I finally have my new computer and gotten as far as installing my usual suite of apps. While I wait for things to install or move from backups to the HD, I started playing with WM4049. This brought me back to the soil-mantled erosion example. After tweaking things for a bit, I decided to add a bit of debris, piped in to the height and texturing… as I was saving the prototype, I decided to look up a proper name or description.
Scree and Clasts
In geology, there is a specific term for the rocks and boulders displaced significant distances by water flow from where they spawned from thermal erosion—such rocks are typically referred to as sediment or clasts, and when transported by water over significant distances, they may be classified as alluvium, fluvial deposits, or glacial erratics, depending on context.
Here’s how these terms break down in geological usage:
Key Terms for Displaced Rocks
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Sediment / Clasts
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These are general terms for fragments of rock broken off from a parent source. Clasts refers specifically to the broken pieces, while sediment includes both clasts and finer particles like silt and sand.
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Alluvium
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Alluvium refers to loose, unconsolidated sediment (including boulders, gravel, sand, and silt) deposited by flowing water, typically in riverbeds, floodplains, or deltas. If thermal erosion (e.g., freeze-thaw cycles) breaks the rock and water carries it away, the transported material becomes alluvium.
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Fluvial Deposits
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These are sediments laid down by river action. The term emphasizes the process of deposition by flowing water. Boulders moved by rivers—especially during floods—are part of fluvial systems.
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Glacial Erratics
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If the displacement is due to glacial meltwater or ice movement, large boulders transported far from their origin are called erratics. These are often geologically distinct from the surrounding bedrock.
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Talus / Scree
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While not transported far, these terms refer to rock fragments that accumulate at the base of cliffs due to thermal or mechanical weathering. If water later moves them downstream, they become part of sedimentary transport.
Thermal Erosion as the Source
Thermal erosion—especially freeze-thaw cycles—can fracture rock, creating clasts that are then mobilized by water. This process is a form of physical weathering, and the subsequent movement is classified as erosion and transport.
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Weathering breaks the rock.
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Erosion removes it from its origin.
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Transport moves it via water, ice, or wind.
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Deposition lays it down elsewhere.