Trees on a hillside have trunks that curve upward near the base, and fence posts lean downslope. Which mass-wasting process is the most likely cause?

Soil creep, the slow continuous downslope movement of the surface soil layer

Rockfall, in which blocks drop freely from a steep face and pile up as talus

Debris flow, in which a saturated mass races down a channel within minutes

Rotational slump, in which a block rotates on a spoon-shaped failure surface

In a region of folded sedimentary strata with alternating resistant sandstone ridges and weak shale valleys, long master streams run parallel along the strike valleys and are joined by short tributaries descending the ridge flanks at nearly right angles. Which drainage pattern is this?

Trellis, because master streams follow the weak strike valleys of tilted alternating strata

Rectangular, because the streams make right-angle turns along sets of intersecting joints and faults

Dendritic, because the streams branch like a tree on flat-lying rocks of uniform resistance to erosion

Radial, because the streams diverge outward from a central high point

Trellis, because master streams follow the weak strike valleys of tilted alternating strata

A river that had reached a graded profile to the sea is suddenly given a lower base level when sea level falls by 20 m. Which sequence of response is expected?

A knickpoint forms at the mouth and migrates upstream, and incision behind it leaves terraces on the old floodplain

The river aggrades throughout its length and builds a thicker floodplain, and terraces cannot form in this case at all

A knickpoint forms at the headwaters and migrates downstream, while the lower valley slowly fills up with sediment

Nothing changes in the valley, because the long profile depends only on discharge and not on the level of its outlet

Which of the following statements about meandering rivers and floodplains are correct?

A point bar builds on the inside of a bend where flow is slow, while the cut bank retreats on the outside where flow is fastest

An oxbow lake is left when erosion narrows a meander neck until the river cuts straight through it

A point bar builds on the inside of a bend where flow is slow, while the cut bank retreats on the outside where flow is fastest

Natural levees are made of the finest clay, which settles farthest from the channel during floods

An oxbow lake is left when erosion narrows a meander neck until the river cuts straight through it

A braided river has higher sinuosity than a meandering river because it splits into many channels

A desert floor with a limited sand supply and one steady wind direction carries isolated crescent-shaped dunes whose horns point downwind and whose steep slip faces lie on the concave side. What are these dunes?

Barchans, which form where sand is scarce and wind blows from one direction

Parabolic dunes, which are U-shaped and held in place by partial vegetation cover

Transverse dunes, which are long continuous ridges set at right angles to the wind

Barchans, which form where sand is scarce and wind blows from one direction

Star dunes, which form where winds blow from many different directions in turn

Which of the following landforms are produced mainly by wind erosion rather than by wind deposition?

Yardangs, streamlined ridges of soft rock carved parallel to the prevailing wind

Deflation hollows, shallow basins from which fine grains have been blown away

Yardangs, streamlined ridges of soft rock carved parallel to the prevailing wind

Parabolic dunes, whose arms are anchored by vegetation

Deflation hollows, shallow basins from which fine grains have been blown away

Loess deposits, buff silt that stands in vertical bluffs

A bedrock knob in a glaciated valley is smooth, polished and gently inclined on the side from which the ice came, and is steep and rough with plucked joint blocks on the downstream side. Which statement about it is correct?

It is a roche moutonnée, an erosional form made by abrasion up-ice and plucking down-ice

It is a drumlin, because it is streamlined in the direction of ice flow and made of till

It is a roche moutonnée, an erosional form made by abrasion up-ice and plucking down-ice

It is an erratic, because ice has carried it a long way from its original source

It is a kame, a mound of sorted sediment built by meltwater at the front of the ice

Which of the following landforms are produced mainly by glacial erosion rather than by glacial deposition?

Cirque, the armchair-shaped hollow at the head of a valley glacier

Hanging valley, a tributary valley left high above a deepened U-shaped trough

Cirque, the armchair-shaped hollow at the head of a valley glacier

Esker, a sinuous ridge of sorted sand and gravel left by a subglacial stream

Hanging valley, a tributary valley left high above a deepened U-shaped trough

Kettle, a hollow left where a buried block of ice melted out of the sediment

A narrow ridge of sand connects a small rocky island to the mainland shore. What is this landform called?

