The mapping activity should be done after students measure their Little Poleta stratigraphic section. In the ESTEM course, we arrived at Little Poleta not long after sunrise, spent the morning completing the Little Poleta stratigraphy exercise, and late morning through early afternoon on this Little Poleta mapping exercise. This timeline allows for an extremely small mapping area, so instructors could also consider a full day for the Little Poleta mapping exercise.

Description and Teaching Materials

Student Handout

Poleta_GeologicMap_HandOut.docx (Microsoft Word 2007 (.docx) 17kB Mar26 20)

Materials:

  • Pencil/eraser
  • Colored pencils
  • Rock hammer
  • Hand lens
  • Suggested: mapboard or clipboard
  • Rulers
  • Acid
  • Brunton compass
  • Base map PoletaTopo.pdf (Acrobat (PDF) 784kB Jun15 20)
  • Tracing paper or vellum paper and tape

Teaching Notes and Tips

Some tips to share with students prior to going out in the field for this exercise:

  • A general discussion of sampling ethics and fieldwork codes of conduct.
  • Topography and distance: expect to walk several miles over the course of the day over uneven ground.
  • Comfortable, broken-in boots with good traction and ankle support are extremely important, as is having enough space in your backpack to carry your gear and keep your hands free.
  • Sharp plants and carbonates are all over the field area--suggest long pants, gaiters(?), and gloves.
  • Watch out for rattlesnakes.
  • No bathrooms available.
  • Zero shade in the field area. Copious amounts of water, sunscreen, sunglasses, hat, bandana, etc. are essential.
  • To avoid losing items like Bruntons, hand lenses, grain cards, etc., please wear them around your neck or hook them to your belt.

Other teaching tips for this mapping area:

  • Bring easy-ups and extra water. The heat and sun can be brutal in this field area. The easy-ups can be used for shade over the course of the day for students who need a break, although they will need to climb the hillside again to get back into their mapping area.
  • The area that students can realistically cover for this exercise depends on whether this is being done as a half-day or full day exercise. For the half-day option, faculty who are most interested in positioning of contacts should start students on the hillside where they measured the stratigraphic section because the stratigraphic relationships are straightforward, and all 5 mapping units crop out on the hillside. Once students reach the top of the ridge, the mapping area becomes much more structurally complex, making it a suitable (but time-intensive) area for students to consider repeating units, missing units, and variations in attitude in a small area. Regardless, faculty should set a time limit and establish map boundaries with students at the beginning of the field day to keep students together and on task.
  • Pick a few important locations in the mapping area for all students to consider to help with their mapping. The structural complexity of this mapping area can quickly become overwhelming, so making sure to help the students manage their time and map at a reasonable resolution is important. Be sure the students can identify the boundaries of the mapping area prior to beginning their mapping.
  • We suggest photographing students' field maps at the end of the mapping day in order to compare their field data to their office map and in the event that a student loses their field map.

Assessment

Assess the geologic map with the rubric for the Mapping Badge.