Erosional and depositional landforms created by various agents

Correct answer: A. Collapse of the roof of a sea arch by waves

A sea stack forms when continued wave erosion weakens and finally collapses the roof of a sea arch. The isolated pillar of resistant rock left beside the cliff is called a stack. A floodplain is made by river deposition, loess by wind deposition, and moraine by glacial deposition; none of these explains this coastal erosional sequence. The sequence is cave, arch, stack, and later stump.

Correct answer: B. Deposition by waves and longshore currents

A sea beach is a coastal depositional landform. Waves and longshore currents transport sand, pebbles, and other loose sediment, then deposit it where wave energy decreases along the shore. Glacial plucking removes rock, wind abrasion wears exposed surfaces, and vertical river erosion cuts downward into a channel. Therefore, only deposition by waves and longshore currents explains beach formation.

Correct answer: D. By swirling pebbles grinding a round hollow into the river bed

Option D is correct. In a turbulent river, swirling water can trap and rotate pebbles in a small depression in the bedrock. The pebbles repeatedly strike and grind the rock, enlarging the depression into a rounded pothole. No particular year applies to this landform.

Correct answer: D. On the outer bend

Option D is correct. In a meander, faster water generally flows along the outer bend, eroding and undercutting the bank to form a cut bank. Slower water on the inner bend tends to deposit sediment, often forming a point bar. No particular year applies to this landform.

Correct answer: C. A deep U-shaped valley carved by a glacier

The correct answer is C. A glacier erodes and deepens a valley into a broad, steep-sided U shape. When the glacier retreats, seawater can flood the lower part of the valley, forming a fjord.

Correct answer: C. By scratches made by rock fragments embedded in a moving glacier

The correct answer is C. Rock fragments carried at the base of a moving glacier are pressed against bedrock and scrape it as the ice moves. These scratches, called glacial striations, can help show the direction in which the ice travelled.

Correct answer: B. Limestone

B, limestone, is correct. Water containing dissolved carbon dioxide is mildly acidic and can dissolve calcium carbonate in limestone. Over time, this enlarges cracks and passages into caves, a process associated with karst landscapes. This type of solution cave is much less typical of granite, basalt, or quartzite.

Correct answer: C. Delta

C, a delta, is correct. Near a river mouth, the river slows as it enters a sea or lake and deposits sediment. The deposits can divide the flow into smaller channels that spread across the delta; these channels are called distributaries. A mushroom rock is shaped mainly by wind, a sea cliff by waves, and a cirque by glacial erosion.

Correct answer: A. Greater wind abrasion near the ground by sand grains

A is correct. Wind-blown sand grains abrade exposed rock most strongly near the ground, where the grains are concentrated. This wears away the lower part faster than the upper part, leaving a narrower base and a broader top—the typical pedestal shape. The other options describe marine, river-deposition, or cave processes.

Correct answer: B. Deposited sediment divides the main channel

B is correct. As a river enters a sea or lake, it slows and deposits sediment near its mouth. These deposits form bars and shallow obstructions that split the flow into smaller channels called distributaries. Tributaries join a river; distributaries branch away from it.

Correct answer: B. When temporary water collects and evaporates in a closed basin

Correct answer: B. A playa lake forms when rainwater or runoff collects temporarily in a closed desert basin with no outlet. In the dry climate, evaporation removes the water, sometimes leaving a flat surface of mud or salts. The other options describe coastal, glacial, or unrelated processes.

Correct answer: C. By repeated wave erosion at a cliff base and retreat of the cliff

Correct answer: C. Waves repeatedly erode the base of a sea cliff, forming a notch. As the overhanging rock collapses, the cliff retreats inland. Repeated erosion and retreat leave a gently sloping rocky surface at the foot of the cliff: the wave-cut platform. The other choices describe glacial, wind, or river processes.

Correct answer: D. Marine erosion opening a passage to the roof of a sea cave

Correct answer: D. Waves erode cracks in a coastal cliff, enlarging them into a sea cave and sometimes opening a passage through its roof. When waves enter the cave, compressed air and spray can burst upward through this opening, creating a blowhole. The other choices describe river, delta, or glacial processes.

