Tasks:
1. How can you tell this is sandstone? (see description to help you!)
2. Do you see evidence of weathering on the Penokean Wall? If so, is it physical or chemical, or both? What are those forces?
3. What weathering forces do you think have the greatest impact on how fast the wall will crumble?
4. Take a photo with your favorite rock formation! (optional)
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You can enter the dry creek area and walk down it to a trail that is near WP3. From there head north to the Penokean Wall and Devil's Elbow.
Sandstone wall
The foundation of what we are seeing today was part of the Penokean Orogeny (an orogeny is when part of the earth's crust folds or deforms into a mountain range because two land masses collide). The Pembine-Wausau island arc and Marshfield Terrane collided to the south between 1.9 and 1.8 billion years ago, forming the Penokee Mountains northeast of here. The (mostly) sandstone cliffs you see here today was deposited as the Penokee Mountains eroded - the eroded (sand) grains coming off of the mountains went into streams and into a shallow sea, which is where this sandstone came from. The sand built up over time and was buried deep under the earth's surface, where the pressure and the movement of water and minerals through the porous sand cemented the sand grains together. Some sandstone metamorpozed into quartzite about 1.7 billion years ago (quartzite is very common in the Blue Hills). Some of it did not, and we can see those sandstone outcrops today.
Sandstone usually has the following properties
- The sand particles are well sorted
- The particle range is in size from 1/16 millimeter to 2 millimeters in diameter.
- The color may be from gray, brown or red.
- The sandstone is relatively soft compared to other rocks.
Weathering
Sandstone is a weaker rock than many others - there is quartzite in the Blue Hills as well, and that is much harder and resists weathering to a greater extent. Sandstone can 'crumble' into sand grains pretty easily, while quartzite breaks off in angular blocks. If you hike to Gundy's Canyon or the Cascading Waterfall, you'll be able to see how angular those pieces are compared to the rounded edges on this feature.
Weathering is the breaking down of rocks through contact with the Earth's atmosphere, water, and biological organisms. Two important classifications of weathering processes exist – physical and chemical weathering.
- Physical/mechanical weathering breaks down the rock without altering its chemical composition - the material is broken down - just the size of the material is affected by this type of weathering. Some examples include frost wedging, abrasion (such as sand-blasting), and plant roots.
- Chemical weathering actually changes the chemical makeup of the rock. Examples include reactions when in contact with water, oxygen and/or acids - this could be rain, acid rain, pollution, the atmopshere, biologically procuced chemicals). Chemical weathering can involve dissolving the rock which forms new substances. Oxidation (rusting) changes the chemical composition of the rock and is also chemical weathering.
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Permission has been granted by Jerrad Macholl of the Rusk County Forestry Department.
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Sources:
https://www.mountainproject.com/area/116608475/the-penokean-wall
https://m.youtube.com/watch?v=nl5j5Aq7pts&t=56s&pp=2AE4kAIB
https://ruskcountywi.com/trees/wp-content/uploads/The-Outsiders-Guide-to-The-Blue-Hills-Natural-Areas.pdf
https://www.geocaching.com/geocache/GC80T52