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Landform Evolution by Running Water and Underground Water

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Question 1Question →

In the geomorphic development of a karst landscape, groundwater percolating through well-jointed limestone dissolves calcium carbonate to form distinct subterranean and surface features. Which of the following sequences correctly orders the evolutionary stages of these landforms from initial surface solution to the most advanced stage of cavern collapse?

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Answer: Swallow hole (gripe) → Doline → Underground cavern excavation → Polje with karst window

Answer

The correct sequence begins with a swallow hole (gripe), followed by doline expansion, underground cavern excavation, and ultimately the formation of a polje with a karst window.
The correct answer accurately outlines the progressive development of limestone terrain under the action of underground water. Surface runoff enters enlarged joints at a swallow hole, expands surface hollows into dolines, excavates subterranean caverns through carbonation, and eventually causes roof collapses that expose underground streams via karst windows and form extensive flat-floored poljes.

Step-by-Step Solution

1
Identify the primary chemical weathering process
Groundwater reacts with carbon dioxide to form weak carbonic acid, dissolving calcium carbonate along limestone joint planes.
Karst landforms develop predominantly through solution (carbonation) by underground water.
2
Trace the initial surface entry points of water
Surface streams disappear underground via sinkholes or swallow holes (gripes).
Joint enlargement at the surface diverts surface runoff into subterranean drainage systems.
3
Analyze intermediate subterranean and surface collapse features
Solution hollows enlarge into dolines, while continuous subterranean solution hollows out massive caverns.
Subterranean stream flow enlarges underground passages along bedding planes and vertical joints.
4
Determine the advanced/mature stage landforms
Cavern roofs collapse to expose underground rivers (karst windows) and create expansive flat-floored depressions (poljes).
Extensive structural collapse of roof rocks marks the late stage of karst landscape evolution.

Key Concept

Karst Geomorphology and Subterranean Landform Evolution
Estimated Time:1m 30s
Question 2Question →

Arrange the following stages in the formation of an oxbow lake in the correct sequential order from first to last.

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Answer

The correct sequence of oxbow lake formation begins with the river forming a pronounced meander loop, followed by continuous erosion and deposition narrowing the neck. The river then cuts straight through the neck during a flood, and finally deposition seals off the abandoned loop to form an oxbow lake.
The correct order follows the progressive geomorphic stages of oxbow lake development: initial meander development with a narrow neck, neck narrowing through lateral erosion and deposition, sudden neck cut-off during a flood event, and final isolation of the crescent lake by alluvial deposition at the entrance and exit of the abandoned loop.

Step-by-Step Solution

1
Identify the initial channel shape
Establish that a meander loop with a narrow neck must exist prior to cut-off.
An oxbow lake can only form from a pre-existing river meander loop in its middle or lower course.
2
Trace the process of meander neck narrowing
Recognize that lateral erosion on hydraulic cut-banks and deposition on slip-off slopes narrow the neck.
Helicoidal flow concentrates energy on the outer banks, bringing the neck ends closer together over time.
3
Identify the breach event
Determine that flood conditions cause the river to cut directly through the neck.
High discharge provides sufficient hydraulic force to carve a new, straight channel along the path of maximum gradient.
4
Conclude with channel abandonment and isolation
Note that alluvium deposition blocks the old meander entrances.
Slower water current at the cut-off entry and exit points encourages deposition, leaving behind a cutoff lake.

Key Concept

Fluvial meander evolution and cut-off mechanisms leading to oxbow lake formation
Question 3Question →

Which of the following landforms is formed primarily by river deposition in its lower course?

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Answer: Delta

Answer

Delta
A delta is formed in the lower course of a river where the gradient flattens and water flow slows down drastically upon reaching a sea or lake. This reduction in velocity causes the stream to deposit its load of sand, silt, and clay at the mouth.

Step-by-Step Solution

1
Identify the stage of the river and process specified in the prompt
The question asks for a feature formed by river deposition in its lower course.
River courses are divided into upper (erosional), middle (transportational/depositional), and lower (predominantly depositional) stages.
2
Evaluate the landform options against the course and process
Deltas are built when a river enters a lake or sea in its lower course, reducing velocity and depositing its sediment load.
Other options represent upper-course erosional features, mass wasting, or metamorphic rock features.

