I thought the 'glacial erratic' in my town park was just a big rock someone dumped there.

For years I walked past this huge boulder in the park thinking it was leftover from some construction project. Then I went to a local geology talk at the library and the speaker used it as an example. He said it's granite, but the bedrock here is all shale, and it was carried over 200 miles by an ice sheet. Seeing the map of the ice flow path made it click. Anyone know of other good examples of erratics in the Midwest?

3 comments

3 Comments

shanejackson1mo ago

Oh man, that's a COOL story! It's wild how something so normal looking has that crazy history. So when you saw the map, did it show the EXACT path from its origin? I'd love to know if they could trace it back to the specific outcrop it broke off from.

terry_barnes1mo ago

Wondered the same thing myself. The map showed a general flow path from the Canadian Shield, but pinpointing the exact rock face is tough after thousands of years of glaciers moving it. They can trace the type of granite to a broad region, but the specific origin gets lost. It's like finding a piece of driftwood on a beach and trying to guess which tree it came from. Still, knowing it traveled that far is the amazing part.

vera_lewis1mo ago

Jump right into shanejackson's question about the exact path. The speaker at the library showed a general flow direction from the Canadian Shield, but said they can usually only trace the erratic's type of granite to a broad region like northern Minnesota or Ontario, not a specific cliff. It's kind of like what @terry_barnes said about driftwood. They know the rock came from that big area because the granite matches, but the exact spot it broke off got lost over 10,000 years and 200 miles of grinding ice. Still, seeing the old glacial maps with arrows sweeping down across the lakes made the whole journey feel real. You start looking at any random boulder in a field and wondering where it actually came from.