Tombolo, a bar that ties an island to the mainland

Spit, a sand ridge attached to the land at one end and free at the other

Barrier island, an offshore sand ridge separated from the coast by a lagoon

Wave-cut platform, a rock surface cut at the foot of a retreating cliff

Tombolo, a bar that ties an island to the mainland

A coast has rocky headlands alternating with sandy bays, and waves approach from the open sea. Which description of wave behaviour and its result is correct?

Refraction focuses wave energy on the headlands and disperses it in the bays, so headlands erode and bays gain sand

Refraction spreads wave energy over the headlands and focuses it in the bays, so the bays erode fastest

Refraction focuses wave energy on the headlands and disperses it in the bays, so headlands erode and bays gain sand

Waves lose all their energy offshore at wave base, so neither the headlands nor the bays are changed by them at all

Longshore drift carries sand from the bays to the headlands, which therefore build seaward as long spits

Two volcanoes are compared. Volcano P has broad convex slopes of only a few degrees and is built almost wholly of thin basaltic lava flows. Volcano Q is a steep concave-upward cone of alternating andesitic lava and pyroclastic layers. Which pair names them correctly?

P is a shield volcano and Q is a stratovolcano

P is a stratovolcano and Q is a shield volcano

P is a cinder cone and Q is a lava dome

P is a shield volcano and Q is a stratovolcano

P is a caldera and Q is a flood-basalt plateau

A large circular depression several kilometres wide, with steep walls, lies where a high volcanic cone once stood. Which process best explains it?

Collapse of the roof into a magma chamber emptied by a very large explosive eruption

Slow erosion of the cone by rivers that cut radial valleys into its flanks over millions of years

Repeated effusion of very fluid basalt that gradually builds up the broad rim of a shield

Piling up of viscous lava at the vent until a steep dome overflows the old crater

Earth Surface Processes: Hillslope, River, Wind, Glacial, Coastal and Volcanic Landforms

This chapter answers the Part A heading “Earth surface processes” of the Geology and Geophysics (GG) paper: “Landforms created by hillslopes, rivers, wind, glaciers, oceans, and volcanoes.” It teaches, agent by agent, the process that moves material and the landform it leaves: weathering and mass wasting on hillslopes, drainage networks and the river long profile, wind-blown dunes and deflation forms, glacial erosion and deposition, waves and coastal forms, and volcanic edifices. The numericals are slope and gradient, drainage density, Strahler order and bifurcation ratio, the threshold of grain motion in a stream from a stated Shields criterion, and the cubic law of wind transport.

1. Hillslopes: weathering, mass wasting and slope form

Rock exposed at the surface is broken down by weathering (physical disintegration and chemical decay) and the debris is carried downslope by gravity, which is mass wasting. A slope fails when the driving shear stress along some surface exceeds the shear strength; water lowers the strength by adding weight and raising pore-water pressure, which is why most slides follow heavy rain. Loose dry debris rests at its angle of repose, about 30° to 35° for angular rubble, and that sets the slope of talus (scree) cones below cliffs. Slope is quoted as gradient = rise ÷ run = tan θ, as percent slope = 100 tan θ, or as the angle θ itself.

Fall and topple: blocks detach from a steep face and drop or tip over; the pile at the foot is talus.
Slide: movement on a surface. A rotational slide (slump) moves on a concave-up, spoon-shaped surface and leaves a crescent scarp and back-tilted blocks; a translational slide follows a planar bedding or joint surface.
Flow: a water-saturated mass (debris flow, mudflow, earthflow) moves like a viscous fluid; a lahar is a volcanic mudflow.
Creep: very slow, continuous movement of soil, millimetres per year, shown by curved tree trunks and tilted fence posts. Solifluction is saturated soil creeping over frozen ground in cold climates.

Creep and slope wash give a typical slope profile that is convex at the hilltop, because transport grows with distance from the divide (in diffusive models the flux is q = −K dz/dx), and concave at the foot, where debris accumulates and streams carry it off. Weak, sparsely vegetated clay and shale cut by rills and gullies form badlands. Worked example: two adjacent contours 20 m apart in elevation are 5 cm apart on a 1:25 000 map. Ground distance = 0.05 m × 25 000 = 1250 m, so gradient = 20 ÷ 1250 = 0.016 = 1.6 %.