Key Concept

Fluvial Deposition and River Stage Features
Estimated Time:45s
Question 4Question →

Arrange the following stages of river capture (stream piracy) in their correct chronological sequence from the initial headward erosion to the final landform development.

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Answer

The correct sequence begins with headward erosion toward the watershed divide, followed by breaching the divide to intercept the weaker stream, the diversion of flow creating an elbow of capture, and finally the formation of a wind gap and misfit stream downstream.
River capture begins when a stream with higher energy or a steeper gradient erodes headward into a watershed divide. Once the divide is breached, the stream intercepts the headwaters of a neighbouring channel. The diverted water creates a sharp bend known as an elbow of capture, while the dry valley below the point of capture becomes a wind gap containing a reduced misfit stream.

Step-by-Step Solution

1
Identify the initial process causing river capture
Headward erosion by the pirate stream with greater erosive power cuts back into the watershed divide.
Unequal gradient or rock resistance causes one stream to extend its valley headward faster than adjacent streams.
2
Determine the moment of interception
The pirate stream cuts through the divide and breaches the upper channel of the adjacent stream.
The divide is eliminated at the lowest col, enabling physical connection between the two drainage systems.
3
Analyze the immediate hydrological redirection
Water from the captured stream flows into the capturing river, forming a sharp right-angled turn called the elbow of capture.
The higher gradient of the pirate stream draws water into its steeper channel.
4
Establish the resulting morphological landforms downstream
The abandoned lower channel retains a wind gap and a misfit stream.
The loss of headwater discharge leaves the former channel oversized relative to its remaining reduced discharge.

Key Concept

River Capture (Stream Piracy) and Drainage Evolution
Question 5Question →

A river system flowing across a mature landscape experiences sudden tectonic uplift, causing a significant base-level drop. This change reactivates intense vertical erosion, prompting the river to cut deeply into its former floodplain and leave step-like benches at identical elevations on both sides of the new valley. Which landform is produced by this geomorphic process?

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Answer: Paired river terraces

Answer

Paired river terraces are formed when river rejuvenation leads to vertical downcutting into a former floodplain, creating symmetrical bench-like landforms on opposite sides of the valley.
Paired river terraces represent former floodplain levels left standing above the current channel after river rejuvenation. Tectonic uplift or a drop in sea level increases the river's gradient and kinetic energy, driving rapid vertical downcutting. As the river carves a deeper inner gorge, remnants of the old valley floor remain as flat bench-like steps at identical elevations on both sides of the valley.

Step-by-Step Solution

1
Analyze the geomorphic trigger described in the stem.
Tectonic uplift drops the base level, initiating river rejuvenation and accelerating vertical downcutting.
Rejuvenation increases the river's energy, allowing it to incise deeply into its existing channel floor.
2
Examine the spatial arrangement of the resulting valley features.
The old floodplain is abandoned above the active channel, forming flat, bench-like steps at matching elevations across the valley.
When downcutting occurs evenly across symmetrical valley sides, paired terraces are formed.
3
Match the observed features to the correct geomorphic landform.
The step-like remnants at equal heights are identified as paired river terraces.
Unpaired terraces occur with lateral meander migration, whereas equal-height benches on both sides confirm paired terraces.

Key Concept

River Rejuvenation and Terrace Formation
Estimated Time:2m 0s
Question 6Question →

In a karst landscape, percolating carbonated groundwater dissolves and expands the vertical joints of exposed limestone bedrock, creating a characteristic pavement structure of flat-topped blocks separated by deep vertical fissures. Which geomorphological term specifically identifies these deep solution fissures?

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Answer: Grikes

Answer

Grikes are the deep vertical solution fissures formed in a limestone pavement.
Grikes is the correct term for the deep vertical channels or fissures formed when carbonated rainwater percolates into limestone joints and dissolves the surrounding rock. This creates the classic karst surface feature known as a limestone pavement.

Step-by-Step Solution

1
Analyze the landform process described in the stem
Rainwater absorbing carbon dioxide forms weak carbonic acid (H2CO3H_2CO_3), which dissolves calcium carbonate (CaCO3CaCO_3) in well-jointed limestone through carbonation.
Chemical solution along joints creates a limestone pavement consisting of two distinct alternating features: blocks and fissures.
2
Distinguish between surface solution features of a limestone pavement
The widened vertical joints/fissures are called grikes, while the raised, flat limestone slabs left standing between them are called clints.
Precise geomorphological classification requires identifying whether the feature represents the eroded trench or the remnant block.

