Initial Publication Date: December 12, 2013

River Rock Identification:

Laura Triplett, Gustavus Adolphus College

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


This activity has been done at a small sand bed river in southern Minnesota, but it can be adapted to any location that has rocks that have been transported long distance via glaciers.


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.


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.


Goals for this activity include identifying rocks in hand sample, reading geologic maps, and identifying source materials for glacial deposits. Students also improve their skills in interpreting glacial flow directions from till source rock locations.

Assessment and Evaluation:

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

Materials and Handouts:

Instructor's Notes (Microsoft Word 25kB May2 08)