|Rocks forming the cliffs that Willow River cuts through were formed about|
half-a-billion years ago.
Underlying the entire county is bedrock that is at least 1.1 billion years old, formed when the North American continent began to split in two. From this rift flowed thousands of feet of lava. In the county, these flaws included the area northwest of where the Willow River runs today while the rest of the county mainly contains clastic rock, which is weathered rock that rivers dumped into the rift’s depression.
Fast forward a half-billion years. At that time – the Cambrian and Ordovician periods – the county sat in a sea off the edge of North America in which northern Wisconsin was a high coastal area. As rivers carried sediment off these eroding mountains, the sand and silt settled in the sea for more than 100 million years. As sea life became more complex near the end of that period, their falling shells settled in the sediment, creating the marine limestone underlying much of the county’s topsoil. Cliffsides of this rock can be seen today at Willow Falls where the Willow River has cut through it; it can be reached via the Willow Falls Hill (Gray) Trail at the state park.
Two ancient faults during that time offset the rocks layers by as much as 400 feet in the county. One fault runs north of Hudson. The other one, known as the Hudson Fault, sits in Willow River State Park on the eastern side of Little Falls Lake. You can walk over the latter fault on the Willow Falls (Blue) Trail at Willow River State Park.
Fast forward nearly another half-billion years. About 2.5 million years ago, the Earth entered a series of ice ages. Much of the landscape seen today in St. Croix County exists as it does because of those glaciers. During one of those glacial periods more than 100,000 years ago, an ice sheet engulfed the entire county.
During the last ice age, which began about 100,000 years ago and ended a mere 8000 years ago, the leading edge of the Superior Lobe of the Laurentide Ice Sheet covered the county’s northwestern corner. It formed moraines – ridges of rock and sediment carried by glaciers – and kettle lakes, where melting ice broken from retreating glacier formed a depression, with the trapped meltwater forming a waterbody. Moraines are visible on Mound (White) Trail at Willow River State Park and a kettle lake on the Siem Trail, located in the Homestead Parklands on Perch Lake.
Most noticeably, the St. Croix River served as a major drainage for meltwater from retreating glaciers with flashfloods scouring out the St. Croix River Valley. The Hudson Pier, at Lakefront Park in downtown Hudson, runs to the center of the river valley. During the height of the glacial drainage, the pier would have been a few hundred feet under the river.
Learn about trail guidebooks available in the Hittin’ the Trail series.