Each agent of erosion and deposition leaves its own marks on the land. Learn them, and a landscape (or a topographic map) tells you what shaped it.
| Agent | Erosion makes | Deposition makes |
|---|---|---|
| Rivers | V-shaped valleys, canyons, meanders (wide bends) | Floodplains, deltas, sandbars; oxbow lakes from cut-off bends |
| Glaciers | U-shaped valleys, sharp peaks (horns), bowl-shaped hollows (cirques), scratched bedrock | Moraines and scattered boulders (erratics), all unsorted |
| Wind | Polished, pitted rocks; hollows where sand was blown away | Sand dunes; layers of windblown dust |
| Waves | Sea cliffs, sea arches | Beaches, sandbars, spits |
Key idea
Rivers cut V-shaped valleys and drop sorted sediment; glaciers carve U-shaped valleys and drop unsorted piles; wind builds dunes.
Why it works
A river cuts only along its narrow bed, down into the rock, while the valley sides slump in: a V. A glacier is a huge tongue of ice that fills the whole valley and grinds its floor and sides at once, rounding it into a U. And because ice is solid, it carries boulders, sand and clay together and drops them together when it melts. Water and wind sort grains by size; ice does not.
Worked example: Reading a valley
A mountain valley has a broad, rounded floor with steep sides, and its floor is scattered with boulders of rock found only far upstream. What shaped it?
- A broad U shape points to a glacier.
- Boulders of distant rock are glacial erratics, carried by ice.
Answer: A glacier
Worked example: Sorted or not?
Deposit 1 has neat layers of sand. Deposit 2 is a jumble of clay, sand, pebbles and boulders. Which was left by a melting glacier?
- Water and wind sort grains by size into layers.
- Ice drops everything together, so the unsorted jumble is glacial till.
Answer: Deposit 2
Common mistake
Expecting glacier deposits to be sorted by size like river deposits. Ice drops boulders, sand and clay together in one jumble, called till. Check: if a deposit has big boulders sitting in fine clay, think ice; neat layers of one grain size point to water or wind.