Question 2 of 3: Part 2 — Short Essay Questions

Question 2: Part 2 — Short Essay Questions (40 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

2.1 — Two schools of thought in geomorphology

Geomorphology's development is usually framed around two contrasting paradigms. The first is the Davisian (historical/genetic) school, championed by William Morris Davis in his "geographical cycle" (cycle of erosion): landscapes are treated as evolving through a predictable sequence of stages -- youth, maturity and old age -- driven by uplift followed by progressive denudation toward a base-level peneplain. The emphasis is on the LONG-TERM, qualitative, evolutionary history of a landscape, inferred by reading present-day landforms as a "stage" in that cycle.

The second is the Gilbertian (process/equilibrium) school, championed by Grove Karl Gilbert, which emphasizes the DYNAMIC, quantitative study of the physical processes actively operating on a landscape (fluvial mechanics, hillslope processes, structural control) and the tendency of landforms toward a graded, dynamic equilibrium between process and form, rather than a fixed evolutionary sequence. Modern process geomorphology is essentially the Gilbertian approach extended with quantitative measurement, rates and thresholds, while the Davisian cycle survives mainly as a conceptual/historical framing device.

2.2 — Five soil-forming factors

Soil formation (pedogenesis) is governed by the five factors identified by Hans Jenny's CLORPT model: Climate, Organisms, Relief (topography), Parent material, and Time.

Climate (temperature and precipitation) controls the rate and type of weathering (chemical weathering dominates in warm, humid climates; physical/frost weathering in cold climates) and the intensity of leaching, which drives horizon differentiation. Organisms (vegetation, soil fauna, microorganisms, and historically human activity) supply organic matter, mix and aerate the profile (bioturbation), and drive nutrient cycling. Relief controls drainage, erosion and deposition rates, and the local water table -- steep slopes shed water and sediment and stay thin/immature, while depressions accumulate colluvium/water and develop poorly drained, organic-rich profiles. Parent material supplies the initial mineralogy and texture, which sets the starting chemistry and the rate at which weathering can proceed (e.g. limestone weathers faster than quartzite). Time is the factor that allows the other four to act cumulatively; a young soil (e.g. on a recently deglaciated or flood-deposited surface) shows little horizon development regardless of how favourable the other factors are, while a soil that has weathered undisturbed for tens of thousands of years can develop deep, well-differentiated horizons.

2.3 — Five major erosional agents

The principal agents that actively modify landscapes are: (1) Running water (fluvial processes) -- rivers erode by hydraulic action, abrasion and corrosion, transport sediment as bedload/suspended/dissolved load, and deposit it as floodplains, deltas and fans; the classic example is a river incising a canyon while building a delta at its mouth. (2) Glacial ice -- moving ice erodes by plucking and abrasion (producing striations, cirques, U-shaped valleys) and deposits unsorted till directly, or sorted outwash via meltwater; a cirque headwall retreating by plucking is a direct example. (3) Wind (aeolian processes) -- wind erodes by deflation and abrasion in arid/unvegetated settings and deposits sorted, well-rounded sand as dunes (e.g. a barchan dune migrating downwind as sand is eroded from the windward slip face and deposited on the lee slip face). (4) Gravity (mass wasting) -- the direct downslope movement of regolith/rock under gravity, ranging from slow creep to catastrophic rockslides, is involved wherever slope angle exceeds the material's shear strength; a talus cone building below a rockfall source is a direct example. (5) Waves and currents (coastal/marine processes) -- wave energy erodes cliffs and headlands and redistributes sediment by longshore drift, building spits and beaches; a spit growing across a bay mouth by longshore transport is a direct example. (Chemical/solution processes acting on soluble bedrock, producing karst, are sometimes cited as a sixth agent, but the five above are the classical set.)

2.4 — Glaciation-related concerns for foundation engineering

Glaciation leaves behind a suite of materials and structures that a geotechnical/foundation investigation must specifically anticipate: (1) Heterogeneous, unsorted till -- a poorly-sorted mixture of clay to boulder-sized clasts with highly variable density and bearing capacity over short lateral distances, making a single test pit or borehole a poor predictor of adjacent conditions. (2) Large erratic boulders embedded within till or at the till/bedrock contact, which can be mistaken for bedrock in a shallow probe, deflect piles, and obstruct excavation and pile driving. (3) Buried bedrock valleys/channels cut by pre-glacial or subglacial drainage and later infilled with drift, producing unexpectedly deep, soft, saturated sediment sequences beneath what the surface topography suggests is a shallow bedrock high. (4) Overconsolidated glacial till beneath former ice loads, which is often stronger and less compressible than its geologic age would suggest (a favourable property), but whose consolidation state can vary sharply where ice thickness/loading history changed locally. (5) Glaciolacustrine varved clays and silts deposited in ice-marginal lakes, which are typically weak, sensitive (strength loss on remoulding) and prone to slope instability, and whose horizontal permeability greatly exceeds vertical permeability due to the fine/coarse seasonal layering, complicating drainage design. A sixth issue worth noting is glacio-isostatic effects (residual uplift/subsidence and raised, potentially unstable, former shoreline deposits) in regions still adjusting to deglaciation.