Key Concept

Karst Surface Solution Features (Limestone Pavement)
Estimated Time:1m 0s
Question 7Question →

Arrange the following sequential stages in the retreat of a waterfall and the subsequent formation of a river gorge, from the initial bedrock arrangement to the final landscape feature.

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Answer

The correct order of stages in waterfall retreat and gorge formation begins with a river flowing over hard rock overlying soft rock, followed by undercutting of the softer rock to form an overhang, then collapse of the unsupported overhang into the plunge pool, and finally continuous headward retreat leaving behind a steep-sided gorge.
The evolution of a river gorge begins when running water encounters alternating rock strata, where a resistant rock layer overlies softer rock. Hydraulic action and abrasion erode the softer stratum beneath, creating an overhang above a plunge pool. When the overhang loses support, it collapses under gravity. As this sequence of undercutting and collapse repeats over time, the waterfall retreats upstream, leaving behind a narrow, steep-sided valley termed a gorge.

Step-by-Step Solution

1
Identify the initial geological precondition.
A resistant caprock overlying less resistant rock layer provides the differential erosion setup.
Differential erosion cannot occur without contrast in rock hardness.
2
Trace the process of mechanical river erosion at the base.
Undercutting creates a plunge pool and an unsupported caprock overhang.
Hydraulic action and abrasion preferentially hollow out the weaker underlying stratum.
3
Determine the structural breakdown stage.
The overhang collapses due to gravity.
Once the undercutting removes critical support beneath the hard caprock, collapse is inevitable.
4
Identify the long-term geomorphic outcome.
Headward retreat creates a gorge.
Repeated cycles of undercutting and collapse cause the waterfall to migrate upstream.

Key Concept

Waterfall Retreat and Gorge Formation
Question 8Question →

In a limestone region, a surface stream suddenly disappears underground through an opening formed by carbonation and solution, leaving the river valley downstream dry. Which karst landform is formed at the exact point where the stream sinks underground?

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Answer: Swallow hole

Answer

A swallow hole (also known as a ponor or sinkhole shaft) is the feature formed where a surface stream disappears subterraneanly into carbonated limestone bedrock.
In limestone regions, surface water containing carbonic acid dissolves vertical joints in the bedrock. Over time, these joints widen into funnel-shaped openings or vertical shafts known as swallow holes (or ponors). When a surface stream reaches a swallow hole, it plunges underground into a subterranean drainage system, leaving the valley lower downstream completely dry.

Step-by-Step Solution

1
Analyze the geomorphic process described in the stem
Groundwater carbonation dissolves joints in limestone bedrock, allowing surface runoff to carve vertical passages underground.
Rainwater containing dissolved carbon dioxide (H2CO3\text{H}_2\text{CO}_3) acts on calcium carbonate (CaCO3\text{CaCO}_3) to dissolve limestone bedrock along vertical joints.
2
Identify the specific landform created at the point of river disappearance
The entrance or shaft through which a river flows into subterranean cavern systems is known as a swallow hole or ponor.
As the swallow hole enlarges, the entire river is diverted underground, leaving the dry valley downstream without water flow.

Key Concept

Karst Drainage Features and Stream Disappearance
Question 9Question →

Match each landform created by running water or underground water with its corresponding course stage or morphological characteristic.

Click a left item, then click its matching right item

Items

Interlocking spurs
Oxbow lake
Stalactite
Polje

Matches

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Answer

Interlocking spurs correspond to the upper course feature formed around hard rock projections; Oxbow lake corresponds to the lower course crescent-shaped water body from a breached meander; Stalactite corresponds to the calcite deposit hanging from a cavern ceiling; Polje corresponds to the extensive flat-floored karst solution depression.
Each feature is correctly matched to its primary geomorphic environment: Interlocking spurs occur in upper river courses due to vertical erosion. Oxbow lakes develop in mature lower courses via meander cutoffs. Stalactites hang from underground limestone cave ceilings, and poljes represent large flat-floored karst surface basins formed by extensive chemical solution.

