 A series of small circular holes are called “potholes,” ground into the rock when a rush of glacial meltwater spun loose rock debris in an eddying current. |  The St. Croix Dalles at Interstate Park formed when a torrent of glacial meltwater blasted through a wall of basalt bedrock. |
The larger holes might have been witches’ cauldrons ground into the bedrock, deep enough to swallow an adult. The smaller ones could be mixing bowls and the tiny ones inverted cups for sipping rainwater with a straw.
Interstate Park along the Minnesota-Wisconsin border is particularly known for its geological wonders called “potholes,” circular indentations in the bedrock formed 12,000 years ago when a torrent of glacial meltwater breached Glacial Lake Duluth (near Lake Superior) and rushed down the St. Croix River. At today’s Interstate Park, the deluge burst through a wall of basalt, carving a 300-foot canyon.
On both sides of the submerged canyon, eddying whirlpools spun rounded stones as big as bowling balls and as small as marbles that downcut over 200 circular holes into the bedrock. Many are deep enough to hide your torso while others would just cover your foot, but the largest potholes measure 60 feet deep and 26 feet across. On the day we visited, some were filled with water from the recent rains, brew pots waiting to be heated. Others were as dry as bones.
* * *
Dianne and I had come through a bit of a torrent ourselves during the previous night’s downpour at another park that had soaked through the corners of our tent, so we laid our gear out to dry at Interstate Park before setting up camp. The delay gave us time to explore and sample the park before diving into its features.
Interstate Park is a dual park system established along the St. Croix River on the Minnesota-Wisconsin border just south of the equally dual border towns of Taylors Falls (MN) and St. Croix (WI). The smaller 300-acre Minnesota component was established in 1895 (Minnesota’s second oldest park, after Lake Itasca) and the larger 1400-acre Wisconsin tract dates back to 1900, that state’s oldest. Together, they constitute the nation’s first two-state park.
Although the two parks are administered by each state separately (e.g., separate state park stickers are required for each park’s entrance), they are knitted together by the St. Croix River. We took the easy river route first, buying tickets for an excursion boat on the Minnesota side. The boat expertly navigated the narrow, swift flow between the bluffs, whose waters raced as if the glacial breach had occurred just last week.
Tuning out the tour guide, I listened instead to the story in the angular canyon walls. The gray, igneous basalt rock that the glacial torrent had burst through formed a billion years ago when North America began to split apart during its continental drift across the tropics. Molten rock upwelled into the gap like a bad case of acid reflex and then cooled in place. While the rift arcs through several midwestern states, only here in its northern reaches did it become the exposed bedrock.
Eons later, when the glacial meltwater torrent came rushing down the St. Croix, it nosed out cracks in the basalt and blasted its way through, creating the valley, bluffs, and potholes.
Later, we kayaked upstream to the canyon, looking for the “cross” in the basalt walls that supposedly led French explorers to give the river the French equivalent name of Sacred Cross. We briefly debated each other’s interpretation of the cross’ location (only to find out later that the river took its name from a cross-shaped island downstream). Then we gave up the quest and let the current take us downstream to where the canyon widened. Soon, the basalt walls disappeared entirely, and the river slowed to a relaxing crawl.
The slower river told the human story. For ten thousand years, the St. Croix served Native Americans as a food source and waterway linking the Mississippi River to Lake Superior via a 20-mile northern portage. Euro-Americans arriving in the 1700s shifted the river’s focus first to fur and then to fir, decimating the beaver population for their pelts and then, after an 1837 treaty ceded the region to U.S. settlers, decimating the forests. An 1886 log jam drew media attention when it clogged the river for seven miles.
By 1914 the logging industry had run its course, and the St. Croix River returned to an earlier, more peaceful existence. It became popular for recreation and relaxation, but modern development still threatened to turn it into just one more polluted river. Another joint Minnesota-Wisconsin effort in the 1960s saw respective Senators Walter Mondale and Gaylord Nelson successfully propose the St. Croix National Scenic Riverway. In a National Park Service video, Senator Mondale proclaimed, “The idea that we’d let these areas be despoiled offends my idea of what America’s about.”
The Scenic Riverway now protects 255 miles of the St. Croix and its tributary, the Namekagon (Ojibwa for “place where the sturgeon spawn”) from unfettered development and pollution.
Even so, environmental threats are never far off. A proposed nickel mine near the St. Croix headwaters in northwest Wisconsin could potentially contaminate the river.
After kayaking, we returned to the bluffs. Amid the pothole formations on the Wisconsin side, near the bluff above the St. Croix River, sits the western terminus marker for the Ice Age Trail that wends 1100 miles east and south across the state, following the end moraines—the farthest edges—of the Wisconsinan glacier, another reminder of the glacial formations that shaped this landscape.
* * *
The North American continent once nearly split in two, and then the rift healed over. A wall of glacial meltwater once roared down the St. Croix River and blasted through a plug of thick basalt, carving bewitching potholes and a beautiful gorge. The furs and the firs were once decimated, but the creation of Interstate Park and the National Scenic Riverway designation has since offered protections to the St. Croix.
Time can heal wounds.
Even our wet camping gear dried out, and we set up our tent.
In the morning, the sun came out and set the St. Croix River sparkling.