2.5 — Five geomorphic factors controlling slope erosion

Sediment erosion on a hillslope is governed by a set of interacting controls: (1) Slope gradient (steepness) -- a steeper slope increases the down-slope component of gravity acting on overland flow, raising runoff velocity and shear stress on the surface; erosion (rilling, gullying, rainsplash transport) increases with slope angle, while very gentle slopes favour deposition instead of removal. (2) Slope length -- a longer slope allows overland flow to accumulate greater discharge and velocity as it moves downslope, concentrating erosive energy (and often triggering rill/gully initiation) near the base; a short slope limits how much the flow can accelerate and converge before reaching the base. (3) Vegetation cover -- a dense canopy intercepts rainfall and dissipates raindrop-impact energy before it reaches the ground (reducing splash erosion), while roots bind soil particles and increase infiltration and shear strength, and stems slow overland-flow velocity; bare or disturbed ground (fire, deforestation, construction) loses all of these protections and erodes far faster for the same rainfall. (4) Soil/material properties (texture, cohesion, permeability) -- cohesive, well-aggregated fine soils resist particle detachment, while loose, non-cohesive material (fine sand, silt) is readily entrained; low-permeability material generates more surface runoff (increasing erosion), whereas high-permeability material allows infiltration and reduces surface erosion (though it may increase subsurface piping). (5) Climate (rainfall intensity and duration) -- short, high-intensity storms deliver greater raindrop-impact energy and generate faster-rising, higher-peak overland flow than the same total rainfall spread over a long, low-intensity event, so erosion increases with rainfall intensity even at a fixed total rainfall volume; this factor is the direct link to drainage runoff named in the question, since the intensity and volume of runoff reaching a slope's rill/gully network is set by how concentrated the rainfall is in time.

2.6 — Wave influence on coastal morphology

Waves shape coastal morphology through several linked mechanisms. Wave refraction bends wave crests as they shoal over irregular offshore bathymetry so that wave energy converges (is focused) on headlands projecting into deeper water and diverges (is dispersed) into bays, meaning headlands are preferentially eroded (forming cliffs, wave-cut platforms, sea caves/arches/stacks) while bays receive net deposition (forming beaches) -- the overall long-term effect is to straighten an initially irregular coastline. Wave-cut erosion at the base of a sea cliff undercuts the cliff, causing episodic collapse and the landward retreat of the cliff line, leaving a gently sloping wave-cut (abrasion) platform in its wake. Where waves approach the shore at an oblique angle, the resulting longshore (littoral) current and longshore drift move sediment progressively along the coast, building depositional features such as spits (where the shoreline changes direction, e.g. at a bay mouth), barrier islands, tombolos (linking an island to the mainland) and beach ridges. Storm waves, with much higher energy than fair-weather waves, can rapidly strip beach sand offshore (net erosion), while lower-energy swell conditions gradually rebuild the beach (net accretion) -- so coastal morphology at any moment reflects the balance of a longer-term sediment budget between these episodic erosional and accretional events, modulated by the direction and intensity of the dominant wave climate.

2.7 — Why glaciers advance or retreat, and their diagnostic landforms

A glacier's terminus position is the direct expression of its mass balance: the net difference between accumulation (snowfall and other ice input in the upper, accumulation zone) and ablation (melting, sublimation, and, for tidewater/calving glaciers, iceberg calving, concentrated in the lower, ablation zone). When accumulation exceeds ablation over a sustained period (a positive mass balance, typically driven by increased snowfall and/or lower summer temperatures), the glacier thickens and its terminus advances as the extra ice is transmitted down-glacier by internal deformation and basal sliding. When ablation exceeds accumulation (a negative mass balance, driven by warmer summers and/or reduced snowfall), the glacier thins and its terminus retreats. Because ice flow takes time to propagate a mass-balance change from the accumulation area down to the terminus, a glacier's length responds to climate with a characteristic lag (the glacier's response time), so the terminus position reflects an integrated, time-lagged record of recent climate rather than the current year's weather alone. Tidewater (calving) glaciers add a further, partly independent control: calving rate depends strongly on water depth at the terminus and on the glacier's own dynamics, so such glaciers can advance or retreat rapidly for reasons only loosely tied to climate.