Step-by-Step Solution

1
Identify fluvial surface landforms and their corresponding river course stages.
Interlocking spurs belong to the upper course characterized by steep vertical headward erosion, whereas oxbow lakes belong to the depositional lower course floodplain environment.
Energy levels and dominant erosion types (vertical vs lateral) define river course features.
2
Identify underground water (karst) features and their formation sites.
Stalactites hang downward from cave roofs due to dripping mineralized groundwater, while poljes are large-scale surface solution depressions in limestone terrains.
Precipitation of calcium carbonate forms cavern speleothems, whereas surface solution and cavern collapse generate broad depressions.

Key Concept

Classification of Fluvial and Karst Landforms
Question 10Question →

Which landform is produced in the upper course of a river when swirling eddy currents carrying pebbles grind cylindrical depressions into the riverbed bedrock?

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Answer: Potholes

Answer

Potholes are circular bedrock depressions created by the swirling action of river load (pebbles and gravel) driven by eddy currents in the upper course.
Potholes are smooth, cylindrical depressions carved into solid bedrock along a river channel. They form primarily in steep upper courses where swift-flowing water creates eddies that whirl pebbles around, drilling deep holes into bedrock depressions.

Step-by-Step Solution

1
Identify the process described in the stem.
The process described is pothole drilling (swirling river currents holding stone tools that erode bedrock).
Corrasion (abrasion) combined with eddy currents in swift-flowing water drills circular holes into irregularities in a stream bed.
2
Associate the process with its corresponding landform and course stage.
Potholes are distinctive erosional features of the upper (torrential) stage of a river.
High stream velocity and turbulent flow in steep upper courses provide the energy required for pebbles to carve circular hollows.

Key Concept

Fluvial Erosional Processes in the Upper Course
Question 11Question →

During stream piracy, an aggressive river tributary undergoes active headward erosion, breaching a watershed divide and intercepting the flow of a neighboring stream. Which landform is created in the abandoned valley immediately downstream of the elbow of capture?

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Answer: Wind gap

Answer

A wind gap is formed in the abandoned valley immediately downstream of the elbow of capture during river piracy.
When a river captures the upper waters of an adjacent stream, the valley downstream from the elbow of capture is deprived of its main water source. The dry gap or pass cut through the ridge where the original river used to flow is known as a wind gap.

Step-by-Step Solution

1
Analyze the geomorphic process described in the stem.
The process is stream piracy (river capture), where active headward erosion enables one stream to breach a watershed divide and divert another stream's flow.
Understanding stream piracy features requires identifying what happens at and downstream of the interception point (elbow of capture).
2
Identify the features associated with stream piracy.
The interception site is the elbow of capture; the diverted upper stream becomes the pirate stream; the stream below the elbow loses its headwaters and becomes a misfit stream; and the abandoned, dry valley pass downstream of the elbow forms a wind gap.
Landform classification in fluvial geomorphology relies on linking specific river processes to their resultant morphological features.

Key Concept

Stream Piracy (River Capture) and Associated Fluvial Landforms
Estimated Time:1m 15s
Question 12Question →

What is the correct sequential order of stages in the evolution of a natural limestone bridge, from initial subterranean dissolution to the final exposure of the arch?

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Answer

The correct sequence begins with percolating carbonated water dissolving limestone joints, followed by the enlargement of underground caverns, localized roof collapse forming karst windows, and finally progressive collapse leaving an isolated roof section as a natural bridge.
Karst landscape evolution follows a strict chronological sequence: initial solution along limestone joint planes by acidulated groundwater creates conduits; continuous subterranean water action enlarges these conduits into vast caverns; progressive ceiling dissolution leads to localized roof failure forming karst windows; and extensive collapse of surrounding roof sections leaves an isolated intact segment spanning the valley as a natural bridge.

Step-by-Step Solution

1
Identify the primary process initiating subterranean karst landform development.
Rainwater enriched with carbon dioxide percolates through limestone joints and bedding planes.
Carbonation is the essential chemical weathering process that initiates underground solution in soluble limestone bedrock.
2
Trace the expansion of subsurface drainage networks into void spaces.
Dissolved joint lines are widened into subterranean stream channels and cavern systems.
Sustained groundwater flow and chemical solution widen fissures into massive cavern chambers over time.
3
Determine the initial structural collapse feature resulting from roof instability.
Localized cavern roof collapse creates karst windows exposing subterranean rivers to the surface.
As cavern chambers enlarge, structural support is lost, causing roof failure in weaker sections.
4
Identify the final residual landform exposed after widespread roof collapse.
A surviving segment of uncollapsed cavern roof remains standing across the valley, forming a natural arch or bridge.
A natural limestone bridge represents a remnant cavern roof feature preserved after surrounding cavern roofs have completely collapsed.

