limestone

A small rockslide near Cascade Falls has exposed the bright orange and yellow limestone layers. According to geologists, similar small erosion events have shaped the creek valley. Photo by Phillip Bock

A  chunk of sandstone fell from the bluff near Cascade Falls earlier this month, exposing the vibrant oranges and yellows of the sandstone underneath and slowly shaping the landscape. The rockslide was minor, but according to geologist, similar events have shaped the valley as we know it over thousands of years.

“What’s happened there recently is what’s been happening over thousands of years to have the waterfall end up being where it happens to be,” Kerry Keen, a professor of geology at UW-River Falls, said. “All waterfalls migrate upstream. The waterfall in Osceola today used to be closer to the St. Croix.”

According to geologists, much of the sandstone around Osceola dates back to the Cambrian period, roughly 500 million years ago. At that time, much of the area was covered by shallow seas, Keen said, which deposited sand and sediment that, over millions of years, compressed into the sandstone we see today.

Glaciers during the last ice age, too, shaped the valley and deposited silt and sand that eventually developed into the rock formations in the St. Croix Valley. Rivers, such as the St. Croix, carved through the soft sandstone over thousands of years, creating the valleys.

“All materials have a hardness,” Keen said, explaining that harder rocks take longer to erode. “Cascade Falls is falling over mostly sandstone of various resistances.”

The St. Croix originally flowed at a higher altitude, Keen said, and, as it carved deeper into the bedrock, a falls was likely created on what is now Osceola Creek. Over the last 14,000 years, the falls have been eroding further into the valley — one small rockslide at a time.

“Eventually the glaciers went away, and the streams since 14,000 years ago have been cutting down and establishing the levels they are at now,” Keen said.

Rock compositions and water flows play a role in how fast erosion occurs. Osceola Creek has a relatively low water flow, and the current rock underneath the falls is harder and more resistant to erosion than some of the upper sandstone layers, Keen noted.

“It takes longer to cut down and work its way back,” he said. “Resistant layers can stick out and last longer, which creates an interesting profile.”

Recent rains, coupled with colder temperatures, likely played a role in the recent erosion event. Rain seeps through cracks in the sandstone and, if frozen, can create wider cracks in the sandstone that eventually cause the rock layers to slide apart.

“It’s been doing this for a long time,” Keen said. “These little pulses of events are probably how it happens.”

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