-- September 2025
Interstate Park along the Minnesota-Wisconsin border is particularly known for its geological wonders called “potholes,” circular indentations in the bedrock formed 12,000 years ago when a torrent of glacial meltwater breached Glacial Lake Duluth (near Lake Superior) and rushed down the St. Croix River. At today’s Interstate Park, the deluge burst through a wall of basalt, carving a 300-foot canyon.
On both sides of the submerged canyon, eddying whirlpools spun rounded stones as big as bowling balls and as small as marbles that downcut over 200 circular holes into the bedrock. Many are deep enough to hide your torso while others would just cover your foot, but the largest potholes measure 60 feet deep and 26 feet across. On the day we visited, some were filled with water from the recent rains, brew pots waiting to be heated. Others were as dry as bones.
* * *
Dianne and I had come through a bit of a torrent ourselves during the previous night’s downpour at another park that had soaked through the corners of our tent, so we laid our gear out to dry at Interstate Park before setting up camp. The delay gave us time to explore and sample the park before diving into its features.
Interstate Park is a dual park system established along the St. Croix River on the Minnesota-Wisconsin border just south of the equally dual border towns of Taylors Falls (MN) and St. Croix (WI). The smaller 300-acre Minnesota component was established in 1895 (Minnesota’s second oldest park, after Lake Itasca) and the larger 1400-acre Wisconsin tract dates back to 1900, that state’s oldest. Together, they constitute the nation’s first two-state park.
Although the two parks are administered by each state separately (e.g., separate state park stickers are required for each park’s entrance), they are knitted together by the St. Croix River. We took the easy river route first, buying tickets for an excursion boat on the Minnesota side. The boat expertly navigated the narrow, swift flow between the bluffs, whose waters raced as if the glacial breach had occurred just last week.
Tuning out the tour guide, I listened instead to the story in the angular canyon walls. The gray, igneous basalt rock that the glacial torrent had burst through formed a billion years ago when North America began to split apart during its continental drift across the tropics. Molten rock upwelled into the gap like a bad case of acid reflex and then cooled in place. While the rift arcs through several midwestern states, only here in its northern reaches did it become the exposed bedrock.
Eons later, when the glacial meltwater torrent came rushing down the St. Croix, it nosed out cracks in the basalt and blasted its way through, creating the valley, bluffs, and potholes.
Later, we kayaked upstream to the canyon, looking for the “cross” in the basalt walls that supposedly led French explorers to give the river the French equivalent name of Sacred Cross. We briefly debated each other’s interpretation of the cross’ location (only to find out later that the river took its name from a cross-shaped island downstream). Then we gave up the quest and let the current take us downstream to where the canyon widened. Soon, the basalt walls disappeared entirely, and the river slowed to a relaxing crawl.
The slower river told the human story. For ten thousand years, the St. Croix served Native Americans as a food source and waterway linking the Mississippi River to Lake Superior via a 20-mile northern portage. Euro-Americans arriving in the 1700s shifted the river’s focus first to fur and then to fir, decimating the beaver population for their pelts and then, after an 1837 treaty ceded the region to U.S. settlers, decimating the forests. An 1886 log jam drew media attention when it clogged the river for seven miles.
By 1914 the logging industry had run its course, and the St. Croix River returned to an earlier, more peaceful existence. It became popular for recreation and relaxation, but modern development still threatened to turn it into just one more polluted river. Another joint Minnesota-Wisconsin effort in the 1960s saw respective Senators Walter Mondale and Gaylord Nelson successfully propose the St. Croix National Scenic Riverway. In a National Park Service video, Senator Mondale proclaimed, “The idea that we’d let these areas be despoiled offends my idea of what America’s about.”
The Scenic Riverway now protects 255 miles of the St. Croix and its tributary, the Namekagon (Ojibwa for “place where the sturgeon spawn”) from unfettered development and pollution.
Even so, environmental threats are never far off. A proposed nickel mine near the St. Croix headwaters in northwest Wisconsin could potentially contaminate the river.
After kayaking, we returned to the bluffs. Amid the pothole formations on the Wisconsin side, near the bluff above the St. Croix River, sits the western terminus marker for the Ice Age Trail that wends 1100 miles east and south across the state, following the end moraines—the farthest edges—of the Wisconsinan glacier, another reminder of the glacial formations that shaped this landscape.
* * *
The North American continent once nearly split in two, and then the rift healed over. A wall of glacial meltwater once roared down the St. Croix River and blasted through a plug of thick basalt, carving bewitching potholes and a beautiful gorge. The furs and the firs were once decimated, but the creation of Interstate Park and the National Scenic Riverway designation has since offered protections to the St. Croix.
Time can heal wounds.
Even our wet camping gear dried out, and we set up our tent.
In the morning, the sun came out and set the St. Croix River sparkling.
-- September 2025
The river still rushes through the St. Croix Dalles today, offering just a glimpse of the glacial meltwater torrent that created the canyon.
An excursion boat navigates a challenging corner of the St. Croix River at St. Croix, Wisconsin.
A larger pothole on the blufftop is deep enough to stand in.
Over 250 miles of the St. Croix and its upstream tributary, the Namekagon, are designated as a National Scenic Riverway.