Key Concept

Subterranean Karst Solution and Cavern Roof Evolution
Question 13Question →

Which of the following landforms is formed in the upper course of a river primarily by active vertical erosion?

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Answer: V-shaped valley

Answer

V-shaped valley
In the upper course of a river, steep slopes cause rapid streamflow that drives energetic vertical erosion. As the river deepens its bed, weathering and mass wasting loosen materials along the sides, producing a steep-sided V-shaped valley profile.

Step-by-Step Solution

1
Identify the dominant river process and course stage
The upper course stage features steep gradients, fast flow velocity, and downward channel incision (vertical erosion).
River kinetic energy in the upper course is mainly directed toward deepening the valley bed.
2
Determine the resulting morphological feature
Deepening of the stream bed combined with weathering of upper valley sides produces a narrow V-shaped valley.
Vertical hydraulic action and corrasion cut downward while mass movement slumps the valley slopes inward.

Key Concept

Upper river course erosion and landform development
Question 14Question →

In Karst geomorphology, subterranean dissolution of limestone leads to distinct underground and surface landforms. Arrange the following stages of subterranean cavern and collapse feature evolution in their correct chronological order from earliest to latest.

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Answer

The correct sequence begins with carbonated water percolating through joints, followed by fracture widening into conduit networks, horizontal cavern development at the water table, speleothem deposition and column growth, and culminates in roof collapse forming karst windows.
The sequence correctly reflects the geomorphological progression from microscopic chemical infiltration of carbonated water into limestone joints, through sub-surface conduit and cavern chamber dissolution at the water table, followed by speleothem dripstone growth inside aerated caverns, ending with cavern roof breakdown resulting in karst windows.

Step-by-Step Solution

1
Identify the initial chemical weathering action.
Rainwater absorbing carbon dioxide forms weak carbonic acid that infiltrates limestone bedding planes and vertical joints.
Chemical dissolution cannot occur underground without initial acidic water entry into rock fractures.
2
Trace the subterranean conduit formation.
Acidic groundwater widens joints and bedding planes into vertical shafts and narrow conduit networks.
Initial dissolution follows structural weaknesses before forming large subterranean chambers.
3
Determine cavern chamber creation at the water table.
Lateral water movement at the phreatic boundary dissolves massive subterranean chambers.
Horizontal flow at the water table concentrates dissolution laterally rather than vertically.
4
Sequence the secondary depositional speleothem features.
Degassing of carbon dioxide in subterranean air spaces deposits calcite stalactites, stalagmites, and coalescing columns.
Depositional dripstone features require air-filled cavern vaults created during water table lowering or vadose flow.
5
Identify the final structural collapse landform stage.
Cavern roof collapse exposes the subterranean river flow as a karst window or collapsed sinkhole gorge.
Extreme underground enlargement coupled with roof thinning causes mechanical instability and ceiling breakdown.

Key Concept

Subterranean cavern evolution and cave roof collapse dynamics in karst topography
Estimated Time:2m 0s
Question 15Question →

Match each landform created by running water or groundwater processes with its correct defining characteristic.

Click a left item, then click its matching right item

Items

Polje
Plunge pool
Braided channel
Stalagmite

Matches

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Answer

Polje matches with the large flat-bottomed depression in limestone regions; Plunge pool matches with the deep basin at the base of a waterfall; Braided channel matches with the wide, shallow river network with mid-channel bars; Stalagmite matches with the calcite pillar growing upwards from a cave floor.
Each feature is correctly matched to its formation mechanism: a polje is a large solution depression in karst topography; a plunge pool is carved at a waterfall base by turbulent water action; a braided channel is formed by sediment deposition splitting river flow; and a stalagmite is an upward-growing cave deposit of calcium carbonate.

