Erosional and depositional landforms created by various agents

The governing concept is coastal erosion. Destructive waves strike the foot of a rocky coast, use hydraulic action and abrasion, and remove material from the cliff base. Continued undercutting makes the upper rock collapse, causing the shoreline to retreat and leaving a steep sea cliff. Therefore, option C is correct. A floodplain is deposited by rivers, a drumlin is associated with glaciers, and loess is wind-deposited silt.

The governing concept is glacial erosion. In plucking, meltwater can enter cracks in bedrock and freeze, helping attach pieces of rock to the moving glacier. As the glacier advances, it pulls or lifts these loosened fragments from the valley floor and transports them. Thus option A correctly describes plucking. A delta is fluvial deposition, dust deposition is aeolian, and a bar is generally a coastal depositional feature, so those alternatives involve different agents.

A is correct. On a wide, gently sloping plain, a river meander can become enlarged and its neck narrowed. The river may cut through the neck, leaving the old loop isolated; deposition can then seal its ends and leave an oxbow lake.

Direct answer: D is the incorrect match. A river can deposit sediment at or near its mouth to form a delta, so river–delta is a valid match. A glacier can erode a bowl-shaped hollow in a mountain, called a cirque, so glacier–cirque is also valid. Wind can shape crescentic dunes in sandy drylands; these are barchans, making wind–barchan correct. A drumlin is an elongated streamlined hill associated mainly with glacial ice movement and glacial deposition beneath or around an ice sheet. Groundwater does not normally form drumlins. Therefore D is the only incorrect pairing. This question tests the main geomorphic agent behind each landform, not merely the location where the landform is seen. Memory cue: delta—river, cirque—ice erosion, barchan—wind, drumlin—glacier.

The governing concept is the changing work of a river from its upper to lower course. In the steep upper course, vertical erosion commonly produces a V-shaped valley. Farther downstream, reduced gradient and increased lateral activity allow floodplains to develop. Near the mouth, falling velocity causes deposition and may create a delta. Hence option A gives the correct sequence; the other sequences mix agents or reverse the normal river pattern.

The governing concept is coastal deposition and the formation of a barrier. Waves and longshore currents transport sand along the coast; where energy falls, the sand accumulates into a bar across or near a bay mouth. Water trapped behind the bar becomes a shallow, partly enclosed body called a lagoon. Therefore option B is correct. A river terrace is fluvial, a cirque is glacial, and a yardang is carved by wind erosion.

The correct answer is A because a river in its youthful upper course has a steep gradient and considerable downward energy. It mainly erodes vertically into its bed, producing a narrow, deep V-shaped valley. Marine deposition, glacial melting, and wind accumulation are different processes and do not explain this characteristic river valley. Lateral erosion becomes more important farther downstream.

The correct answer is C. In the middle and lower course, the river gradient is gentler and its flow moves mainly from side to side. Faster water erodes the outer bank, while slower water deposits sediment on the inner bank, gradually enlarging bends called meanders. The steep source region is dominated by vertical erosion, while glacial zones and sea caves are unrelated settings.

The correct answer is A. During repeated floods, river water overtops its banks and spreads across the adjoining low-lying valley floor. As the water loses velocity, it deposits layers of fine alluvium over a broad area. Repeated deposition gradually builds a level or gently undulating floodplain. Sea-wave erosion, floating icebergs, and frost action in deserts are not the principal processes responsible for this fluvial depositional landform.

The correct answer is C. A delta forms where a river enters a sea or lake and suddenly loses gradient and carrying capacity. If the river brings abundant sediment and waves or currents do not remove it quickly, sand, silt, and clay accumulate at the mouth. Continued deposition may build distributary channels and a deltaic plain. Strong currents, little sediment, and glacial erosion work against delta formation.

The correct answer is A. Moving glacier ice carries embedded rock fragments that scrape and abrade the valley floor and sides. Pressure also allows the glacier to pluck loosened blocks from the bedrock. This combined erosion widens and deepens an earlier river valley, changing its narrow V-shaped cross-section into a broad, steep-sided U shape. Wind deposition, river deposition, and sea-wave deflection cannot produce this glacial profile.

The correct answer is B. A cirque, also called a corrie or cwm, is a bowl-shaped hollow formed near the head of a mountain glacier. Processes such as frost shattering, abrasion, and plucking deepen and enlarge the hollow around the zone where snow and ice accumulate. It is therefore an erosional glacial landform, not a river, wind, or marine depositional feature. The association with a bowl-shaped hollow is a useful identifying clue.

