Posted by: soniahs | July 12, 2011

Geology at Interstate State Park

I know it’s a cliche to say “It seemed bigger when I was younger,” but I’ll say it anyway in this post. One of the places we visited on our recent trip to the Great Lakes was Interstate State Park, on the St. Croix River between Minnesota and Wisconsin near the town of Taylors Falls. We actually visited the MN park– there’s also a park on the WI side.

St. Croix River at Interstate State Park (Y. Fernandez)

This park was a pretty memorable spot that we visited frequently when we were younger, and it’s still pretty interesting. In this area, the St. Croix River form a deep gorge through a layer of basalt- very old basalt. The rock here was laid down during a continental rifting event around 1.1 billion years ago. Later, sandstone and siltstone were laid down by a shallow sea during the Cambrian. Much, much later, glacial meltwater during the last Ice Age carved the St. Croix River valley.

Cliffs along the St. Croix (Y. Fernandez)

While the most obvious geological feature here is the cliffs, with their irregularly-fractured angles, there are a bunch of other features. An informational kiosk at the parking lot has a map of features like glacial potholes. These formed when rocks were caught in whirlpools and eddies in the river, and eventually scoured out vertical shafts in the rocks. Some of these pothiles were big enough to walk into:

Looking up from the interior of a pothole- the hole is about a meter across.

Others were smaller in size. This one reminded me of a lava tree mold:

This pothole was in a boggy area, probably with more potholes concealed underneath (Y. Fernandez)

This is an interesting place to stop if you’re interested in geology. There were also a lot of birds. A stiff breeze when we were there meant that the paddleboat tours were closed down for the day. It would be fun to go back at some point and canoe down the river- though not during the spring thaw season…


  1. […] While at Interstate State Park, we witnessed a dramatic rescue of a gosling, swimming through the rapids of the St. Croix River. […]


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