Mass movements, erosion, and deposition

Gravity pulls material downslope in mass movement. Exam tip: connect it with exogenic geomorphic processes.

The direct answer is B, slowly tilted poles and bent fences. Soil creep is the extremely slow, gradual downslope movement of loose soil and weathered material under gravity. Because the movement is slow, it may not be noticed from one day to the next. Over months or years, however, objects fixed in the ground slowly lean downhill. This creates tilted poles, bent fences, curved tree trunks, and sometimes small step-like features on a slope. Option A, sudden deep gorge formation, is incorrect because a gorge is mainly produced by strong river erosion, not the slow mass movement called soil creep. Option B is correct because poles and fences record the long-term movement of the soil beneath them. Option C, a mountain collapsing in one day, describes a sudden landslide or collapse, not creep. Option D, a river immediately forming a delta, concerns deposition at a river mouth and is unrelated to slope movement. The reasoning is therefore: soil creep is slow movement, slow movement bends or tilts objects gradually, and tilted poles or bent fences are visible evidence. A useful memory trick is “creep bends, landslide breaks.” In an examination, look for the option showing gradual movement and long-term deformation, not a sudden dramatic event.

Landslides can be rapid and sudden while soil creep is very slow. Exam tip: treat speed as the main criterion.

Water saturation lowers particle friction and adds weight on slopes. Exam tip: remember water as a major landslide factor.

Debris flow forms from loose material mixed with water on steep slopes. Exam tip: link rainfall with slope.

Rockfall is sudden falling of material from steep jointed rocks. Exam tip: place it under rapid mass movement.

In slump material moves downward along a curved slip surface. Exam tip: treat it as a specific landslide type.

Frost action widens cracks and supplies loose material. Exam tip: see weathering as preparation for mass movement.

In the mature stage rivers form meanders and lateral erosion increases. Exam tip: link meanders with lateral erosion.

In abrasion the river uses its load to grind bed and banks. Exam tip: remember it as a mechanical erosion process.

In attrition particles collide and become smaller and rounded. Exam tip: treat it as wear during transport.

Hydraulic action uses water pressure and compressed air in cracks to break rock. Exam tip: write water force as the main cause.

Solution is more effective on rocks with soluble minerals. Exam tip: take limestone as a key example.

In the upper course high gradient directs river energy to downcutting. Exam tip: link steep gradient with vertical erosion.

A V-shaped valley forms when a river downcuts and slopes weather back. Exam tip: link it with the upper course.

When energy falls carrying capacity decreases and deposition begins. Exam tip: link falling velocity with deposition.

As river energy decreases it deposits heavier particles first. Exam tip: understand deposition through energy loss.

A delta forms where a river deposits abundant sediment in calm shallow water. Exam tip: treat sediment supply as the key condition.

A floodplain forms by repeated overbank deposition and lateral shifting. Exam tip: link it with a mature river.

The correct answer is A, an alluvial fan. A stream leaving a steep mountain valley suddenly encounters a gentler slope and a wider plain. Its velocity and carrying capacity decrease, so it deposits gravel, sand, and silt. Because the stream often spreads outward from a narrow valley mouth, the deposits form a roughly fan-shaped landform called an alluvial fan. B is wrong because a waterfall is mainly an erosional feature associated with a sudden drop in the channel. C is produced by wave erosion along a coast. D is a bowl-shaped hollow carved by glacial erosion. The key clue is “stream leaves a steep valley and spreads over a plain”: this indicates sudden loss of slope and deposition at the mountain foot. Memory cue: mountain stream plus fan-shaped deposit equals alluvial fan.

Current is faster on the outer bend so erosion is greater there. Exam tip: remember outer bend as cut bank.

Velocity is lower on the inner bend so sediment is deposited. Exam tip: link inner bend with point bar.

When the meander neck is cut off the isolated bend becomes an oxbow lake. Exam tip: remember neck cut-off.

Base level is the limit below which a river generally cannot erode. Exam tip: take sea level as general base level.

Waves use rock fragments as tools against the coast. Exam tip: treat it as marine mechanical erosion.