Paleoclimate Reconstruction: Walker Lake-Bloody Canyon Moraines of Mono Basin, California

Sarah Hall; Peter Lippert; Casey Moore
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McGill University; University of California, Santa Cruz; University of California, Santa Cruz

Intended Audience

This field activity is designed for undergraduate Earth Science students with some knowledge of common rock-types, geomorphology, orienteering and mapping skills.

Location

Walker Lake, Mono County, California. Coordinates: 37.869°, -119.165° (starting point of exercise, room to park a few vans). From CA 395S, turn off right onto Parker Lake Rd at 37.878°, -119.1086°. Dirt roads lead up to the starting point.
This is a good starting activity for various other exercises involving the moraines in the region. For example, in many locations, the range-bounding Sierra Nevada Fault offsets the moraines. Using known moraine ages and measured fault offsets, students can calculate fault slip rates (at other locations – Hilton Creek, 37.565°, -118.788°).

We have incorporated the use of satellite imagery into this activity prior to the field trip. Using ASTER data, students map the extents of glacial features on the images. They note the crests and flanks of each moraine. From this data they are able to make quantitative measurements of the mean slope of each moraine and easily map a topographic profile across the moraine (of course, this can also be done by hand with the topographic map). With additional time and computer access, this activity could be enhanced with a diffusion modeling exercise for a more quantitative relative dating technique. With the availability of handheld computers, students may map the moraines in the field directly onto the images they prepared ahead of time.

The most basic version of this activity is for students to map and gather data from each of the three main moraine crests south of the lake, but as an added activity, students can map the "mystery" moraine east of the lake near the eastern extent of the younger moraines (GPS: 37.893°, -119.128°) and try to correlate it with one of the three main advances.

Summary

In this activity students map and describe glacial geomorphic features related to Quaternary glaciations in the Sierra Nevada of central California. In this region, multiple sets of glacial moraines and corresponding bogs and lakes are evidence of various advances during the Pleistocene. With time, glacial features are modified by erosional processes resulting in morphological changes that we can use to relatively date the various features. Using cross-cutting relationships, moraine morphology, boulder description/counting, and various other relative dating techniques students will "date" the moraines. In this locality there are three main moraine crests that are well-preserved and available for students to hike along and gather data (classified as Tioga-Tenaya, Tahoe, and Mono Basin moraines). At a locality along each of the three different moraine crests, students will observe the boulders along the ridge crest (within ~5m of the crest) within a known area. The students will record the number of boulders, the degree of weathering of the boulders, the lithology, the size (~diameter), roundness, presence of lichen (describe), and the presence of striations on each boulder. They will also note the style and amount of vegetation on the moraine and soil characteristics (amount of gruss around boulders?) The students should make a table in their field notebooks so that it is easy to compile the data at the end of the day. Generally students work in groups of 2 or 3. Since many of these observations are somewhat qualitative it works well for them to be able to discuss their thoughts while gathering data. It is helpful to use a few boulders as examples to set some ground rules before starting the activity – this will help the class be the same page with regard to the techniques.

Context

Previously, this activity has been conducted with a group of ~20 undergraduate students, ~3 graduate student TAs, and 2 faculty members as one activity during a ~2 week long field school in Owens Valley, CA. This activity was used in early May and there was still a bit of snow on the ground. It is possible that prior to mid-May, vans may not make it all the way up the dirt roads. The younger moraines are quite steep and pose a significant hike. However, for trips planned during mid-May-Sept it should be possible to drive most of the way up to the starting point. During the activity students will be walking along the crest of the moraine, so there is no serious elevation change, however, trees and boulders are obstacles. Participants should wear good hiking boots, layered clothing (can be cold and windy as well as brutally sunny), and bring lots of water. Remind the students to stay hydrated – this is at a higher elevation than many students are use to (2500m). Plan for at least 6 field-hours dedicated to this activity as well as ~1 hour of "data analysis".

Goals

The main goals of this activity are: 1) learn to recognize glacial geomorphic features and understand the processes that have formed and modified them, 2) experience the usefulness and limitations of relative dating techniques, 3) learn about absolute dating methods for these features (cosmogenic 10Be, 36Cl, 26Al of moraine boulders, 14C of bog/lake basal peat) and how samples for these different techniques are collected, and 4) practice gathering, logging, and analyzing field data.

Design

It is important to give a few background mini-lectures before or during the exercise. One valuable discussion simply involves a drawing of the glacial geomorphic features in an alpine glacial system (cirques, arêtes, moraines, bogs, lakes – these are all visible in this location, both in the field and on the satellite images). It is also useful to briefly discuss till and glacial striations, although it is very easy to show examples of these during the activity.

Two other important discussion points involve the relative and absolute dating methods. It is a good idea to talk about the difference between "relative" and "absolute" and give many examples of each. Maybe assign a short reading on relative dating techniques before the activity (Burke and Birkland, 1979 is a good option). Explain the different relative dating techniques and how to evaluate (i.e. "ping" vs "thud" in the hammer test). Before the activity decide to either define a region on each moraine for the students to cover (need a known area to survey on each moraine). Alternatively, if the students have a GPS, have them define their own location and area. It is important to know the extent of the area covered so that data is comparable between moraine sets.

Take the time to debrief after the activity about the limitations of relative dating and how great it is that we now have many absolute dating techniques! We have used "student presentations" here to have them lead the discussion of the absolute chronologies. Groups of students read papers on the absolute chronologies for moraines in the region and present the methods and results in a quick ~5 min presentation to the group.

Notes and Tips

This locality is free, easy access, and relatively easy hiking. Encourage students to bring layers, rain gear, and snacks. Prepare for the students to be slightly stressed about the qualitative aspects of the relative dating techniques (but that is part of the learning!) Avoid planning this activity for a time when there will be snow on the ground (before early May, after ~Oct?). Call the Mammoth Lakes Visitor's Center for information on road access and conditions. Start with the youngest moraine. The older moraine is more diffuse and it is hard for the students to see where the crest of the moraine is located. The youngest moraines are quite well preserved and abundant with boulders

Assessment and Evaluation

We have the students hand in their maps, summarized data tables, and a short (1-2 page depending on the level) write-up of the activity. Usually they have worked in small groups so they hand their data in as a group. All students are required to make their own map. Once they have compiled their data (calculated number of boulders/m, etc,), we log the class data on the board (or white board, clipboard, piece of paper) and discuss the results as a class. This is usually where we debrief about relative dating techniques. We do assign grades based on their "reports" and this largely reflects the quality of the maps and written section. It is easy to gauge whether or not the students are grasping the activity while in the field. Usually the first moraine is difficulty for them, but by the second moraine, most of them understand the idea of the exercise.

Materials and Handouts

To each student we provided the Handout, Topographic Map, Satellite image, and one piece of vellum. The required materials (per student) are listed on the Student Handout. Copies of these materials are uploaded below.
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