“Rock and roll” isn’t about music for Mike Petefish when he’s picking rocks in Claremont, Minnesota. “When I see a rock like this one with a certain bluish color, I know it’ll be dense and heavier for its size than any other, and all I can do is rebury it cause I sure can’t lift it with a backhoe,” he says.

Picking rocks takes more man-hours than planting for Petefish, in southern Minnesota. This whopper was his “rock of the ages.” He and Craig Johnson (in photo) first thought it was the size of a football until they dug deeper. 

Dodge County Minnesota is right at the limit of the Continental Glacier from the most recent Ice Age over 10,000 years ago, says Minnesota State Geologist Harvey Thorleifson. Cultivation and frost push them to the surface.

The bluish, dense rocks like the one pictured can be iron- and magnesium-rich rocks, which explains their density, he says. They’re common along the North Shore of Lake Superior near Duluth, Minnesota, Thorleifson says.