Exercises: Landform modelling

Name four agents of landform modelling and a characteristic landform produced by each.

What is differential erosion? Give an example of a landform explained by it.

Limestone rocks develop karst topography. Name two surface karst landforms (exokarst) and two underground karst landforms (endokarst).

What is a karstic cave and which two processes, the dissolution and the precipitation of carbonate, take part in its excavation and in its decoration?

What is a meander and in which stretch of a river does it develop best? Explain why lateral erosion dominates over vertical erosion in that stretch.

What is a dune and what is the difference between its windward side and its leeward side? What does the inclination of its internal layers indicate about the wind that formed it?

Name three glacial landforms, such as the cirque, the U-shaped valley or the moraine, and briefly describe each one.

What is a cliff and which process causes its retreat? Name a landform associated with marine erosion, such as an arch, a sea stack or a wave-cut platform.

In karst, the dissolution of limestone follows the reaction $\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca}^{2+} + 2\ \text{HCO}_3^{-}$. Explain what role the $\text{CO}_2$ dissolved in rainwater plays and why karst develops better in humid climates than in arid climates.

Profiles 1 and 2 show two valleys with very different geometries. Identify which agent (a river or a glacier) carved each one and explain how the geometry of the valley reflects its formation process.

In a region, limestone and marl layers alternate with a gentle dip. Predict what structural landform might develop (cuesta) and describe its profile: why is one slope much gentler than the other?

Stalactites and stalagmites grow by precipitation of the carbonate that the water carries dissolved. Explain the chemical process, the reverse of dissolution, and why an "active" cave needs a continuous supply of water.

What are fluvial terraces and how do they form? Explain what history of river incision, through changes in base level or climate, their stepped levels record.

Distinguish cuesta relief from eroded anticline relief: which structure controls each one and how they are distinguished by the dip of the strata on the two slopes.

What are gullies and why do they develop rapidly on hillsides of soft materials, without vegetation and with torrential rainfall? Relate them to the badlands landscape.

Explain how a wave-cut platform forms at the foot of a cliff and what happens to it if sea level falls (raised marine platform).

An environmental restoration plan proposes curbing erosion in a badlands basin in Guadix-Baza through two actions: the massive terracing of slopes using heavy machinery and the intensive planting of Aleppo pine.

1. a) Justify why terracing on silty-clayey and gypsiferous materials in a semi-arid climate can drastically accelerate piping processes (tunnel erosion or suffosion) and slope collapse instead of curbing them.
2. b) Analyze the response of the hydrological system to a flash storm (high-intensity, short-duration rainfall) considering the infiltration rate of these clayey materials and the sparse initial plant cover.
3. c) Design an alternative soil conservation and hydrological-forestry management strategy that is consistent with the natural dynamics of this landform.

Two areas with the same granitic lithology (same mineralogical composition and initial tectonic fracture density) are compared: the Ría de Vigo (temperate-humid oceanic climate, 1600 mm annually with no dry season) and La Pedriza de Manzanares (continentalized Mediterranean mountain climate, 800 mm annually with winter frosts and a very dry summer).

1. a) Interpret the differential genesis of both landscapes by explaining the predominant physical-chemical mechanisms: why does deep chemical weathering ('lehm' or arenization) with isolated corestones predominate in Galicia, whereas tors with large smooth rock domes, exfoliation sheets, and angular scree dominate in Guadarrama?
2. b) Formulate a well-founded hypothesis: if climate change were to make the Galician climate much more arid and colder (periglacial), what would happen to the existing weathering mantle and what new landform would be exposed after a period of active erosion?

During the excavation of a road cutting in a mountain valley at an altitude of 1200 m, two superposed unconsolidated sedimentary formations are exposed:

• Deposit A (base): Massive diamictic material, lacking stratification or grain-size sorting, with a silty-clayey matrix (40%) enclosing clasts of all sizes (from millimeters to metric boulders), predominantly angular and faceted, some with longitudinal striations.

• Deposit B (top): Material with well-defined cross-stratification, very well sorted (medium gravels of 2 to 5 cm), sandy matrix below 10%, and rounded, imbricated pebbles oriented downstream.

A preliminary technical report erroneously classifies both deposits as 'flood alluvium'.

1. a) Refute the report by rigorously assigning each deposit to its transport agent (glacial vs. fluvial), justifying how the physical properties of the agent (viscosity, density, and flow regime) determine grain-size sorting and the degree of roundness of the clasts.
2. b) Reconstruct the environmental evolution of the valley that explains the stratigraphic superposition of deposit B over deposit A.

