Unraveling Geological History: Glaciers and Faults at Discovery Park, Seattle

This page created by Trileigh Tucker, Seattle University.
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Initial Publication Date: June 30, 2009 | Reviewed: August 4, 2022


This is the last lab and field exercise in Introduction to Geology. At this point in the term, students are already familiar with the characteristics and typical sequence of Seattle's glacial layers; they have also studied faulting and know that the Puget Sound area has major faults and a history of earthquakes. The goal of this field exercise is that students use their understanding of these subjects, along with careful field observations, to collaboratively deduce the complex history of a series of bluffs exposed along the Seattle shoreline. A lab handout is provided.

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Learning Goals

  • Stratigraphy
  • Faulting indicators and mechanisms
  • Local geologic history
  • Data gathering and recording techniques
  • Identification and interpretation of sedimentary layers or rock units based on field observation
  • Correlating observations across outcrops
  • Combining data from different field areas to unravel regional geologic history

Context for Use

This field exercise is excellent for beginning geology students later in the term, and its basic structure could easily be applied to any field area that comprises outcrops (or parts of outcrops) with substantially different characteristics. We have about 2 hours on site; 3 hours would be ideal. The only equipment needed are pencils and sketch paper, but hand lenses would be helpful - even essential in some field areas. Binoculars are useful for looking at high-up sections of the outcrop but not required. (However, if I had a departmental supply of them I would routinely ask students to use them.)

This is the last field exercise in Introduction to Geology, and thus students are already familiar with local glacial stratigraphy and basic concepts of faulting, and know that our region has an earthquake history and associated faults. From prior field exercises, they are comfortable with field sketching and what constitutes an appropriate level of detail for sketches. They have also had considerable experience with group brainstorming about ideas for interpreting observations.

The exercise could be adapted for people without this background knowledge, given a significant increase in available time at the exposure and with hand samples of each geologic unit available so that learners could become familiar with these before working on broader stratigraphy. It would also be fairly easy to give effective instructions and support for sketching and brainstorming in this single session. The primary difficulty for students in this adaptation would probably be the amount and diversity of material to be absorbed in a single session.

Description and Teaching Materials

In this field exercise, students are taken in university vans to an off-campus geologic exposure comprising three subareas: a lower stratigraphic section (southeastern part), a mixed section obscured by heavy vegetative growth (center), and an upper stratigraphic section (northwestern part). The goal of the exercise is for students, first individually and then in teams, to characterize each subarea separately and then integrate their initial observations and hypotheses into an overall picture of the exposure's complex geologic history.

Students are each given a lab handout (attached below) containing detailed information about procedure and writeup instructions. Each student also receives a blank outline of the exposure's shape, with three subareas labeled A, B, and C, to be filled in with sketches of each subarea. Students form their own teams or are assigned by the instructor to teams. Each team is also given 2-3 copies of an aerial photo of the exposure from Google Maps, with locations of subareas A, B, and C noted.

Once on site, students are given an initial introduction by the instructor indicating the general setting of the exposure in our region, the limits of the exposure to be analyzed, and the locations of subareas A, B, and C. I describe the lab procedure and assign durations for each task in the exercise, along with a time at which the class should reconvene as a whole for an end-of-exercise analysis and summary. I also give some general guidelines and cautions (described below under "Teaching notes and tips") about the exercise.

After individuals and teams have completed their on-site observations and analyses, they regroup as a whole class. I then lead them in an analysis in which each team shares observations and their multiple hypotheses about the relationships among the three subareas. I challenge or support these where appropriate, but only after all groups have shared their ideas, so that any challenges are depersonalized. Each team inevitably comes up with at least one excellent observation and/or hypothesis that we can then weave into the overall picture. I finish by giving them the currently-accepted geological understanding of the outcrop's history.

Teaching Notes and Tips

Because they are familiar with the typical sequence of glacial layers in our area, students often simply glance at their section of the outcrop and assume that they are seeing all of those same layers in that sequence--thus missing the observations most critical to deducing the site's geologic history. Therefore, at the beginning of the exercise it's important to caution them not to make undue assumptions, while at the same time not giving away the puzzle that constitutes the core of the exercise.

This caution is an important part of their learning the process of science: that even as we become familiar with the scientific story, we always have to watch for assumptions and biases that inhibit new insights. I sometimes draw parallels with developing a meaningful relationship with a person: to know them as they truly are, we need to stay alert to what they're really saying rather than what we're used to hearing!

Some students, especially with the current ubiquity of small cameras, want to substitute photography for sketching. Because sketching is much more effective in helping students look closely at outcrops, I discuss this and emphasize that I'll be evaluating their lab-report sketches, not photographs. I tell them that they're welcome to use photographs in addition to their field sketches in their lab reports, as ways to highlight or further describe what they've seen-but not as substitutes for detailed sketches.

Well-executed sketches are also important, as I tell the students, for helping other team members understand what's going on at the specialist's outcrop-it's not just for the instructor to grade. This helps the students understand the importance of careful observation and good record-keeping in the process of science.

Time management is a challenge for students in this exercise, since there are several sequenced activities--individual observation, team consultation, and whole-group debriefing-that each may occur in a different location. At the beginning of the exercise, I give students specific durations for each component. However, because this exercise is late in the term and in a particularly beautiful setting, it's easy for students to become distracted and lose track of time. I patrol the groups to give them time reminders and help them with observations or ideas when needed to keep them moving along.
Finally, the particular outcrop that I use for this exercise is characterized by steep slopes and thus students sometimes want to explore higher than is safe for them. The more engaged they are with the geological puzzle, the more likely it is they'll want to take risks to figure it out. To counter this possibility, I note the danger and encourage them to find at the foot of the bluffs samples that probably fell from the higher sedimentary layers.


Assessment for this field exercise happens informally during the exercise, and formally afterward. During the exercise, I circulate among the groups, listening to their discussions to see whether they're making helpful observations and comparing subareas effectively. If they're way off track, I make comments to steer them back, or express approval if they're doing well. However, it's important to me not to interfere with their observations or thought processes in a way that inhibits their creativity or leads them to think they must get the "right answer."

Students are formally assessed based on their lab reports. Each student writes her or his own section of the group's lab report, which is worth 35% of the grade, and the team analysis of geological relationships among the three subareas is worth 65% of the lab grade. Because this field exercise emphasizes process learning, my assessment of students' work values this aspect over content learning. As noted in the handout for this exercise, individual components are evaluated on sketch quality and detail of individual observations, and the team portion is assessed on thoughtfulness of their hypotheses for both individual areas and relationships among them, and on their lists of further evidence that could help them choose among their hypotheses.

References and Resources