Geology & Fossils
Minnesota’s rocks have generally been eroded down to the Paleozoic Era. Thus, you won’t find dinosaurs in Minnesota. Rather, fossil hunters find ancient marine creatures with a scattering of Ice Age fossils on top.
The best place to see the rocks that lie under Minnesota’s soils is in the southwest corner of the state. Here, the Minnesota River has eroded through debris left by glaciers and exposed some of the world’s oldest rocks. These rocks are granites, igneous rocks that are not kind to fossils. But there were few living things to be fossilized when these rocks were formed 2½ to 4½ billion — or 2,500 to 4,500 million — years ago anyway!
In Minnesota’s southeast corner, rivers have cut deep into the land, creating the state’s best fossil grounds. Here, rocks from the Ordovician Period (435-500 million years ago) rich with marine invertebrate fossils, such as bivalves, corals, and cephalopods, are common.
The two most famous blankets of fossil-bearing rocks in southeast Minnesota are the Platteville Limestone and Decorah Shale. Hundreds of very common fossil species are known in the Plattesville and Decorah formations, some named for Minnesota, Minneapolis, or St. Paul.
Fossilized marine invertebrates can be collected along the Mississippi and St. Croix river valleys from the “Twin Cities” of Minneapolis and St. Paul south to the Iowa border. Most Minnesotans live within 20 miles of a place where they can find some of these fossils within five minutes of searching.
The Ice Age
Minnesota was heavily scoured by glaciers during the Ice Age. As the last Ice Age glacier receded some 10,000 years ago, Lake Agassiz drained away to the north, leaving behind a fertile plain in northwest Minnesota called the Red River Valley. Numerous flat, sandy ridges mark beaches that once ringed the retreating lake, which was finally reduced to Canada’s Lake Winnipeg.
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