⚠️ Percent is not degrees

A slope of 100 % is 45°, not 90°, because tan 45° = 1. A 30° slope is about 58 %, and a 1 in 20 gradient is 5 % (about 2.9°). Always check whether a question gives rise ÷ run, a percentage or an angle.

2. Drainage basins, stream order and drainage patterns

A drainage basin is the area that sheds water to one outlet, bounded by a divide. In Strahler ordering headwater streams are order 1; where two streams of the same order u meet, the stream below is order u + 1, and a junction of unequal orders keeps the higher order. Horton's law of stream numbers: the number of streams falls geometrically with order, and the ratio Rb = N(u) ÷ N(u+1) is the bifurcation ratio, usually 3 to 5 in basins not dominated by structure. For a basin of highest order Ω with constant Rb, the count of first-order streams is Rb^(Ω−1). A basin with 64, 16, 4 and 1 streams of orders 1 to 4 has Rb = 4 at every step.

Drainage density Dd = total length of channels ÷ basin area (km per km²). It is high where the ground is impermeable, weakly vegetated, easily eroded or very wet, and low on permeable, well-vegetated terrain. A basin of 12.5 km² with 40 km of channels has Dd = 3.2 km/km². High Dd means a flashy flood response because water reaches channels quickly.

Drainage patterns and what controls them
Pattern	Appearance	Typical control
Dendritic	Tree-like branching at acute angles	Uniform resistance: flat-lying or massive rock
Trellis	Long parallel master streams, short right-angle tributaries	Folded alternating hard and soft strata
Rectangular	Right-angle bends in main and tributary streams	Intersecting joints and faults
Radial	Streams run outward from a centre	Dome or volcanic cone
Centripetal	Streams run inward to a low point	Basin, crater or sinkhole depression
Parallel	Straight, near-parallel streams	Uniform steep regional slope
Annular	Ring-like streams around a centre	Eroded dome with alternating hard and soft rings
Deranged	No integrated network, many lakes	Young terrain after glaciation

3. River processes, long profile and fluvial landforms

A river carries dissolved load, suspended load and bed load (rolling, sliding and saltating grains). For uniform flow the bed shear stress is τ_b = ρ g h S (water density ρ = 1000 kg/m³, depth h, bed slope S). A grain of diameter D begins to move when the Shields parameter τ* = τ_b ÷ [(ρ_s − ρ) g D] reaches a critical value τ*c, roughly 0.03 to 0.06 for gravel in turbulent flow. The largest grain moved is therefore D = τ_b ÷ [τ*c (ρ_s − ρ) g]. Worked example (take g = 9.8 m/s², ρ_s = 2650 kg/m³, τ*c = 0.05): for h = 1.5 m and S = 0.002, τ_b = 1000 × 9.8 × 1.5 × 0.002 = 29.4 Pa and D = 29.4 ÷ (0.05 × 1650 × 9.8) = 0.0364 m, about 36 mm. D scales linearly with h and S.

ℹ️ Hjulström curve

The velocity needed to entrain grains is lowest for fine sand (a few tenths of a millimetre). Silt and clay need a higher velocity because cohesion binds them, even though they are smaller, while coarse grains need it because of their weight. Once moving, fine particles stay in suspension at velocities far below their entrainment velocity.

The long profile (elevation against distance) is concave-up: steep with coarse bed load in the headwaters, gentle and fine-grained downstream where discharge is large. A graded river balances its transport capacity against its sediment supply. Base level is the lowest level to which a river can cut (sea level ultimately, or a lake or resistant ledge locally). A fall of base level or uplift creates a knickpoint, a steep step such as a waterfall, that migrates upstream, and the river incises behind it, leaving terraces; a rise of base level makes the river aggrade.

