After weathering breaks rock into sediment, something has to move it. Erosion is the carrying away of sediment by moving water, wind, ice or gravity. Wherever the carrier slows down, it drops the sediment: deposition.
Key idea
Moving water, wind and ice pick up sediment and carry it; when they slow down, they drop it, largest grains first. Every grain eroded somewhere is deposited somewhere else.
Why it works
The faster water or wind moves, the bigger the grains it can push or hold up. A flooded mountain stream rolls boulders; a slow river carries only fine mud. When a river enters a lake, it slows down, so it drops gravel first, near the shore, then sand, then silt, and the finest clay settles far out in calm water. This sorting by size is a fingerprint of water and wind.
- Water (the biggest mover): river deltas, sandbars, beaches, floodplains.
- Wind: sand dunes and layers of windblown dust.
- Ice: glaciers scrape up rock of every size and drop it in heaps called moraines.
- Gravity: landslides, rockfalls and slow soil creep down slopes.
Worked example: A delta
A muddy river flows into the sea. What forms at its mouth, and why?
- The river slows down as it spreads into the still sea.
- Slower water can't carry as much sediment, so it drops it, building a delta.
Answer: A delta, because the water slows and drops sediment
Worked example: Which settles first?
A fast stream carrying gravel, sand and clay flows into a pond. In what order do they settle?
- Big, heavy grains need fast water to stay moving.
- So the gravel drops first, then the sand, and the clay last, far from the inlet.
Answer: Gravel, then sand, then clay
Common mistake
Thinking erosion only destroys land. Every grain carried away is set down somewhere, building deltas, beaches, dunes and floodplains. Check: the Mississippi Delta is made of sediment eroded from much of the central United States.