Riverbank rock identification

Laura Triplett
Gustavus Adolphus College
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Students collect rocks from a sand-bed river, identify them, and place them on a bedrock geology map of Minnesota to identify the original rock sources.

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I use this in an upper-level geomorphology course that is required for geology and environmental studies majors, so many of them have only had one intro-level geology course before. I have also used it in an introductory geology course.

Skills and concepts that students must have mastered

Students will benefit most if they can identify rocks in hand sample and know how to read a geologic map. It is a good exercise for introducing concepts of sediment transport by glaciers and by rivers.

How the activity is situated in the course

As part of a fieldtrip examining glacial till outcrops.


Content/concepts goals for this activity

1) Identifying rocks in hand sample
2) Reading geologic maps
3) Identifying source materials for glacial deposits

Higher order thinking skills goals for this activity

1) Interpreting glacial flow directions from till source rock locations

Other skills goals for this activity

Description of the activity/assignment

This exercise is part of a field trip lab in which we see map different till deposits. At a small, meandering sand-bed river in Minnesota, students are directed to wander along the riverbank and pick up gravel-sized rocks. When they each have a handful, I lay a table-sized, laminated map of Minnesota bedrock geology on the ground. I ask the students to identify their rocks, then find the corresponding bedrock units on the map and throw their rocks down in those regions. Soon we have a pile of rocks in northwestern Minnesota into Canada (weathered shales, weathered limestones), northern and northeastern Minnesota (granites, gneisses, basalts and rhyolites if we are lucky) and central Minnesota (dolomite, sandstones, granites). This visually demonstrates that glaciers transported stones from many distant locations and brought them here to southern Minnesota. In addition, we talk about how the river is down-cutting through consecutive layers of till and mixing those particles together in the stream deposits. The exercise gives students practice in identifying rocks, reading maps, and synthesizing different fields of geology to answer a geomorphic question about glacial transport.
Designed for a geomorphology course
Has minimal/no quantitative component

Determining whether students have met the goals

So far I only assess the activity by observing student participation in the activity and discussion.

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