Upper course: V-shaped valleys, rapids, waterfalls with plunge pools, potholes and gorges.
Meanders: erosion on the outer cut bank where flow is fastest, deposition of a point bar on the inner bank; sinuosity = channel length ÷ valley length. A neck cut-off leaves an oxbow lake.
Floodplain: overbank floods build coarse natural levees beside the channel and fine back-swamp mud farther away. Braided rivers (high bed load, erodible banks, variable discharge) split around mid-channel bars; meandering rivers carry finer load in cohesive banks.
Alluvial fan where a stream leaves a mountain front and loses confinement; delta where it enters standing water: river-dominated (bird-foot, Mississippi), wave-dominated (arcuate, Nile), tide-influenced (Ganga–Brahmaputra).

4. Wind: transport, erosional forms and dunes

Wind moves grains by suspension (dust), saltation (sand hopping on ballistic paths, the dominant mode for sand) and surface creep (coarser grains pushed by the impact of saltating ones). Air is about 800 times lighter than water, so wind transports only sand and finer material. Above the threshold, sand flux rises roughly as the cube of the shear velocity, q ∝ u*³ (Bagnold), so tripling u* raises the flux 27 times.

Deflation removes fines, leaving deflation hollows (blowouts) and a stone desert pavement (lag).
Abrasion by saltating sand near the ground carves faceted pebbles (ventifacts), streamlined ridges parallel to the wind (yardangs) and pedestal rocks.
Loess is wind-laid, unlayered silt that stands in vertical cliffs, sourced from glacial outwash and desert margins.

Principal dune types
Dune	Wind and sand supply	Form
Barchan	One dominant direction, limited sand	Crescent, horns point downwind
Transverse	One direction, abundant sand	Wavy ridges at right angles to the wind
Linear (seif)	Two oblique wind directions	Long ridges along the net transport direction
Star	Winds from many directions	Central peak with radiating arms; builds up, hardly migrates
Parabolic	One direction, partial vegetation cover	U-shape, arms trail upwind, nose moves downwind

Every migrating dune has a gentle windward (stoss) side, about 10° to 15°, and a slip face on the lee side at the angle of repose, about 30° to 34°. Grains avalanche over the crest, so the dune advances downwind and leaves cross-bedding whose foresets dip downwind. The Thar desert carries barchans, linear and transverse dunes among other forms.

5. Glaciers: erosional and depositional landforms

A glacier is a moving mass of ice formed from compacted snow (firn). It moves by internal deformation and by basal sliding on meltwater at the pressure-melting point. Above the equilibrium line (net mass balance zero) snow accumulates; below it ice ablates, and the glacier advances when accumulation exceeds ablation. Ice erodes by plucking (quarrying of jointed rock) and abrasion by rock fragments held in the base, which leaves polish and striations. Gangotri, whose snout Gaumukh feeds the Bhagirathi, is a Himalayan valley glacier.

Landforms of glacial erosion
Landform	Description
Cirque (tarn when a lake fills it)	Armchair hollow at the head of a valley glacier
Arête and horn	Knife-edge ridge between cirques; pyramid peak where several cirques meet
U-shaped valley	Steep walls, flat floor, truncated spurs
Hanging valley	Tributary valley left high above the deepened trunk valley, often with a waterfall
Fjord	Glacial trough drowned by the sea, with a shallow sill at the mouth
Roche moutonnée	Bedrock knob, smooth and gently sloping up-ice, steep and plucked down-ice

Deposits are of two kinds. Till is dumped directly by ice, unsorted and unlayered from clay to boulders. Outwash is sorted and stratified, laid down by meltwater. Moraines are ridges of till: lateral, medial (where two lateral moraines merge below a junction of valley glaciers), terminal (end), which marks the greatest advance, and ground moraine. Drumlins are streamlined oval hills of till, elongated along the flow with the steeper blunt end facing up-ice. Eskers are long sinuous ridges of sand and gravel from subglacial streams, kames are meltwater mounds, kettles are hollows left by melted buried ice, and varves are annual coarse-summer and fine-winter couplets in glacial lakes. Erratics are boulders carried far from their source.

⚠️ Drumlin or roche moutonnée?

Both are streamlined along the ice flow, but the drumlin is built of till and its steeper, blunter end faces up-ice. The roche moutonnée is eroded bedrock: gentle and abraded up-ice, steep and plucked down-ice. Till and moraines are unsorted, whereas eskers, kames and outwash are sorted meltwater deposits.

