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The Dells of Eau Claire:
The majority of people that visit this county park have absolutely no idea of what it took for Mother Nature to create this landscape. Actually, very few people have ever even seen a landscape that looks anything close to this! I have tried to explain some of the factors involved in the creation of this landscape below. I hope this helps solve the mystery of Marathon County.
Volcanics:
The rocks found here were formed almost two billion years ago by massive volcanoes that were located in central Wisconsin. These volcanoes spewed huge amounts of magma, which eventually cooled and hardened into jagged horizontal layers. These layers are made of Rhyolite, one of the hardest rocks found on the planet. After millions of years of tectonic shifting, these horizontal Rhyolitic layers have become tilted more toward vertical.
Glacial Scouring:
Eventually the volcanic eruptions subsided. Millions of years later along came the ice ages with there multiple glacial advances and retreats. These erosive glacial movements soon ground the jagged volcanic mounds down into the smooth rolling hills that we see today.
Water Erosion:
1500 years ago, global warming began, causing the glaciers to melt and recede. This caused the release of massive torrents of water which carved deep gorges into the tilted layers of Rhyolite. In places, the river tumbles and spills, cutting across the rock’s cleavage planes. At others, the river flows smoothly running parallel with the cleavage planes.
Potholes:
Silt, sand and soil, held in suspension by rapidly flowing and swirling river waters, erodes localized weaknesses in the Rhyolite bedrock, creating small depressions or holes. Small rocks and pebbles then become trapped within these holes and with a continued corkscrew grinding action, enlarge them. Eventually larger rocks, called grinders, become caught in these eddy pools and expand the holes even farther, creating what are called potholes.
Scallops
Scallops are asymmetrical depressions eroded into rock faces from the effects of fast moving and swirling river waters. They have a sharp lip that drops abruptly into a depression on their downstream side. Scallops can be used to tell the direction of river flow even after the river has long since changed course and moved on to other channels.
Joints:
Almost all types of rock contain joints. Joints are basically fractures that divide rock into multiple sections but do not penetrate through the entire rock. Pieces of rock are still attached to each other with enough strength that they still move as one. Joints generally occur in multiples and normally at regular distances, parallel to each other. The mechanical properties and thickness of the rock layers govern the spacing and number of joints. Some rock has joints in just one direction while other types of rock have joints in two or three different directions. Here you will see parallelogram jointing in three distinct directions. These joints have separated sufficiently as to allow small amounts of water to enter into them. With freezing, ice wedging now occurs and the pieces break completely free from each other. Flowing river water, with its erosive properties, also works at widening these joints until more separation and breakage occurs.
At The Dells:
Along this river you will find naturally occurring block-type rock formations coupled with smooth rounded edges. It took volcanism, glacial scouring, jointing, chemical weathering, water (fluvial) erosion and many other processes and factors, working in unison, to form this landscape.
Coordinates listed are for Southside parking and a good spot to start. The entire hike is less than a mile and a half and makes a complete loop. From the parking coordinates, walk north until you reach the river (400 feet). At the river turn left and follow the trail to the West. Once you reach the bridge, cross over and head back to the East along the North side of the river. Once you reach the dam, cross back over to the South side, and head west once again, until your back where you started. If you follow these directions, the waypoints listed below will be in order as you hike the trail.
Email your answers to the questions, to me, using the link in my profile only. If your answers are not recieved by me in an appropriate amount of time, your log will be deleted. Photos are accepted and appreciated as long as the answers to the questions are not revealed. You do not have to wait for confirmation from me before logging this cache as completed. Most of all……learn……and enjoy the view.
1. Why do you think the sand (erosional particles) were deposited at this location and not farther back upstream?
2. Does the water flow across, or parallel to, the joints in the rock at this location? How does that effect erosion and what does the eroded area look like?
3. Does this hillside pillar show signs of fluvial erosion, such as potholes or scallops?
4. Does the water flow across, or parallel to, the joints in the rock at this location? How does that effect erosion and what does the eroded area look like?
5. How large is the pothole at this location? Are there any Grinders remaining inside?
6. At this abandoned river channel, what was the original direction of river flow?
Additional Hints
(No hints available.)