C is correct. The larger main glacier generally erodes its valley more deeply than the smaller tributary glacier erodes its side valley. After the ice melts, the tributary valley floor is left higher than the main valley floor; a waterfall may descend from it.

A is correct. An esker is typically a long, winding ridge of sand and gravel deposited by meltwater flowing through or beneath a glacier. When the ice melts, the deposited sediment remains as a ridge.

Yardangs are streamlined ridges carved in desert rock by persistent wind erosion. Wind-blown sand abrades exposed surfaces, while softer or weaker rock is removed faster than resistant rock. This differential erosion leaves long, narrow ridges aligned with the prevailing wind direction. Option C is correct. Flood deposition, glacial melting, and marine organisms do not explain this characteristic desert landform.

A spit is a narrow ridge of sand or shingle projecting from a coast into a bay, estuary, or sheltered water. Longshore drift transports sediment parallel to the shoreline, and deposition occurs when the coast changes direction or wave energy falls. Snowfall, deep river erosion, and earthquake uplift do not provide the sustained sediment movement needed to build this coastal depositional feature.

A bar or barrier ridge is a coastal depositional feature formed when waves and longshore currents accumulate sand or shingle across the entrance of a bay, lagoon, or river mouth. Continued deposition can partly or completely block the opening. River incision, glacial scraping, and wind erosion remove or cut material rather than creating this coastal sand barrier, so option D is the only suitable explanation.

In a karst region, slightly acidic groundwater enters joints and dissolves soluble limestone. Over time, underground cavities and channels enlarge. If the roof becomes too weak to support its weight, it subsides or collapses, producing a sinkhole at the surface. Sea-wave sand, glacial debris, and desert dust are depositional materials and do not explain the solution-collapse process responsible for karst sinkholes.

Gorges and canyons are deep, narrow valleys produced mainly by a river cutting downward into its bed and the surrounding bedrock. This dominant vertical or downward erosion, called river incision, is especially effective where uplift or resistant rock maintains a steep gradient. Wind deposition, coral growth, and groundwater deposition build materials or organisms rather than excavating a deep river valley. Hence option A is correct.

Option B is correct. When a river carrying abundant sediment loses speed or discharge, it deposits some of its load in bars within the channel. The flow divides around these deposits into multiple channels. Changes in flow can erode or rebuild the bars, so the channels shift and intertwine. No particular year applies to this landform.

In a meander, water moves more slowly along the inner bend because friction is greater and the current has less transporting power there. Sand, silt, and gravel are consequently deposited on this inside bank, gradually building a gently sloping point bar. The outer bend has faster flow and commonly develops a cut bank through erosion. Thus option C is correct; the waterfall and cirque locations are unrelated.

Direct answer: A floodplain is mainly formed by river deposition, so option A is correct. During floods, a river may overflow its channel and spread across the adjoining valley floor. Once the water spreads out, its speed and carrying power decrease. The river then drops alluvium such as silt, fine sand, and clay. Repeated flooding gradually builds a broad, fairly flat floodplain beside the channel. Option B, a cirque, is a bowl-shaped hollow carved by glacial erosion near the head of a mountain valley. Option C, a mushroom rock, is shaped mainly by wind abrasion, often because sand-laden wind erodes the lower part more strongly. Option D, a sea cave, is cut into a coastal cliff by wave erosion. Thus only the floodplain is primarily a river-depositional landform. A useful cue is: floods spread and settle sediment, building plains.

Erosion removes soil or rock, and the eroded material is commonly transported by water, wind, ice, or waves. When the transporting agent loses energy—for example, when river velocity falls—the agent can no longer carry all of its load. The excess material is then deposited in a suitable place. Thus deposition commonly follows transport under reduced energy, making option A correct; the other statements contradict this sequence.

The correct answer is B. Strong tidal currents in many estuaries keep sediment moving or redistribute it, so it does not build up into a large delta. A delta forms where a river loses speed as it enters quieter water and deposits sediment. This is the usual contrast; conditions can vary from one river mouth to another.

A lagoon is a shallow coastal water body partly or completely separated from the open sea. Longshore drift and wave action can deposit sand to build a bar, spit, or barrier beach across a bay or along the coast. The enclosed water becomes a lagoon, so option A is correct. The other choices describe river, glacial, and wind erosion rather than coastal barrier deposition.