6. Oceans and coasts

Waves feel the bottom at a depth of about half their wavelength; in shallow water they slow, steepen and break. Waves arriving at an angle push water obliquely up the beach (swash) while the backwash runs straight down the slope, so sand zigzags along the shore as longshore drift. Refraction bends wave fronts toward the shore and focuses energy on headlands, which erode, while bays receive sand.

Erosional forms: sea cliff and notch, wave-cut platform, and from a jointed headland the sequence cave, arch, stack, stump.
Depositional forms: spit (a ridge joined to land at one end), baymouth bar, tombolo (a bar linking an island to the mainland), barrier island with a lagoon behind, and tidal flats with mangroves, as in the Sundarbans.
Sea-level change: emergent coasts show raised beaches and marine terraces; submergent coasts show rias (drowned river valleys) and fjords.
Coral reefs: fringing, barrier and atoll. Darwin's sequence is a subsiding volcanic island passing from fringing reef to barrier reef to atoll; the Lakshadweep islands are mostly atolls.

7. Volcanic landforms

The form of a volcano follows the viscosity and gas content of its magma. Fluid basalt flows far and builds gentle edifices; viscous, gas-rich andesite and rhyolite pile up near the vent or explode.

Volcanic edifices
Edifice	Magma and style	Form
Shield volcano	Fluid basalt, effusive	Broad dome with slopes of only a few degrees (Mauna Loa)
Stratovolcano (composite)	Andesite to dacite, alternating lava and pyroclastic layers	Steep, concave-upward cone (Fuji, Vesuvius)
Cinder (scoria) cone	Basaltic scoria thrown from a vent	Small cone with slopes near the angle of repose
Lava dome	Very viscous rhyolitic or dacitic lava	Steep-sided mound over the vent
Caldera	Large explosive eruption empties the chamber	Wide circular depression by roof collapse
Flood-basalt plateau	Fissure eruptions of very fluid basalt	Stacked flows in stepped (trap) terrain, e.g. the Deccan Traps

Basalt lava has two surface forms: smooth, ropy pahoehoe and rough, clinkery aa; under water it forms pillow lava. Cooling contraction makes columnar joints. Fragmental products are ash, lapilli and bombs, and hot pyroclastic flows leave welded tuff. The Deccan Traps are the product of fissure eruptions about 66 million years ago, and Barren Island in the Andaman Sea is India's only historically active volcano.

Key takeaways

Hillslopes fail when driving stress exceeds shear strength, and water is the usual trigger. Falls, slides, flows and creep leave talus, scarps, lobes and convex-concave slopes. Gradient = tan θ, percent = 100 tan θ, so 100 % is 45°.
Strahler order rises where equal orders meet; Rb = N(u) ÷ N(u+1) and N1 = Rb^(Ω−1). Drainage density = channel length ÷ area. Drainage patterns record structure: dendritic uniform, trellis folded strata, rectangular joints, radial domes.
A grain moves when τ_b = ρ g h S exceeds τc (ρ_s − ρ) g D, so D = τ_b ÷ [τc (ρ_s − ρ) g]. The long profile is concave-up; a base-level fall makes a knickpoint that retreats upstream and leaves terraces.
Wind moves sand by saltation, flux rising as u*³. Deflation hollows, pavement, ventifacts and yardangs are erosional; barchan, transverse, linear, star and parabolic dunes and loess are depositional, with slip faces at the angle of repose.
Glaciers erode by plucking and abrasion to make cirques, arêtes, horns, U-valleys, hanging valleys and fjords, and deposit till (moraines, drumlins) and sorted meltwater deposits (eskers, kames, outwash). Drumlin is depositional, roche moutonnée erosional.
Waves refract onto headlands and drift sand along the shore to build spits, tombolos and barriers. Volcano shape follows magma: fluid basalt makes shields and flood-basalt plateaus, viscous andesite makes stratovolcanoes and domes, and caldera collapse leaves a wide depression.