Step-by-Step Solution

1
Identify the karst surface feature formed by massive solution and cavern collapse.
Polje matches the description of a large flat-bottomed depression in limestone landforms.
Poljes represent an advanced stage of karst landform evolution where subterranean rivers and solution hollows coalesce into a wide flat plain.
2
Analyze the upper-course fluvial feature located beneath vertical drops in water channels.
Plunge pool matches the deep basin formed at the base of a waterfall.
Hydraulic action and abrasion by swirling sediments rapidly deepen the channel floor directly below waterfalls.
3
Classify the depositional fluvial channel form containing mid-stream islands or bars.
Braided channel matches the shallow network created by heavy sediment deposition.
When stream energy drops relative to its heavy sediment load, channels split and recombine around sediment bars.
4
Determine the subterranean dripstone feature that builds upward from cave floors.
Stalagmite matches the upward-growing calcite pillar.
Dripping water loses carbon dioxide upon entering the cavern atmosphere, precipitating calcium carbonate on the floor beneath the drip point.

Key Concept

Classification of Fluvial and Karst Geomorphic Features
Estimated Time:1m 30s
Question 16Question →

Match each geomorphic landform developed through fluvial or groundwater action with its precise evolutionary process.

Click a left item, then click its matching right item

Items

Blind Valley
Polje
Potholes
Oxbow Lake

Matches

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Answer

Blind Valley matches with abrupt river termination at a swallow hole; Polje matches with expansive solutional-tectonic depression; Potholes match with cylindrical bed grinding by eddy currents; Oxbow Lake matches with meander cut-off and depositional sealing.
Each feature is correctly matched with its specific geomorphic evolution mechanism: Blind Valley with river capture into sinkholes, Polje with large-scale structural limestone dissolution, Potholes with fluvial eddy-current corrasion, and Oxbow Lake with flood stage meander cut-off and deposition.

Step-by-Step Solution

1
Analyze karst surface drainage features
Identify that a Blind Valley ends abruptly where surface streams plunge underground into sinkholes or ponors.
Karst hydrology frequently redirects surface drainage subterraneanly through swallow holes.
2
Evaluate major subterranean and surface karst depressions
Identify Polje as the expansive, flat-bottomed depression resulting from structural faulting combined with chemical solution.
Poljes are distinctive large-scale karst features requiring long-term tectonic and carbonation processes.
3
Analyze upper course fluvial erosional mechanisms
Connect Potholes with vertical mechanical abrasion (corrasion) by swirling pebbles trapped in channel floor depressions.
Eddy currents spin bedload pebbles to hollow out smooth cylindrical pits.
4
Evaluate lower course depositional and erosional processes
Connect Oxbow Lake with neck chute cut-offs during flood stages followed by silting up of the abandoned meander ends.
Meander necks narrow through lateral erosion until high discharge forces a straight cut-off.

Key Concept

Classification of Fluvial and Karst Geomorphic Processes and Landforms
Question 17Question →

During the structural evolution of a karst drainage basin, percolating meteoric water enriched with dissolved carbon dioxide descends vertically through unsaturated jointed limestone until it reaches an impermeable stratum. Which of the following correctly identifies the dominant process operating at the water table interface and the resulting subterranean landform?

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Answer: Chemical solution enlarging horizontal conduits along bedding planes to form phreatic caverns

Answer

Chemical solution enlarging horizontal conduits along bedding planes to form phreatic caverns
The correct answer identifies chemical solution enlarging horizontal conduits along bedding planes to form phreatic caverns. As carbonated water descends through joints in permeable limestone and reaches the saturated water table, its flow becomes predominantly horizontal along bedding planes. The continuous solvent action of carbonic acid dissolves calcium carbonate, expanding horizontal fissures into extensive subterranean caverns.

Step-by-Step Solution

1
Analyze the rainwater movement and chemical agent in karst environments.
Meteoric water absorbs carbon dioxide to form weak carbonic acid (H2CO3H_2CO_3), which dissolves calcium carbonate (CaCO3CaCO_3) in limestone.
Limestone dissolution is driven primarily by chemical carbonation rather than mechanical stream abrasion.
2
Determine the physical behavior of groundwater at the water table interface.
Vertical percolating (vadose) water reaches the saturated (phreatic) zone and shifts to horizontal lateral flow along bedding planes.
Water cannot easily penetrate deeper impermeable strata, concentrating solvent activity laterally.
3
Identify the resulting geomorphic feature created at this horizontal interface.
Persistent chemical solution along horizontal planes excavates extensive subterranean cavern networks and cave systems.
Phreatic cavern evolution along bedding planes is the primary landform developed where horizontal groundwater flow dominates.