To gain buildable land and 'protect from flooding' a town located on the loop of a lower-course meander, a hydraulic engineering project proposes channelizing the riverbed by excavating a straight artificial canal that cuts off the neck of the meander and building levees on its banks.

1. a) Predict the morphodynamic consequences of this artificial cutoff upstream and downstream of the channelized section, considering changes in river gradient, flow energy, headward erosion, and sedimentation.
2. b) Analyze the geological and geotechnical risks to which new buildings constructed on the neck cutoff meander and the floodplain would be exposed (taking into account fill permeability, differential compaction, and the water table).
3. c) Justify why sediment retention and rigid channelization run counter to the current guidelines of the Water Framework Directive, and propose a non-structural flood risk mitigation measure.

The Ebro Delta has lost more than 95% of the sediment supply it historically received due to the trapping of sands and silts in the Mequinenza and Ribarroja reservoirs built in the mid-20th century. The delta also suffers an average subsidence of around 3 mm/year and the onslaught of easterly sea storms with high-energy waves.

1. a) Use the principles of Galloway's morphogenetic triangle (fluvial-dominated, wave-dominated, or tide-dominated) to predict how the morphometry of the Ebro Delta is changing and will change as its original sediment budget is disrupted.
2. b) Explain the accelerated retreat of the Fangar spit and the Trabucador barrier, and argue why subsidence combined with relative sea-level rise exacerbates the salinization of coastal aquifers and rice paddies.
3. c) Critically evaluate two proposed actions: building a continuous concrete seawall or implementing a sediment 'bypass' (slurry transport of mud from the reservoirs) combined with dune restoration.

In the Torcal de Antequera, the relief displays orthogonal rectilinear corridors ('callejones') and limestone towers resembling 'stacks of coins' (protruding tabular strata separated by deeply dissolved horizontal grooves).

1. a) Deduce how the interaction between Alpine compressive tectonics, lithostatic decompression (orthogonal jointing), and primary lithological variations (millimeter/centimeter alternation of pure micritic limestones and marly limestones or calcarenites) has guided differential karst dissolution to generate this unique architecture.
2. b) If a deep cave located beneath this site were to undergo a continuous drawdown of the regional water table due to groundwater overexploitation, predict with reasoned arguments the effects on the mechanical stability of the overlying massif and on karst hydrodynamics.
3. c) Evaluate which direct human impacts threaten the conservation of karst dissolution microforms (such as solution flutes or runnels and biogenic cavities) and propose two technical management measures for their preservation.

In western Scandinavia, a fjord exhibits a unique bathymetry: the central basin reaches a bedrock depth of -850 m below sea level, but before connecting to the open ocean, the depth rises abruptly to a submarine bedrock threshold (sill) located at just -60 m. On the steep walls of the fjord, two hanging valleys discharge, with their current bedrock floors located at +300 m above sea level.

1. a) Justify why fluvial dynamics alone could never excavate a depression at -850 m below sea level and deduce the physical factors (ice thickness, lateral confinement, and flow velocity) that enabled the main glacier tongue to overdeepen such a basin.
2. b) Explain the physical process that gives rise to the altitudinal difference between the hanging valleys (+300 m) and the bottom of the fjord (-850 m), as well as the morphogenetic cause of the terminal sill located at -60 m.
3. c) Following deglaciation and the consequent loss of lateral ice support, predict which gravitational and geotechnical slope processes will affect the fjord walls, and reason the secondary catastrophic geological hazard that a massive landslide on these walls can generate for coastal settlements.

A geological cross-section at the margin of a sedimentary basin shows a monoclinal sequence dipping 15° to the east. The stratigraphic sequence consists of an alternation of:

• Layer A: Resistant and permeable bioclastic limestones (25 m thick).

• Layer B: Soft, plastic, and impermeable marls and clays (60 m thick).

• Layer C: Hard cemented sandstones (20 m thick).

• Layer D: Easily friable shales and gypsum (80 m thick).

The main drainage network flows north-south, perpendicular to the dip direction.

1. a) Determine the geographic orientation (which cardinal direction it faces) of the scarp or front and of the dip slope (cuesta back slope) of the resulting landforms, justifying your deduction based on the geometry of the strata.
2. b) Predict which types of slope movements (translational slides, rotational slumps, or rockfalls) will occur preferentially on the scarp compared to the dip slope of the cuesta, analyzing the influence of the dip of the bedding planes and the presence of the hydrogeological contact between limestones and marls.
3. c) Justify why scarp retreat over geological time can lead to the formation of 'witness hills' (buttes) and isolated pinnacles (outliers), and predict their future geomorphological evolution.