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  • Writer's pictureAsh, Lead Naturalist

A Geological Tour of Willow River State Park

Updated: Nov 11, 2020

Created By: Courtney Schlosser, Kari Hussay, Ashley Goldbeck & Additional Park Staff


Willow River State Park is an outstanding place to enjoy nature’s beautiful scenery and the geological processes they make. The exposed bedrock within the park consists of Cambrian Eau Claire Sandstone located below the Little Falls dam and Ordovician Prairie Du Chien dolomite located alongside the Willow Falls. The Cambrian time period refers to rocks that are approximately 500-570 million years old. The Ordovician period refers to rocks 450-500 million years old. Marine fossils such as gastropods, Trilobites, and Stromatolites may be found within these rock formations.

There are many examples of erosion that can be seen at the park. Erosion, or the wearing away process, is caused by the action of water, wind, and even the air we breathe. Weathering is one example of a type of erosion that is responsible for the breakdown of rock over time. There is also evidence of glaciation within the park. The Laurentide Continental Ice Sheet is the most recent form of glaciation, which occurred 10-15,000 years ago. As it advanced through areas of Wisconsin, it picked up rocks along the way. When the glacier(s) receded it left behind debris known as glacial erratic, which can also be seen here at the park.

Glacial drift, landforms, erosion and weathering have modified the landscape throughout the park. This guide will help you discover and learn about some of the geological phenomenons within the park, which can be seen right off of many of our established trails.

This field guide includes a map with each geological stop marked upon it to help you locate these featured areas. All geological stops must be walked to, but you will likely want to drive to different designated parking areas if you wish to visit them all in one day. There are a total of 10 stops, which are identified on the attached map. These stops include:

1. Mass Movement – Creep

2. Weathering Processes

3. Exposed Prarie du Chien Formation

4. Bacterial Growth

5. Glacial Erratic

6. Oxbow Lake Development

7. Exposed Eau Claire Formation

8. Point Bar and Cut Bank Erosion

9. Kettle Lakes

10. Moraine Topography

Let's get started shall we? Lace up those hiking boots and get ready to explore!


Geological Stop #1

Willow Falls Hill Trail

Length to Site: .2 miles (from Willow Falls lot)

Terrain: Very Steep, paved

Geological Formation Seen Here: Mass Movement (Creep)


The very steep hill along this trail leads you to the extraordinary waterfalls. On your way down, you will witness very steep ravines carved throughout the landscape on either side of the trail. When the soil on such steep terrain becomes saturated from heavy rainfall it becomes weak and fluid like. Mass downward movement of material driven by gravity often occurs on steep hills where instability is present. Keep an eye out for oddly bent tree trunks as you hike down. The particular mass movement seen along the right side of the trail is known geologically as the term creep. Creeps are triggered by weather related factors such as freeze, thaw or water seepage. The evidence for creep here is the bending of the tree trunks. The trees are trying to grow straight up and down but are being pulled downslope instead, causing the peculiar curved shape seen in these trunks.

Geological Stop #2

Willow Falls (right side)

Length to Site: .4 miles (from Willow Falls lot)

Terrain: Very steep, paved, then gravel

Geological Formation Seen Here: Mechanical Weathering


Overtime, surface conditions break down rocks. This process is known as weathering. There are two main types of weathering: Mechanical and chemical. I will also talk about differential weathering later on.

Mechanical weathering includes processes that break down a rock without changing its chemical makeup. An example of mechanical weathering can be seen along the right side of the trail near the willow falls where you can see tree roots growing up and through the rocks. Ultimately, breaking the rocks apart.

Oppositely, chemical weathering includes processes that break down a rock or mineral by altering its chemical composition. If you look closely, many of the rocks in the park have lichen growth on them. Lichen secretes organic acids, which remove ions from the rock for nutrients, changing the chemical makeup of the rock. The mineralogy of the rock determines the types of lichen growing on them. Lichenometry is the way geologist can tell time. The size of the lichen growth represents the length of time the rock has been exposed. The larger the lichen the longer its been exposed.

Geological Stop #3

Willow River Falls

Length to Site: .4 miles (from Willow Falls lot)

Terrain: Very steep, paved, then gravel

Geological Formation Seen Here: Exposed Prarie du Chien


The bedrock of the Willow Falls is mostly a sedimentary rock known as dolostone. There are also beds of weaker sandstone, together these make up what geologists call the Prairie du Chien Group. These rocks formed 450-500 million years ago in what was once a warm shallow ocean that covered Wisconsin and Minnesota during the Ordovician period. When creatures in the oceans died, their shells and bodies containing calcium carbonate (CaCO3) would fall to the ocean floor making a thick and limey mud to lithify into the rock we know today as dolostone.

The willow falls is also an example of differential weathering, which occurs when rocks and minerals erode at different rates, as an effect of variations in composition and resistance of a rock. This results in an uneven surface where more resistant material protrudes above softer or less resistant parts. In the case here, we see softer rock like the lightly cemented sandstone beds weathering faster then the harder more consolidated dolostone. As the water flows over the rock, it weathers the rock at different rates making up the different steps or “tiers” of the falls.

Geological Stop #4

Willow River Falls

Length to Site: .4 miles (from Willow Falls lot)

Terrain: Rugged, varied and slippery near edges

Geological Formation Seen Here: Bacteria growth along left side the Willow Falls


A bacterial growth is forming on top of the dolostone along the left side of the Willow Falls. This bacterial community is dependent on the nutrients that come from the rock. Dolomite contains calcium, magnesium, carbon and oxygen. However, dolomite may contain some iron impurities. If you look closely there is water seeping out of the rock, bringing with it iron oxide, which is staining the rock face a rusty orange color.

