Driftless and Crater Information

The Decorah Crater

470 million years ago, the eastern edge of what is now Decorah was hit by a 650-foot meteorite. The force of that impact is estimated at 1,000 megatons of TNT (the bomb dropped on Hiroshima during World War II was around .015 megatons), creating a 3.5-mile-diameter crater.
At that time, much of North America, including what is now Iowa, was covered in ancient oceans. Over time, sediment was laid down within the crater basin, creating layers of shale.

It was these layers of shale that led to the discovery of the meteor crater. Jean Young, an independent geologist from the Decorah area, had studied samples from well-drillings for years. After noticing unusual patterns in the shale layers, she contacted the Iowa Geological Survey and they mapped the samples, eventually outlining the crater’s basin. Scientists tested the layer of material underneath the crater for shocked quartz (shattered crystals) and conducted rock density and electromagnetic aerial surveys, confirming the impact structure.  

But all those layers of shale held an even greater surprise: ancient marine animals captured as fossils, many of them new to science and all quite different from those found outside the crater (northeast Iowa in general is fossil-rich because of that ancient ocean that once covered it).

The most dramatic discovery was Pentecopterus decorahensis, a six-foot-long sea scorpion that pre-dated the oldest known sea scorpion by 10,000 years. To get at the fossils, inconveniently located underneath the Upper Iowa River, scientists temporarily diverted the river from the excavation site, recovering more than 5,000 specimens, including plants, shrimp-like animals, and the oldest known fossils of jawless fish. These discoveries have led to new insights into what life was like during the Ordovician period.

What is the “Driftless” Area?

DriftlessTo understand why northeast Iowa, southeast Minnesota, and southwest Wisconsin are collectively known as the “Driftless Area,” you must first look back hundreds of thousands of years.

Throughout time, parts of North America have been covered by glaciers, towering masses of dense ice formed over centuries by the compaction of layer upon layer of snow. Glaciers are so massive they move under their own weight. Over time, they have repeatedly advanced and retreated from the upper half of North America, down through the Upper Midwest and Great Lakes region. As they did so, the ice sheets scoured, ground down, and flattened the landscape, leaving behind silt, sand, gravel, boulders, and other debris known as “glacial drift” when they receded.

The Driftless Area was missed by the most recent glaciation, and maybe others before it. The glaciers advanced and retreated around, but not over, the region. And so, the land was not ground down and covered by deposits of glacial drift. Hence, the “Driftless” area.

You can see this geological evidence as you explore the Driftless Area: the deep valleys, exposed bedrock, and towering bluffs are all there because the rivers and streams of the area have had millions of years to carve and erode the underlying bedrock, their work never erased by the flattening of the land and laying down of new sediment.
The area is also defined by its unique karst topography. Its underlying bedrock of limestone, dolomite, and other rocks are slowly dissolved by water, creating caves, sinkholes, springs, and coldwater streams unique to this part of the Midwest, creating remarkable natural wonders just waiting for you to discover.

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