Whitewater Rafting Duluth, Minnesota: Geology of the St. Louis River

Chances are if you decided to click on the link for this blog you might laugh at bad geology puns like the simple but proven oldie “geology rocks”. Really? You want geology puns? Let me dig some up! Insert laugh. You are not a nerd if you like puns or want to learn about geology. At Swiftwater Adventures we love to share our knowledge of natural history with those that are curious. Whitewater rafting in Duluth, Minnesota can be a great way to learn and experience the geology of the St. Louis River. Let’s drift (pun intended) back into time.

While whitewater rafting or kayaking along the St. Louis River you will notice huge exposed rock outcrops, some that are long vertical slabs that run east to west for miles. This jagged rock is like nothing else in northeastern Minnesota and besides the Ely Greenstone (aged 3.2 billion years) is some of the oldest rock in the state.

The geology of the St. Louis River dates back about 1.8 billion years, just as primitive life was beginning on Earth, referred to by geologists as the Precambrian Era. During this time ancient seas deposited silt, sand, and gravel that built up into layers over millions of years. The immense weight of these layers compacted the silt to form shale and the sand and gravel into sandstone, sedimentary rocks. Then, millions of years later, heat and pressure from tectonic movements converted the sandstone into graywacke and the shale into slate, both metamorphic rocks.

Today, geologists have coined the bedrock of the St. Louis River as the Thomson Formation. The same tectonic actions that created the Thomson Formation are also responsible for folding, tilting, and exposing these rocks. These formations are angled towards the north and south and can be easily observed while whitewater rafting near Duluth, Minnesota on the Louis. Some of these folds run for miles.

During the last glacial period, Glacial Lake Duluth deposited thick layers of red clay, silt, and sand over this landscape. Once the Superior Lobe, a huge ice dam, broke the St. Louis River, now swollen with glacial melt waters, ripped away the glacially deposited sediments and once again exposed the Thomson Formation. Skyline parkway in Duluth runs along the shoreline of Glacial Lake Duluth which was about 500 feet higher than present lake levels. Lake Superior has only been in its present form for less than 7,500 years.

Sediment loads deposited where the river meets Lake Superior, in cooperation with wave action and deposition, formed Park Point, the largest freshwater spit on Earth.

Along the river you can see different colored and sized boulders that are obviously different than the slate and graywacke known as Thomson Formation. Many of these are boulders were deposited into the St. Louis River from glacial action. As glaciers moved across the landscape they would freeze and thaw. Through this process glaciers would rip chunks of rock from the bedrock in Canada and the north shore of Lake Superior. Then through glacial movements (advance and retreat) were deposited in the St. Louis River.

Today, this section of river flows through the lowest points of the angled bedrock— water follows the path of least resistance. Drops in elevation create the rapids you will run if you go whitewater rafting while in Duluth, Minnesota on the beautiful and ancient St. Louis River. Call to book your trip!

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