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Uncovering the St. Croix River's History Through Its Geology
- Cecilia Gustafson
- Jun 26
- 3 min read
The St. Croix River is a unique area, not only for its natural beauty, but also for its complex geological history. It is home to deep valleys and the world’s largest natural potholes (not just the ones on the road)! From one billion years ago to the present day, lava, water, and weather have shaped this area into what you see today.
Over one billion years ago, lava flows surrounded the area. These lava flows created many rock types we find around the river today as they cooled and developed. During the Precambrian period, layers of bedrock, shale, sandstone, and
even some crystalline rock formed. Zooming forward in time to about eleven hundred million years ago, the continent was moving, which created a rift from Lake Superior to Iowa. Through this split, lava flowed, and when it cooled, it created a layer of strong, dark rock known as basalt.
Later, during the Cambrian and Ordovician periods, layers of marine sandstone, shale, and limestone were deposited. As a result, the St. Croix Valley has many sedimentary or water-deposited rock formations. Within these rocks are fossils of a variety of small sea animals. See if you can find these fossils of trilobites and primitive mollusks at either of the Interstate Parks! Around 100 million years ago, the Cambrian Sea retreated from what is now Minnesota and Wisconsin.
The majority of erosion that shaped the area into its present-day form was caused by glaciers about 2 million years ago during the Quaternary Period. This period is divided into the Pleistocene Epoch (2 million to 10,000 years ago) and the Holocene Epoch (10,000 years ago to present). The Pleistocene epoch is more commonly known as the Ice Age. The Laurentide Ice Sheet covered Minnesota for most of the Ice Age, during which it retreated and reformed multiple times. Towards the end of the Pleistocene and the beginning of the Holocene, glaciers began melting. Glacial Lake Duluth, the ancestor of present-day Lake Superior, overflowed and sent down rushing water into the valley, gouging out the area the St. Croix flows through.
Wind and water aren't the only factors that have shaped the St. Croix River Valley; there have been significant human impacts over the last 200 years too. Logging in the late 1800s and 1900s caused erosion along the riverway and destroyed huge swaths of habitat. Thankfully, we no longer clog up the river with logs, but even without massive logging operations today, the river is not safe from human impacts. Lately, habitat destruction, introduction of invasive species, climate change, pollution, and urbanization have become drivers of river issues that are affecting the way the river flows today. It's no exaggeration that the effects humans have had on the riverway may be seen for the next one billion years of our Earth's history.
Next time you take a hike or paddle on the St. Croix River or its watershed, take a look not only at the trees and the animals, but also at the rocks. It took millions of years to be shaped and placed in the way you see them today, and they can be just as bright and exciting as all the birds in the sky.
Life finding a way along the rocks of the St. Croix River - Photo Credit: Craig Blacklock
Sources
Andersen, Osh(Barbara), et al. “Transformation of a landscape in the upper mid-west, USA: The history of the lower St. Croix River valley, 1830 to present.” Landscape and Urban Planning, vol. 35, no. 4, Sept. 1996, pp. 247–267, https://doi.org/10.1016/s0169-2046(96)00304-0.
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