Glacial Geology in the Field

Goals

• Become aware of the complexity of glacial depositional processes
• Determine the provenance of glacial sediments

Skills

• Outcrop observation
• Description of sedimentary deposits
• Stratigraphic column

This field lab works well with several content areas of an introductory geoscience course: climate change, surface processes (glaciation and rivers), formation of sedimentary rocks.

• For courses focused on earth history, glacial geology may be the final major field lab piece in the regional history of the area. Thus it connects with the field lab Assembling a Geologic History.
• For courses focused on earth system science, a glacial geology field lab is a graphic reminder of how differently the earth surface appears in glacial periods than now. The lab can be integrated into the sections of the course that deal with historic climate changes (Little Ice Age, advance of Alpine glaciers) as well as long geologic ice age periods.
• For courses focused on earth resources, students can consider the economic uses of sand and gravel, commonly acquired from glacial deposits.

Minimum time required is two to three hours, depending on the complexity of the exposures and the number of tasks for the students.

• For all work in gravel pits: hard hats and boots, glasses rather than contact lenses (and consider further eye protection).
• For stratigraphic descriptions of glacial material: shovel, trowel, cloth tape and nail.
• For measuring directional indicators (oriented pebbles, scratches on bedrock, etc.): compass and calipers (to locate pencil-shaped grains).
• For determining provenance (origin of glacial materials): hand lens, rock identification chart, shovel.

Teaching materials might include the following:

• Description of how to do a pebble count and a form for recording these data.
• Description of how to measure pebble orientation in situ (to determine transport direction in some kinds of tills)
• Description of how to measure a stratigraphic section

This lab can focus on either or both of two subjects: 1) glacial provenance (based on the rock types in the glacial material) and 2) depositional environment of the glacial material (based on observations of sedimentary beds and stratigraphy).

Students need some background in rock identification to solve a provenance problem. In addition, a rock type that may not be particularly abundant or obvious often identifies the glacial provenance. For instance, in southern Minnesota, all the glacial sediments have granite, metamorphic rocks (including greenstone) and carbonates. Those from northwest Minnesota have fragments of Pierre Shale (and mainly small fragments, at that) and those from northeast Minnesota have fragments of vesicular basalt, red sandstone, and a few other distinctive rocks from the Lake Superior Basin. However, you can turn such complexities into "teaching and learning moments" for the students.

Depending on the particular exposure or gravel pit, you and the students may be describing till, ice-contact stratified drift (for instance, in a kame), and/or outwash. Various layers may be clast-supported or matrix-supported, laminated or not, deformed or undeformed by glacial movement and ice melting, and of various grain sizes. The variety adds levels of interest and also complexity. You may want to direct students to particular locations to begin their observations.

This is a good lab to work on sketching of geologic features, as the layering and varying grain sizes of glacial materials are obvious from a distance. For detailed stratigraphic descriptions, you may want to recommend that students tackle a meter or two at most, depending on the length of the lab.

Slopes in gravel pits are inherently unstable and students should be cautioned not to climb on vertical slopes that are currently being mined. If the pit is inactive, students will be able to climb up slumped deposits and carefully excavate fresh sections adjacent to those slumps for description. In all cases, students climbing on gravel pit slopes need to be aware of others above and below them. Consider bringing hardhats for students (and you) to wear.

The assessment method chosen will depend on the particular purposes of the field lab and the course. You may ask students to include copies of their pebble counts, stratigraphic sections, sketches and other observations. In a lab report, students can be asked to interpret the provenance and the depositional environment of the glacial sediments. If you've had groups of students study different parts of an exposure, each group could present its results orally.

You can also devise exam questions based on the field lab activities.