Key Concept

Karst Cavern Evolution and Underground Water Solution Processes
Estimated Time:2m 0s
Question 18Question →

A river undergoes rejuvenation when its erosive energy is renewed due to a fall in sea level or tectonic uplift of the land. Which of the following landforms is formed as a direct result of river rejuvenation?

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Answer: River terraces

Answer

River terraces
River terraces are step-like benches abandoned above a new valley floor when a river experiences rejuvenation, providing it with renewed energy to erode vertically into its former floodplain.

Step-by-Step Solution

1
Identify the primary geomorphological trigger described in the stem.
The trigger is river rejuvenation caused by base-level lowering or land uplift.
Rejuvenation increases the stream's gradient and potential energy, triggering renewed vertical downcutting.
2
Analyze how active downcutting affects existing valley features.
The river incision carves a deep inner valley into its old floodplain, leaving step-like benches along the valley sides.
These step-like features, known as paired or unpaired river terraces, mark the former levels of the valley floor before rejuvenation occurred.

Key Concept

River Rejuvenation and Terrace Formation
Question 19Question →

Match each landform generated by running water or groundwater processes on the left with its precise formative geomorphic mechanism on the right.

Click a left item, then click its matching right item

Items

Polje
Braided stream channel
Natural levee
Uvala

Matches

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Answer

Polje matches with the massive, flat-floored karst depression formed by combined structural faulting and solutional planation. Braided stream channel matches with the network of shallow channels separated by alluvial bars created by excessive bed-load deposition. Natural levee matches with the elongated ridge of coarse sediment accumulated along river banks during overbank flooding. Uvala matches with the compound closed depression formed by the progressive coalescence of several sinkholes.
Each landform is paired precisely with its defining formative geomorphic mechanism based on standard physical geography. Poljes represent massive fault-assisted karst solution floors; braided channels stem from channel bed load choking; natural levees result from coarse sediment settling at flood margins; and uvalas form through compound sinkhole coalescence.

Step-by-Step Solution

1
Analyze karst surface depressions by scale and origin
Identify that dolines merge into uvalas, while the massive, structurally controlled karst depression with a flat floor is a polje.
Karst terrain landforms progress systematically from individual sinkholes to uvalas, culminating in structurally downfaulted poljes.
2
Evaluate fluvial depositional mechanisms in middle and lower river courses
Distinguish between channel-splitting alluvial bars (braided stream) and bank-building ridge deposits from overbank floods (natural levees).
Braiding occurs within the river bed due to load exceeding transport capacity, whereas levees build up along the stream margins during flood spillover.
3
Match each landform term directly to its precise process definition
Pair Polje to right_1, Braided stream channel to right_2, Natural levee to right_3, and Uvala to right_4.
Each feature corresponds strictly to its geomorphic definition in senior secondary fluvial and karst geography.

Key Concept

Classification and process attribution of surface fluvial depositional features and subterranean/surface karst solution landforms.
Question 20Question →

During upper-course fluvial erosion, rock fragments and coarse sediment carried by a river continuously grind against the riverbed and channel walls, scouring and wearing them down through mechanical friction. Which process of river erosion does this action describe?

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Answer: Corrasion

Answer

Corrasion (abrasion) is the process of river erosion where transported rock fragments grind against and wear down the riverbed and banks.
Corrasion, also known as mechanical abrasion, takes place when boulders, pebbles, and sand particles trapped in the river current scrape, scour, and wear down the bed and banks of the river channel.

Step-by-Step Solution

1
Analyze the mechanism described in the stem
The stem describes mechanical grinding of the riverbed and banks using the river's sediment load as an abrasive agent.
Identifying whether the process involves load-on-channel friction, load-on-load collisions, chemical solution, or fluid pressure isolates the correct geomorphic term.
2
Distinguish corrasion from other fluvial erosion processes
Corrasion (abrasion) uniquely refers to load wearing away channel boundaries, whereas attrition is load wearing load, corrosion is solvent dissolving rock, and hydraulic action is fluid force alone.
Matching the definition to official Senior Secondary School (SSS) geography terminology ensures correct classification.

Key Concept

Fluvial Erosion Processes (Corrasion, Attrition, Corrosion, Hydraulic Action)
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Landform Evolution by Running Water and Underground Water Practice Questions — JAMB UTME | Examkin