Welcome to The Natural Recession 💵💰
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This rock exposure near Ashland, Kentucky lies within the Appalachian Plateau, a region shaped by millions of years of sediment deposition, uplift, and erosion. The rock layers visible here were deposited during the Pennsylvanian Period, when eastern Kentucky was covered by river systems, floodplains, and swampy lowlands. Over time, layers of sand, silt, and mud were buried, compacted, and cemented into sedimentary rock.
Today, erosion has cut into these layers, revealing a striking rock face with horizontal bands and a curved, concave profile. This shape tells a story about the composition of the rock and how different layers respond to weathering.
Why the Cliff Curves Inward
1. Softer layers erode faster
Shale and siltstone are made of very fine particles and tend to:
- Absorb water easily
- Break apart when wet
- Fracture during the freeze thaw cycles
As rainwater runs down the cliff face, it preferentially attacks these softer layers. Over time, they erode backward into the cliff, creating recesses.
2. Harder layers form protective ledges
Sandstone is composed of coarser sand grains cemented together, making it:
- More resistant to weathering
- Less prone to rapid breakdown
As the shale beneath erodes away, sandstone layers remain intact longer and begin to overhang, forming ledges. These ledges help shield the softer rock below from direct rainfall, further enhancing the inward curve.
3. Gravity and block failure
Once the softer layers beneath are undercut:
- The overlying sandstone loses support
- Large blocks crack along natural fractures
- Sections eventually fall away
This cycle repeats over time, gradually migrating the cliff face backward and maintaining its concave profile.
4. Chemical weathering and staining
Water moving through the rock dissolves minerals, especially in the shale layers. Iron-bearing minerals oxidize when exposed to air and water, producing the rusty orange and reddish streaks seen along bedding planes. These stains highlight where water flows most frequently and where weathering is most active.
What Rock Color Tells Us About Weathering
The different colors visible in the rock layers at this site provide important clues about rock composition and how each layer responds to weathering in this area.
- Light tan to yellow layers are primarily sandstone. Their lighter color comes from quartz-rich sand grains and relatively low organic content. Sandstone is generally more resistant to erosion, so these layers tend to form ledges and overhangs.
- Dark gray to black layers are shale or siltstone, made of very fine particles that often contain organic material. These darker layers absorb water more easily and break down faster, making them more easily eroded. As a result, they commonly form recessed bands in the cliff face.
- Rusty orange or reddish staining along some layers indicates iron oxidation, where iron-bearing minerals react with air and water. These stains often mark zones where water flows frequently and where chemical weathering is actively occurring.
Together, these color differences show that erosion does not affect all rock layers equally. Softer, darker layers wear away faster, while lighter, harder layers remain intact longer which is a key reason this rock face has developed its distinctive concave shape.
LOGGING REQUIREMENTS
Please complete the tasks and questions below, and send me your answers
1) Look at the rock face and identify two different layers in terms of the color of each layer and whether each layer forms a ledge or a recess.
2) Of those layers, which one appears to erode faster, and what evidence do you see that supports your answer?
3) Take a photo of yourself (or personal item) at this location. You can attach it to your found it log (required).