Practice questions (17)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Trees on a hillside have trunks that curve upward near the base, and fence posts lean downslope. Which mass-wasting process is the most likely cause?
1. Soil creep, the slow continuous downslope movement of the surface soil layer
2. Rockfall, in which blocks drop freely from a steep face and pile up as talus
3. Debris flow, in which a saturated mass races down a channel within minutes
4. Rotational slump, in which a block rotates on a spoon-shaped failure surface
Show answer
Answer: A — Soil creep, the slow continuous downslope movement of the surface soil layer
Curved trunks and tilted posts record slow, continuous creep: the tree keeps growing upright while the soil moves, so the base bends. A rotational slump is the tempting alternative, but it is sudden, leaves a crescent scarp with a back-tilted block, and does not bend trunks into a smooth curve. Rockfall and debris flow are rapid events and leave talus and lobes respectively.
On a topographic map of scale 1:25 000, two adjacent contour lines whose elevations differ by 20 m are 5 cm apart. What is the average slope between them, as a percentage (rise ÷ run × 100), to one decimal place? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 1.6
Ground distance = 5 cm × 25 000 = 125 000 cm = 1250 m. Slope = 20 m ÷ 1250 m = 0.016, which is 1.6 %. The usual slip is to use the map distance of 5 cm as if it were metres, or to report the angle (about 0.9°) instead of the percentage.
In a region of folded sedimentary strata with alternating resistant sandstone ridges and weak shale valleys, long master streams run parallel along the strike valleys and are joined by short tributaries descending the ridge flanks at nearly right angles. Which drainage pattern is this?
1. Rectangular, because the streams make right-angle turns along sets of intersecting joints and faults
2. Dendritic, because the streams branch like a tree on flat-lying rocks of uniform resistance to erosion
3. Radial, because the streams diverge outward from a central high point
4. Trellis, because master streams follow the weak strike valleys of tilted alternating strata
Show answer
Answer: D — Trellis, because master streams follow the weak strike valleys of tilted alternating strata
Trellis drainage is the signature of folded or tilted alternating hard and soft strata: master streams occupy the weak strike valleys and short tributaries cut across the ridges. Rectangular drainage is the tempting alternative, since it also shows right-angle junctions, but there the bends come from joints and faults in rock of broadly uniform resistance, and the long parallel strike-valley streams are absent.
A drainage basin has 81 first-order streams, and its bifurcation ratio is a constant 3 at every order. If the highest-order channel is a single stream, what is the Strahler order of the basin outlet?
Numerical answer — type the value.
Show answer
Answer: 5
With Rb = 3 the counts fall as 81, 27, 9, 3, 1 for orders 1 to 5. Equivalently N1 = Rb^(Ω−1) gives 81 = 3^4, so Ω − 1 = 4 and Ω = 5. Answering 4 comes from counting the four divisions instead of the five orders.
A catchment has an area of 12.5 km² and the total length of all its channels is 40 km. What is its drainage density in km/km², to one decimal place? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 3.2
Drainage density = total channel length ÷ basin area = 40 km ÷ 12.5 km² = 3.2 km/km². Inverting the ratio gives 0.31, which is the area per unit length and not the density.
A steady, uniform river flows 1.5 m deep over a bed of slope 0.002. Take g = 9.8 m/s², water density 1000 kg/m³, grain density 2650 kg/m³ and a critical Shields parameter of 0.05. What is the diameter of the largest grain that the flow can just move, in millimetres, to the nearest millimetre? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 36
Bed shear stress τ_b = ρ g h S = 1000 × 9.8 × 1.5 × 0.002 = 29.4 Pa. At the threshold τ* = τ_b ÷ [(ρ_s − ρ) g D] = 0.05, so D = 29.4 ÷ (0.05 × 1650 × 9.8) = 29.4 ÷ 808.5 = 0.03636 m = 36.4 mm, which rounds to 36. Using ρ_s instead of (ρ_s − ρ) gives about 23 mm, the common slip.
A river that had reached a graded profile to the sea is suddenly given a lower base level when sea level falls by 20 m. Which sequence of response is expected?
1. A knickpoint forms at the mouth and migrates upstream, and incision behind it leaves terraces on the old floodplain
2. The river aggrades throughout its length and builds a thicker floodplain, and terraces cannot form in this case at all
3. A knickpoint forms at the headwaters and migrates downstream, while the lower valley slowly fills up with sediment
4. Nothing changes in the valley, because the long profile depends only on discharge and not on the level of its outlet
Show answer
