Poleta Stratigraphy

Sarah R. Hall, College of the Atlantic

Calla Schmidt, University of San Francisco

Becca Walker, Mt. San Antonio College

Summary

Based on field observations at Little Poleta, students identify and describe sedimentary rock units and measure the thicknesses of 5 different stratigraphic units. They then establish the field site's major lithologies and illustrate spatial relationships between rock units.

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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.
  • Measure the thicknesses of different stratigraphic units in the field area.
  • Construct a stratigraphic column illustrating the thicknesses, lithologic differences, and spatial relationships between units.

Other Skills Goals for this Activity:

Students will...

  • Use a Brunton compass to measure strike and dip.
  • Use a Jacob staff to measure stratigraphic thickness.
  • Record Jacob staff measurements systematically in a field notebook and translate one's field notes to an office copy of a stratigraphic column.

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 (i.e., selecting an area with less straightforward stratigraphic relationships, a higher resolution section in which students focus on subtle changes in grain size, sedimentary structures, etc., rather than simply distinguishing between the 5 mapping units), but the area that was selected for this activity makes for a good introduction to bedrock section measuring. The Little Poleta bedrock mapping activity followed this section measuring exercise.

See the other part: the Poleta: Bedrock Geologic Map activity.

Prerequisite Skills and Concepts:

Students in this course had experience with constructing stratigraphic columns for soils (CZO soil stratigraphy) and sediment (Mono North paleoshoreline stratigraphy exercise) prior to beginning this exercise. Specifically, they were familiar with using the X-axis of a stratigraphic column to represent different grain sizes/sediment characteristics and the Y-axis to represent unit thickness. In addition, students completed the following work during the field course prior to visiting Little Poleta:

  • 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.
  • 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.

How the Activity is Situated in the Course:

This activity followed the Walker Lake, McGee Creek, and Panum Crater activities. View the E-STEM field course timeline for more information.

Description and Teaching Materials

Student Handout

Poleta_Stratigraphy_HandOut.docx (Microsoft Word 2007 (.docx) 55kB Jun27 20)

Necessary Materials:

  • Field notebooks
  • Bruntons (3 per team so that each person can have one and one can stay on the Jacob staff)
  • Jacob staffs
  • Hand lenses
  • Grain size cards
  • Acid
  • Rock hammers
  • Field maps
  • Radios
  • Office maps (some students might want map boards) PoletaTopo.pdf (Acrobat (PDF) 784kB Jun15 20)
  • Sample bags if students are collecting samples

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: for the section measuring exercise, there is not a huge distance to cover, but expect to walk up and down a relatively steep hill a few times during the exercise 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. The Jacob staffs should be disassembled and put into someone's backpack when finished with the exercise. Do not use the Jacob staffs as walking sticks, please.

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.
  • We suggest meeting with students prior to the start of the section measuring exercise to establish a note-taking protocol and division of field labor with them. For example, perhaps all of the field data will be recorded in one person's field notebook. If student A is the first person measuring, student A calls out each time that he/she/they measures one Jacob staff, and student B records that in the field notebook. Perhaps student A measures 5 Jacobs staffs, then they switch, and student B becomes the measurer and student A becomes the recorder. During the meeting, it will also be helpful to remind students that they should be taking strike and dip measurements periodically to make sure that they continue walking perpendicular to strike while measuring their section.
  • Initially, spread students out along the base of the outcrop to make sure that it isn't too crowded and record which groups started where. Students who start on the west edge of the hillside will not have unit 5 in their section--only 6, 7, 8, and Harkless. Students who start on the east edge of the hillside may have some initial challenges in measuring unit 5 because of its outcrop character. If possible, positioning a few faculty or TAs along the outcrop will alleviate these initial issues. For purposes of assessment, we suggest having a faculty member and/or TA measure several sections across the hillside.
  • We suggest spot-checking students' field notebooks early on to make sure that they are recording the necessary data to construct their office strat column.

Assessment

Assess the stratigraphic column with the rubric for the Mapping Badge.