Glacial, sedimentary, structure, wisconsin

Glaciated breccia: A three-part rock story

This piece of glaciated breccia from the Brussels Hill meteorite impact structure in Brussels, Wisconsin is probably one of my favorites in my collection. It’s not the oldest, nor is it necessarily the prettiest, but it holds one of the best stories.

This rock began its life ~440 million years ago during the Silurian Period. Back when Wisconsin was inundated by warm shallow seas, carbonate rocks like dolostone (what composes this breccia) were deposited on the seafloor.

We aren’t positive when it occurred, but sometime after the seas retreated from Wisconsin but before the last glaciers arrived, a meteorite hit in what is now the Door Peninsula of Wisconsin. The high pressures from this impact brecciated the target rocks—meaning broke up the rocks into fragments. In the above photo, you can see all the individual fragments of rocks that then became cemented together in a matrix.

Finally, these brecciated rocks were glaciated during the Pleistocene Wisconsin glaciations. As the glaciers slowly carved over the landscape, they left a trail of glacial striae or striations—scratches into bedrock made by rocks that were carried at the bottom of glaciers. In the above, you can see the white, vertical striae that were carved by the moving glaciers.

So all from this one rock, you can learn its humble origins as a regular sedimentary rock, its mid-life crisis of being broken up and fragmented by a meteorite impact and its final stage of being smoothly polished by the glaciers.

Always be sure to listen to what your rocks have to say, they have fascinating stories to tell!

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