Glacial Erratics

The video provides a walkthrough of a field that contains some large stones. The first image shows the same field while the second image shows a large stone in Split Rock Park in Chickasaw County south of Fredericksburg, Iowa.

Glacial erratics are a remnant from the most recent times when glaciers covered much of the surface of Iowa. Glaciation is a slow Earth process that changes the surface of the Earth through erosion and deposition. One of the types of deposits are the various rocks and boulders that are “erratically” found on the surface that were transported through the massive power of the glaciers. Glacial erratics are a common occurrence in farm fields of the Iowa Erosional Surface landform (Landforms of Iowa by Jean Prior). These rocks originated in regions to the north and were moved here during the most recent period of glaciation, 10,000-14,000 years ago. These rocks are embedded in the glacial till and each season with the freeze-thaw cycle more rocks are thrust to the surface. Erratics are said to be “plucked” through weathering and then transported and deposited with the glacial load. These rocks are a nuisance for farm machinery and may cause much damage so they are often picked up and taken off the surface of the field. 

Where do the rocks in these fields come from?

Why do farmers pick up rocks from fields?

Students could:

• Watch the video and brainstorm how they think these rocks get here, where might they have come from?
• Share if any of them pick up rocks at farms or even make contact with a farmer from northeast Iowa that could talk about the rocks. 
• Compare glaciation that impacted Iowa to current conditions of glaciers in the polar regions or on mountains.
• Explore the large boulder in the middle of the street in Nora Springs, Iowa or Split Rock Park in Chickasaw County south of Fredericksburg.

• Iowa Geological Survey |The Iowan Surface: An article that touches on why glacial erratics are present in Iowa and how glacial activity shaped the landscape.
• Iowa Academy of Science | Large Glacial Erratics in Northeast Iowa: Article providing teachers with basic info and photos of large erratics.
• Oxford Education | How do Glaciers Shape the Landscape? : Information on how glaciation is a slow Earth process.
• Minnesota Conservation | The Glacier Did It: Teacher guide for many glacial remnants, including erratics.