Answer: A — A knickpoint forms at the mouth and migrates upstream, and incision behind it leaves terraces on the old floodplain
A fall of base level steepens the lower reach, so the river erodes downward there first; the steepened step is a knickpoint that retreats upstream, and incision into the former floodplain leaves terraces. Aggradation is the response to a rise in base level, which is the tempting reversal. A knickpoint cannot migrate downstream because the disturbance starts at the outlet and travels up the channel.
Which of the following statements about meandering rivers and floodplains are correct?
1. A point bar builds on the inside of a bend where flow is slow, while the cut bank retreats on the outside where flow is fastest
2. Natural levees are made of the finest clay, which settles farthest from the channel during floods
3. An oxbow lake is left when erosion narrows a meander neck until the river cuts straight through it
4. A braided river has higher sinuosity than a meandering river because it splits into many channels
Show answer
Answer: A — A point bar builds on the inside of a bend where flow is slow, while the cut bank retreats on the outside where flow is fastest; C — An oxbow lake is left when erosion narrows a meander neck until the river cuts straight through it
The inside of a bend is slow and collects the point bar, while the outer bank is cut by the fastest flow; a neck cut-off isolates the old loop as an oxbow lake. Levees are the coarsest overbank deposit, laid next to the channel where the flood first loses speed, and the fine mud goes to the back swamp. Braiding reflects high bed load and steep, erodible banks; it makes a wide, low-sinuosity channel, not a highly sinuous one.
A desert floor with a limited sand supply and one steady wind direction carries isolated crescent-shaped dunes whose horns point downwind and whose steep slip faces lie on the concave side. What are these dunes?
1. Parabolic dunes, which are U-shaped and held in place by partial vegetation cover
2. Transverse dunes, which are long continuous ridges set at right angles to the wind
3. Barchans, which form where sand is scarce and wind blows from one direction
4. Star dunes, which form where winds blow from many different directions in turn
Show answer
Answer: C — Barchans, which form where sand is scarce and wind blows from one direction
A crescent with horns pointing downwind, formed in a one-directional wind with little sand, is a barchan. Parabolic dunes are the tempting confusion because they are also curved, but their arms trail upwind, held by vegetation, and the nose advances downwind. With abundant sand the barchans merge into transverse ridges.
Which of the following landforms are produced mainly by wind erosion rather than by wind deposition?
1. Yardangs, streamlined ridges of soft rock carved parallel to the prevailing wind
2. Parabolic dunes, whose arms are anchored by vegetation
3. Deflation hollows, shallow basins from which fine grains have been blown away
4. Loess deposits, buff silt that stands in vertical bluffs
Show answer
Answer: A — Yardangs, streamlined ridges of soft rock carved parallel to the prevailing wind; C — Deflation hollows, shallow basins from which fine grains have been blown away
Yardangs are cut by abrasion and deflation, and deflation hollows are the basins left where fines have been removed: both are erosional. Parabolic dunes and loess are accumulations of sand and silt laid down by wind, so they are depositional even though wind built them.
Sand flux in a steady wind, well above the threshold, may be taken as proportional to the cube of the shear velocity u. By what factor does the flux increase when u rises from 0.20 m/s to 0.60 m/s? Give the number only.
Numerical answer — type the value.
Show answer
Answer: 27
Flux ratio = (0.60 ÷ 0.20)³ = 3³ = 27. Taking the ratio of speeds directly (3) or its square (9) are the usual slips; the cubic law is what makes strong winds move most of the sand.
A bedrock knob in a glaciated valley is smooth, polished and gently inclined on the side from which the ice came, and is steep and rough with plucked joint blocks on the downstream side. Which statement about it is correct?
1. It is a drumlin, because it is streamlined in the direction of ice flow and made of till
2. It is a roche moutonnée, an erosional form made by abrasion up-ice and plucking down-ice
3. It is an erratic, because ice has carried it a long way from its original source
4. It is a kame, a mound of sorted sediment built by meltwater at the front of the ice
Show answer
Answer: B — It is a roche moutonnée, an erosional form made by abrasion up-ice and plucking down-ice
A knob of bedrock that is abraded and gently sloping up-ice and plucked and steep down-ice is a roche moutonnée. A drumlin looks similar in plan, but it is a pile of till, not bedrock, and its steeper, blunter end faces up-ice, the reverse of this knob. Erratics are transported boulders and kames are sorted meltwater deposits.