If you look at the bacteria present in this community (cocci, coccus, baccili and strepto bacillus. They appear to be a rust/orange color like the sedimentary rock they live in here at the falls.

Geological Stop #5

Nelson Farm Trail

Length to site: .5 miles (from Willow Falls lot)

Terrain: Flat, gravel

Geological Formation Seen Here: Glacial Erratic


Evidence of glaciers can be seen throughout the park but the Nelson Farm Trail heading is a great location to see these massive granite rocks which have been transported from their original foundation and left to rest here along the lefthand side of the trail. These large boulders are now known as glacial erratics. Glaciers have the ability to move rocks great distances; these particular boulders came from northern regions, carried by the Laurentide Continental Ice Sheet, which was the most recent glaciation that took place 10,000-15,000 years ago. You can also see large boulders like these along the edge of the yellow pioneer trail and near the nature center.

Geological Stop #6

Nelson Farm Trail

Length to site: 1.5 miles (from Willow Falls lot)

Terrain: Flat, gravel

Geological Formation Seen Here: Oxbow Lake Development


As rivers flow over time they often change course. Water always wants to take the path of least resistance; if a bend in the river becomes too wide, the main current will begin to form an easier route. The new route eventually cuts off the other one off from the main flow and is abandoned. Forming what is called an oxbow lake. You can see one is forming here alongside the trail. There is also another oxbow lake forming down river along the purple Trout Brook Trail.

Geological Stop #7

Little Falls Dam

Length to site: .5 miles (from Nature Center/ Beach parking lot)

Terrain: Flat, paved

Geological Formation Seen Here: Exposed Eau Claire Formation (west of the river below the dam)


Below the Little Falls Dam you will spot the great Eau Claire rock formation, which is approximately 500-570 million years old and was formed in an ancient shallow sea. If you look closely at the bedrock, you’ll see it is tilting to the right. This dip is a result of the fault breaking below and tipping the rock to the side.

The rocks are cross cut by a reactivated Keweenawn-age fault. The northeast facing fault is know as the Hasting fault and is an extension of the Lake Owen fault. This fault occurred because of the Midcontinent Rift System, which pushed the crust apart approximately one billion years ago. This old fault was reactivated and moved again after the sedimentary rocks in the park were deposited from glacial activity.

The bedrock here is most visible in the fall when most of the trees are bare of their leaves. If your interested at getting a closer look at this sandstone formation there is a trail that leads you down below the dam where you are able to look directly across the river at it.

Geological Stop #8

Purple Trout Brook Trail Bridge

Length to site: 1 mile (from Nature Center/ Beach parking lot)

Terrain: Flat, gravel

Geological Formation Seen Here: Point Bar And Cut Bank Erosion


This trail follows the flowing waters of the Willow River and offers great examples of the force moving water has. Overtime, water is powerful enough to erode rock, soil and stream banks as it twists and turns downstream. These twists and turns are known as meanders. Meanders have two sides, the point bar and the cut bank. The point bar is an area of deposition formed on the inside of the meander. Water flows slowly here, allowing for particles to settle. The cut bank side is formed as water flows around the outer curve and increases speed. As the water moves faster, it is becomes erosive and eventually forms a steep bank. On the cut bank side of a meander you will sometimes seen large rocks piled along the river. This structure is known as a riprap, which is a man-made preventative measure to minimize bank erosion.

Geological Stop #9

Brown Oak Ridge Trail

Length to site: .7 mile (from Nature Center/ Beach parking lot)

Terrain: Hilly, gravel

Geological Formation Seen Here: Kettle Lake And Moraine Topography


Kettles lakes are holes or depressions formed by ice chunks, which have broken off of glaciers and then were partially buried by melt water sediments. These depressions can fill up with water, making kettle lakes. Kettle lakes today are typically found in outwash plains or flat regions of melt water at the edge of glaciers. Here we have evidence of an old kettle lake that would have been along the edge of a retreating glacier. If you continue hiking up the trail towards the last stop on our tour, it would be difficult not to notice the hummocky topography leading up to the most obvious moraine topography in the park. Hummocky topography consists of irregular mounds of glacial debris, a result of thinning ice. The large ice blocks can cause debris to squeeze out of the ice depositing in the valley between these ice blocks. This deposition of debris forms the steep ridges and hummocky landforms called moraines. Moraines are typically made of unsorted glacial drift called till. This till can be found near several of the geological stops within the park.

Geological Stop #10

Orange Knapweed Trail

Length to site: 1 mile (from group camp overflow parking lot)

Terrain: Slightly hilly, mowed trail

Geological Formation Seen Here: Terminal Moraine


terminal moraine, also called an end moraine, is a type of moraine that forms at the edge of a glacier, marking its maximum advance. At this point, debris that has accumulated by being pulled, scraped or pushed by ice, is dumped into a heap. Here under all of this prairie grass, lies evidence of massive glacial activity in the area many years ago. Below is a diagram depicting what the process looks like.


This concludes the geological stops within this field guide. We hope you enjoyed learning about and discovering some of Willow River State Parks beautiful scenery and the geological landforms. Now that you are equipped with some background knowledge on these different types of landforms, we encourage you to see if you can identify similar geological features elsewhere in the park and even beyond this area. There are several state parks in Wisconsin that hold a history rich in geological features like the ones discussed in this field guide, and more!

Have fun exploring and as always, feel free to share your experiences with us. We love hearing from you!

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