Earth Science Lesson: Weathering Basins (Gnammas)
What Are They?
A weathering basin — also called a solution pit or gnamma — is a naturally formed, bowl-shaped depression in the surface of exposed bedrock.
These hollows develop over centuries or millennia as rainwater, air, and living organisms slowly dissolve, erode, and smooth the rock.
They form most often in sandstone, quartzite, or granite, and can range from a few centimeters to over a meter wide.
How They Form
- Water Accumulation:
Rainwater collects in tiny irregularities on the rock’s surface, beginning the cycle of erosion.
- Chemical Weathering:
Slightly acidic rainwater (carbonic acid from CO₂ and organic decay) dissolves the mineral cement between sand grains.
- Physical Erosion:
Freeze–thaw cycles and wind abrasion enlarge the hollow.
- Biological Activity:
Lichens and moss on the rim produce mild acids that chemically dissolve the rock.
- Sediment Trap:
Leaves, pine needles, and soil collect in the hollow, trapping moisture and forming a micro-ecosystem that speeds up weathering.
At This Location
You are standing in front of a large sandstone boulder showing a classic example of a weathering basin in its upper surface.
Look closely and you’ll see:
- A rounded cavity that likely holds rainwater after storms.
- Lichens and moss along the rim and surface.
- A smooth inner wall without tool marks.
- Leaf litter and soil buildup inside the depression.
These are the hallmarks of a naturally formed gnamma.
Geological Context: Heber Springs Sandstone
This rock is part of the Atoka Formation, a Pennsylvanian-aged (~300 million years old) sandstone found across north-central Arkansas.
These rocks were deposited as river and delta sands into an ancient inland sea.
Over millions of years, tectonic uplift and erosion exposed them to the surface — where weathering processes like this basin began to shape the rock.
Your Task: Investigate the Basin: Use scientific observation to decide:
~ Was this basin shaped by natural weathering, or could it have been carved by human activity?
Observe Each Process and Record What You See:
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Process
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Evidence to Look For
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1. Water Accumulation
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Does the hollow collect rainwater? Is it positioned to trap runoff?
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2. Chemical Weathering
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Are the surfaces smooth and naturally curved, or sharp and carved?
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3. Physical Erosion
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Do the edges show flaking, cracks, or softening over time?
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4. Biological Activity
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Are lichens, moss, or roots present on the rim or inside?
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5. Sediment Trap
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Do you see leaves, pine needles, or soil collecting inside?
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Now compare your findings to the human-made clues:
- Sharp or symmetrical edges
- Tool marks or uniform carving
- Multiple similar hollows nearby
If none of these above are present, your evidence supports natural formation.
The Science of Weathering
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Type of Weathering
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Description
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Example in This Feature
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Chemical
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Breakdown by acids and reactions with rainwater
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Acidic water dissolves sandstone cement
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Physical
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Breakdown by force (freezing, wind, roots)
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Freeze–thaw expansion enlarges basin
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Biological
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Breakdown by plants, fungi, and lichens
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Lichens release organic acids on the rim
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Life on the Rock
Lichens and moss are small but powerful geological agents:
- They produce mild acids that chemically etch the rock.
- They trap dust and moisture, forming tiny soil pockets.
- Their slow growth marks long exposure and clean air.
This living layer shows how life interacts with the Earth’s surface — a connection between the biosphere, hydrosphere, and lithosphere.
Logging Tasks
To claim your find:
- Describe:What features of the basin suggest it was formed naturally rather than carved?
- Identify: What types of biological growth (lichen, moss, roots, etc.) do you see on or inside the rock?
- Explain:Which of the three types of weathering (chemical, physical, biological) do you think has been most active here? Why?
- Optional Photo: Post a photo of yourself or a personal item near the boulder (no spoilers of the exact feature, please).