About 500 million years ago, a shallow sea covered the area, laying down layers of sand and minerals that make up much of the sandstone bluffs now seen along the river.

citation

Wikipedia contributors. (2023, November 7). St. Croix River (Wisconsin–Minnesota). In Wikipedia, The Free Encyclopedia. Retrieved February 6, 2024, from https://en.wikipedia.org/w/index.php?title=St._Croix_River_(Wisconsin%E2%80%93Minnesota)&oldid=1183996131

St. Croix River (Wisconsin–Minnesota) (Wikipedia) The St. Croix River (/ˈseɪnt ˈkrɔɪ/ SAYNT KROY; literally “holy cross” in French) is a tributary of the Mississippi River, approximately 169 miles (272 km) long, in the U.S. states of Wisconsin and Minnesota. The lower 125 miles (201 km) of the river form the border between Wisconsin and Minnesota. The river is a National Scenic Riverway under the protection of the National Park Service. A hydroelectric plant at the Saint Croix Falls Dam supplies power to the Minneapolis–St. Paul metropolitan area.