Which of the following landforms are produced mainly by glacial erosion rather than by glacial deposition?
1. Cirque, the armchair-shaped hollow at the head of a valley glacier
2. Esker, a sinuous ridge of sorted sand and gravel left by a subglacial stream
3. Hanging valley, a tributary valley left high above a deepened U-shaped trough
4. Kettle, a hollow left where a buried block of ice melted out of the sediment
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Answer: A — Cirque, the armchair-shaped hollow at the head of a valley glacier; C — Hanging valley, a tributary valley left high above a deepened U-shaped trough
The cirque is gouged by plucking and abrasion at the glacier head, and the hanging valley records the deeper erosion of the main glacier compared with its tributary: both are erosional. An esker is built of meltwater sediment, and a kettle is a depression within glacial deposits where buried ice melted, so both belong to deposition.
A narrow ridge of sand connects a small rocky island to the mainland shore. What is this landform called?
1. Spit, a sand ridge attached to the land at one end and free at the other
2. Barrier island, an offshore sand ridge separated from the coast by a lagoon
3. Wave-cut platform, a rock surface cut at the foot of a retreating cliff
4. Tombolo, a bar that ties an island to the mainland
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Answer: D — Tombolo, a bar that ties an island to the mainland
A bar of sand or gravel that links an island to the mainland is a tombolo. A spit is the tempting alternative because it is also a sand ridge on the shore, but a spit is attached only at one end and ends in open water, so it does not connect to an island.
A coast has rocky headlands alternating with sandy bays, and waves approach from the open sea. Which description of wave behaviour and its result is correct?
1. Refraction spreads wave energy over the headlands and focuses it in the bays, so the bays erode fastest
2. Refraction focuses wave energy on the headlands and disperses it in the bays, so headlands erode and bays gain sand
3. Waves lose all their energy offshore at wave base, so neither the headlands nor the bays are changed by them at all
4. Longshore drift carries sand from the bays to the headlands, which therefore build seaward as long spits
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Answer: B — Refraction focuses wave energy on the headlands and disperses it in the bays, so headlands erode and bays gain sand
Waves slow in the shallow water off a headland and bend toward it, so their energy converges there and the headland is eroded; in the bays the fronts spread out and the low energy lets sand settle. The reversed statement is the tempting error. Waves do not lose all energy at wave base, since that depth only marks where they start to feel the bottom.
Two volcanoes are compared. Volcano P has broad convex slopes of only a few degrees and is built almost wholly of thin basaltic lava flows. Volcano Q is a steep concave-upward cone of alternating andesitic lava and pyroclastic layers. Which pair names them correctly?
1. P is a stratovolcano and Q is a shield volcano
2. P is a cinder cone and Q is a lava dome
3. P is a shield volcano and Q is a stratovolcano
4. P is a caldera and Q is a flood-basalt plateau
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Answer: C — P is a shield volcano and Q is a stratovolcano
Fluid basalt spreads far and builds a broad shield with slopes of a few degrees, while viscous andesite and explosive pyroclastic layers build a steep composite cone. The stratovolcano and shield pairing reversed is the tempting trap. A cinder cone is small and steep with slopes near the angle of repose, and a caldera is a collapse depression, not an edifice.
A large circular depression several kilometres wide, with steep walls, lies where a high volcanic cone once stood. Which process best explains it?
1. Collapse of the roof into a magma chamber emptied by a very large explosive eruption
2. Slow erosion of the cone by rivers that cut radial valleys into its flanks over millions of years
3. Repeated effusion of very fluid basalt that gradually builds up the broad rim of a shield
4. Piling up of viscous lava at the vent until a steep dome overflows the old crater
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Answer: A — Collapse of the roof into a magma chamber emptied by a very large explosive eruption
A caldera forms when a large volume of magma is erupted so fast that the chamber can no longer support its roof, which subsides along ring faults. River erosion cuts radial valleys and does not create a wide circular basin with steep walls, which is the tempting but wrong choice. Effusion and dome growth build positive relief rather than a depression.