Erosional and depositional landforms created by various agents

Correct answer: C. Lithology refers to the nature and characteristics of rocks. Waves repeatedly attack a coast and enlarge cracks and joints; weak or heavily jointed rock is eroded more readily than strong, massive rock. A sea cave can begin where such a weakness meets the wave-attack zone. River floods, glacier thickness and loess colour do not explain this process.

Correct answer: B. Waves can erode a notch into the base of a coastal stack, removing support and making the stack unstable. Continued erosion may cause it to collapse or leave a shorter remnant called a stump. A notch therefore weakens the base rather than making the stack more stable.

A marine terrace is a relatively level former shoreline or wave-cut platform that now stands above the present sea. Its elevation may result from tectonic uplift of the land, a fall in relative sea level, or both. Thus it records earlier coastal conditions. Option C is correct; loess, karst collapse and river potholes are formed by different agents and do not explain a raised marine surface.

Barrier islands are elongated, sandy depositional bodies commonly aligned parallel to a coast. Waves, longshore currents and storms transport and redistribute sediment, while changes in relative sea level influence their migration, attachment or separation from the mainland. Lagoons or marshes may form behind them. Therefore option D gives the relevant coastal controls; the other options describe glacial, karst or fluvial processes.

Tidal flats are shallow, usually sheltered coastal surfaces where water velocity falls during slack water and other low-energy phases of the tidal cycle. As flow energy decreases, its ability to keep suspended particles moving also decreases, so silt and clay settle onto the flat. Strong waves and swift currents generally resuspend or transport fine material rather than deposit it. Evaporation creates salt crystals, not mineral silt or clay, making option A correct.

Correct answer: A. Salt-marsh plants slow tidal water as it passes through their stems and leaves. This allows suspended silt and clay to settle, while roots help hold the sediment in place. With a continuing sediment supply, these deposits can gradually raise the marsh surface.

Correct answer: A. In an estuary, river water mixes with salty seawater. Dissolved salts can reduce the electrical repulsion between fine clay particles, allowing them to clump into larger aggregates called flocs. These larger aggregates settle more readily in relatively calm water, promoting fine-sediment deposition.

Correct answer: A. Along an irregular coastline, wave refraction changes where wave energy is concentrated. As wave fronts approach a projecting headland, they become closer together and their energy is focused, so erosion is stronger there. In a bay, wave fronts spread out and lose energy; the weaker water movement allows sand and shingle to accumulate, forming beaches or other depositional features. Thus the same coastal system can contain eroding headlands and depositional bays. B removes the main agent, while C and D describe environments that do not explain the stated coastal pattern. Memory cue: concentrated energy erodes; reduced energy deposits.

Correct answer: A. Glacial hollows can form when buried blocks of stagnant ice melt, or through erosion by moving ice. In karst regions, groundwater dissolves soluble rock such as limestone, forming hollows; the collapse of a weakened cavity roof can enlarge them. Thus, similar-looking depressions can have different origins.

Correct answer: B. When a stream leaves a steep valley and spreads across an alluvial fan, its flow usually loses energy. Coarse, heavy material tends to settle nearer the fan’s upper part, while finer sediment can be carried farther before it settles. This sorting by particle size is selective deposition.

The correct answer is A. A bergschrund is a deep crevasse near the head of a mountain glacier, where the moving glacier ice separates from relatively stationary ice or the rocky headwall. It can be a difficult and dangerous obstacle for climbers.

The governing idea is the balance between weathering, erosion, and episodic deposition in arid geomorphic systems. A pediment is mainly a gently sloping, bedrock-controlled erosional surface, so it does not usually carry a thick soil mantle. However, limited mechanical weathering supplies debris, and occasional sheet floods spread a thin layer of regolith across the surface. Thus A is correct; coral, glacial till, and cave calcite belong to different environments.

B is correct. Winds arriving from several directions can build a high central mass of sand with ridges, or arms, radiating outward. This is the characteristic form of a star dune. A long single ridge is more typical of a linear dune formed under a different wind pattern.

The governing concept is the depositional work of waves, currents, and longshore drift along a coast. A spit is a projecting ridge of sand or gravel, and a barrier bar is a shore-parallel deposit; either may partly enclose a lagoon as sediment accumulates. Thus option A gives the clearest depositional coastal group. Cirques and horns are glacial, sinkholes and poljes are karst, and yardangs or ventifacts are wind-shaped features.

The governing concept is the diagnostic relationship between a geomorphic agent and its erosional landforms. Mountain glaciers excavate bowl-shaped cirques, sharpen ridges into arêtes, leave pointed horns where cirques intersect, and widen river valleys into U-shaped troughs. Therefore option B is the only group showing strong glacial erosion. Loess, barchans, and nebkhas are wind deposits, karst forms result from solution, and spits or beach ridges are coastal deposits.

The governing concept is karstification, in which slightly acidic groundwater dissolves soluble limestone along joints and bedding planes. This produces closed depressions called dolines, swallow holes or ponors, and caves; inside caves, mineral-rich water may deposit stalactites from the roof. Therefore option C is correct. Drumlins and eskers are glacial, deltas and point bars are fluvial, and cliffs or stacks are mainly marine erosional forms.

Yardangs are streamlined ridges carved by wind abrasion and deflation, and ventifacts are stones faceted or polished by wind-borne sand; both represent aeolian erosion. Loess, however, is a blanket-like deposit of wind-blown silt, so it represents aeolian deposition. The other groups are mainly glacial, coastal or karst landforms and do not combine both wind processes. Therefore, option A is correct.

The correct answer is B. A river terrace is a former floodplain left above the active river channel after the river cuts down into its valley. Multiple terrace levels generally record repeated phases in which floodplain surfaces formed or remained relatively stable, followed by renewed river incision. Higher terraces are often older, though local conditions can affect their sequence.

The correct answer is A. In a river’s upper course, vertical erosion is generally stronger than lateral erosion, so the river cuts a narrow V-shaped valley. Resistant ridges project into the valley, and the river winds around them rather than cutting straight through. The alternating projections on opposite sides appear to interlock.

The correct answer is B. Hydraulic action forces water and compressed air into cracks in a rocky coast, widening them over time. Abrasion occurs as sand and pebbles carried by the waves scrape the rock. Together, these processes can enlarge a coastal crack into a sea cave and erode a notch at the foot of a cliff.