The soil transported by the gravitational forces is called _______
Try it first: pick an option, then read the explanation.
Detailed Explanation: Colluvial Soil
Colluvial soil is formed when rock and soil fragments, loosened by weathering on a slope, move downhill under the direct action of gravity — through processes such as slow creep, slumping, or landslide-type movement — and accumulate at the base of the slope or hillside. Unlike alluvial or aeolian soils, no water or wind is the primary transporting medium; gravity alone does the work.
A Practical Example: Black Cotton Soil
Colluvial deposits are often cited alongside black cotton soil as a practical example in Indian geotechnical textbooks. Black cotton soil is highly cohesive and plastic in its natural (relatively dry) state, giving it noticeable strength — but this strength drops sharply once it comes into contact with water, since the montmorillonite-rich clay swells and softens dramatically. This behaviour makes colluvial/black cotton soils notoriously problematic for foundations, roads, and canals unless properly treated or bypassed.
The other options describe soils moved by different agents: alluvial soil (option a) is transported by flowing water; loess (option c) is a fine, wind-blown (aeolian) silt deposit; and till (option d) is unstratified material deposited directly by melting glacial ice. Since the question specifies gravitational transport, colluvial soil is the correct answer.
| Soil Type | Transporting Agent | Typical Example |
|---|---|---|
| Alluvial | River / running water | Gravel, sand |
| Aeolian | Wind | Loess, dune sand |
| Glacial | Ice / glacier | Till, drift |
| Colluvial | Gravity | Talus, black cotton soil |
Key Concepts for Students
- Gravity is the defining agent for colluvial soil — no water or wind transport is involved, only downslope movement under the soil's own weight.
- Talus is a related term: the accumulated rock/soil debris at the foot of a slope is often called talus, and is a classic colluvial deposit.
- Black cotton soil is expansive: it loses strength dramatically on wetting due to swelling clay minerals, making it a frequently tested "problem soil" in exams.
- Colluvial slopes require special attention: they are prone to further movement (creep, slides) especially when saturated, which is critical for road and hillside construction.
- Memory hook: "co-" as in "collapse downhill" — colluvial soil collapses/creeps downhill under gravity.