This cache was placed with permission from the Hormel Nature Center. For rules and hours, please visit their website here.
Beginning millions of years ago, Minnesota went through a massive series of events that bulldozed the land. Glaciers advancing and then retreating carried rocks, pushed dirt, and carved lakes and rivers across the state. The last event was the Wisconsinian Age where a wall of glacial ice stretched across much of the state into present day Iowa. When it retreated 12,000 to 14,000 years ago, it left behind piles of rocks and dirt that are called Glacial Till.
Austin, MN sits in the area called the Des Moines Lobe of the last glacier area. This massive lobe is responsible for the glacial formations we can see today such as moraines, glacial erratics, and the lakes and rivers in the area. When the glacier advanced into the area, it brought with it dirt and rocks. Once the ice melted away, the dirt and rocks it brought with it was left behind. In areas where the glacier pushed the earth, it formed what are called Moraines. These are long and skinny ridges in the landscape best seen by a high elevation or on a topography map. Rocks encased in the glacier were left sitting where they were, creating what we call Glacial Erratics. These are stones brought from distant lands that do not match the surrounding stone types. Lastly, when the ice melted away, the water created had to go somewhere. As water rushed across the landscape, it carved away the soft, loose soil until it reached the much more resistant bedrock layer. Huge glacial rivers slowly died down and became the rivers we know today. Pockets of earth dug out by the glaciers were also filled in creating lakes as well!
Before you is a massive Glacial Erratic. This erratic weighs 125 tons and is 3,740 cubic feet. That’s almost 4 school busses of rock! Pressure from the earth turned the granite into gneiss. This boulder was then plucked up by the Des Moines Lobe of the glacier and transported to the area. We know the boulder is not from here as it differs from the bedrock seen in the area. Austin consists of a Limestone/Dolostone bedrock. These types of stone are a sedimentary rock, where layers of sandy soil were compressed with immense pressure to create it. Picture pages of a book pressed between the covers. The boulder here however is a metamorphic rock type. The granite which was formed via lava cooling, was put under heat and pressure that changes its characteristics. Throughout the state, different types of erratics can be found depending on the direction of the glacier that last visited. Areas where the glacier originated from the North-East tend to have a reddish colored erratic, whereas glaciers from the North-West typically brought grey erratics with them.
Throughout the park, you can see evidence of the last glacier. A walk along the river gives you a peek into the layer of soft soil deposited here by the glacier, and if conditions are right, the layer of bedrock may appear! A walk to the outlook tower gives you an idea of the hills of glacial till left behind with a bird’s eye view on top of the tower.
Carried many miles only to break down here!
The rock types we are studying here are gneiss and limestone. Both these stone types are affected by erosion in similar and different ways. The following types of erosion are as follows:
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Freeze-Thaw:
Water seeps into cracks in the rock, freezes, expands, and exerts pressure, eventually causing the rock to fracture and break apart.
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Temperature Changes:
Repeated heating and cooling can cause the minerals in rocks to expand and contract at different rates, leading to stress and fracturing.
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Abrasion:
Wind and water can carry particles that rub against the surface of rocks, gradually wearing it away.
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Root Expansion:
Plant roots can grow into cracks in the rock and exert pressure, causing the rock to split.
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Hydrolysis:
Water can react with certain minerals in gneiss, such as feldspar, and alter their chemical composition, leading to the formation of new clay minerals. Water seeping through limestone crates a calcium-carbonate rich solution that when exposed to open air, forms solid formations such as stalagmites and stalagtites in cave systems.
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Oxidation:
Iron-bearing minerals in rocks can react with oxygen, forming iron oxides (rust), which weaken the rock.
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Dissolution:
Certain minerals in rocks, like calcite, can dissolve in acidic water, leaving behind voids and weakening the rock structure.
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Plant and Animal Activity:
Plants can penetrate cracks in the rock and their roots can exert pressure, while animals can burrow and erode the rock. This can also be seen by humans walking the same path over limestone that wears down, or by carvings in the stone.
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Requirements to log this Earthcache:
Please send me the answers to the following questions either via email or messenger system.
1) From reading the cache page, what direction do you believe the erratic originated from?
2) Go to the waypoint named "Bedrock Example". Observe the Limestone used to make this seating area. What differences do you see from the glacial erratic? Include colors, grain size, and any erosion.
3) Go for a hike to the lookout tower. While on your hike, do you observe any of the signs of glacial activity mentioned in the cache page?
4) Please include in your log a photo of yourself or a personal item of something you found unique or interesting while logging this earthcache. A small description of what you found is appreciated!