Correct answer: B. Moving sediment parallel to the coastline

Correct answer: B. Longshore drift is the net transport of beach sediment along the coastline. Waves often approach the shore at an angle, carrying material diagonally up the beach; the backwash returns it downslope. Repetition creates an overall movement parallel to the coast. The other choices describe glacial, karst, or river processes.

Correct answer: D. A closed depression formed by solution or collapse

Correct answer: D. A doline is a closed depression in a karst landscape, commonly formed when groundwater dissolves soluble rock such as limestone. It can also form when the roof of an underground cavity collapses. The other options describe coastal, glacial, or wind-related features, not a karst depression.

Correct answer: D. When a stalactite and a stalagmite join

A cave pillar is a depositional feature produced by the precipitation of calcium carbonate in a limestone cave. A stalactite grows downward from the roof, while a stalagmite grows upward from the floor. If these two formations continue growing until they meet, they form one continuous column or pillar. Therefore option D is correct; the other choices describe unrelated desert, fluvial, or coastal processes.

Correct answer: B. Deposition of transported material begins

The governing concept is the relationship between an agent’s energy and its transporting capacity. When flowing water, wind, waves, or ice loses energy, its velocity or carrying power generally falls. It can no longer carry the full sediment load, so the transported particles settle and deposition begins. Erosion does not necessarily increase, rocks do not melt, and groundwater does not stop; therefore option B is correct.

Correct answer: B. Stalactite hangs from the roof and stalagmite grows upward from the floor

In a limestone cave, mineral-rich droplets lose carbon dioxide and deposit calcium carbonate. Deposits attached to the cave roof grow downward and are called stalactites; deposits built upward from the floor are stalagmites. Hence option B states the defining difference. The other choices incorrectly assign these features to deltas, wind, glaciers, or sea waves.

Correct answer: C. It has a wide floor and steep sides due to glacial erosion

A V-shaped valley is commonly cut by a river, which erodes mainly downward in its upper course. A glacier, in contrast, occupies and abrades the entire valley floor and sides, widening and deepening it into a broad U-shaped cross-section with steep walls. Therefore option C is correct; the other options identify unrelated wind, marine, or delta processes.

Correct answer: D. When a sea arch collapses and leaves an isolated rock pillar

The correct answer is D. Waves erode a rocky headland, enlarging cracks into caves and sometimes a sea arch. When the arch roof collapses, an isolated rock pillar remains; this is called a sea stack. The other options describe river deposition, a wind-shaped dune, and a cave deposit, respectively.

Correct answer: B. Deposition of sediment along the coast by longshore drift

The correct answer is B. Longshore drift transports sediment along a coast. Where the coastline changes direction or the water loses energy, some of that sediment is deposited and can build a narrow ridge projecting from the shore into the sea. This ridge is called a spit.

Correct answer: D. In the direction of the prevailing wind

The correct answer is D: the horns point in the direction the prevailing wind blows. A barchan is a crescent-shaped dune that moves downwind, and its two pointed horns extend ahead of its central body. Thus, the horns point downwind, not upwind.

Correct answer: A. An erg is a sand sea, while a reg is a stony or gravel-covered desert surface

The correct answer is A. An erg is a broad desert area dominated by wind-blown sand, often with extensive dunes. A reg is a desert surface covered mainly by stones or gravel, often left after wind removes finer material. The key difference is the dominant surface material: sand in an erg and coarse debris in a reg.

Correct answer: B. Scratching of bedrock by particles embedded at a glacier’s base

The correct answer is B. As a glacier moves, stones and other particles embedded in its base are dragged over the bedrock and scratch it. These long, narrow grooves are called glacial striations, and their alignment can indicate the direction of former ice movement.

Correct answer: A. Where a mountain stream suddenly enters a plain

An alluvial fan forms where a stream leaves a steep, confined mountain valley and suddenly spreads onto a relatively flat plain. The sharp reduction in slope and carrying capacity makes the stream deposit gravel, sand, and silt in a fan-shaped body, often at a mountain front. Therefore, option A is correct; the other choices describe marine or glacial settings unrelated to alluvial fans.