The two histories leave different landform signatures. An advancing glacier bulldozes and overrides its own proglacial deposits, producing a push (thrust) moraine of deformed, often folded/faulted sediment at its leading edge, and can override and streamline earlier till into drumlins or flutes aligned with ice flow. A retreating glacier instead leaves an undisturbed depositional record: a staircase of recessional moraines marking successive still-stands during overall retreat, kettled, hummocky ice-contact topography (from the melt-out of stagnant, debris-covered ice blocks), and outwash trains/valley trains that incise progressively as the ice margin retreats up-valley and base level for the meltwater stream falls. A single recessional moraine at the ice front therefore does NOT by itself indicate an advance -- it more often marks a temporary still-stand within an overall retreat.

2.8 — Hydrological changes in a hydrograph downstream of urbanization or deforestation

Urbanization (paving, roofing, storm-sewering) and deforestation both reduce the land surface's capacity to intercept and infiltrate rainfall, and both effects show up clearly in a downstream gauging-station hydrograph. Peak discharge increases: with less interception and infiltration, a larger fraction of a given rainfall event is converted directly to surface runoff rather than being stored in soil or vegetation, so the hydrograph's flood peak rises for the same storm. Time to peak (lag time) shortens and the rising limb steepens: impervious surfaces and engineered drainage (storm sewers, curb-and-gutter, straightened channels) route water to the channel far faster than natural overland flow, infiltration and subsurface flow paths did, so the hydrograph peaks sooner after the rainfall begins and rises more sharply. The recession limb also steepens: because less water was ever stored in the soil/groundwater system, there is less delayed subsurface drainage to sustain a gradual decline, and discharge falls back toward pre-storm levels more quickly. Baseflow (dry-weather flow) decreases: reduced infiltration means less groundwater recharge, so the sustained, low-flow "floor" of the hydrograph between storm events drops, and in extreme cases previously perennial streams can become intermittent. Taken together these changes increase the stream's overall flashiness (a higher peak-to-baseflow ratio and more erratic response to individual storms) and increase the total runoff volume delivered per storm, which in turn increases downstream channel erosion and instability (channel widening/incision) as the channel adjusts to a flow regime it was not formed under.

2.9 — Flows, slumps and glides: style of movement and materials

Flows, slumps and glides are all gravity-driven mass movements, but they are distinguished by the geometry of their failure surface, the internal coherence of the moving mass, and the material/water conditions that favour each.

A flow (debris flow, earthflow, mudflow) has NO discrete rupture surface -- the whole moving mass deforms continuously and internally, behaving as a viscous-to-plastic fluid rather than a rigid block; internal structure and stratigraphy are completely destroyed. Flows typically require elevated water content (or very fine, easily remoulded, sensitive material) to reach the fluid consistency needed for continuous deformation, and velocities span a wide range from very slow (creeping earthflow, mm/day) to catastrophically rapid (debris flow, tens of km/h). Flow deposits are unsorted, lobate, and often preserve flow-related surface structures (pressure ridges, levees along the flow margins, a bulging toe) but no coherent internal layering.

A slump moves as one or more largely INTACT blocks along a curved, concave-up (listric) rupture surface, so the motion is ROTATIONAL: the block rotates backward as it translates downward and outward, producing a characteristic back-tilted bench or scarp at the head and a bulging, often hummocky toe where the rotated mass overrides the original ground surface. Because the block remains coherent, its original stratigraphy/bedding is often still recognizable within the slumped mass, only rotated. Slumps are most common in relatively homogeneous, cohesive fine-grained material (clay, silt, till) on oversteepened slopes such as riverbanks, coastal bluffs or cut slopes, and movement is typically slow-to-moderate, though it can be reactivated episodically (e.g. after heavy rain raises pore pressure on the rupture surface).

A glide (translational slide) also moves as a largely intact, coherent mass, but along a roughly PLANAR failure surface controlled by a pre-existing plane of weakness -- a bedding plane, joint, fault, or the contact between weathered overburden and bedrock -- rather than a curved surface, so the motion is TRANSLATIONAL (the block slides essentially parallel to that plane) rather than rotational. Because the controlling surface is a discrete structural weakness, a glide can mobilize suddenly and move rapidly once the friction/pore-pressure balance on that plane is exceeded, and it commonly occurs in bedrock or in a residual-soil/colluvium mantle overlying a dipping structural discontinuity.

In short: flows have no discrete failure surface and are fully disaggregated (fluid-like); slumps and glides both move as largely intact blocks, but a slump ROTATES along a curved surface while a glide TRANSLATES along a planar, structurally-controlled surface.

2.10 — Secondary weathering products

Chemical weathering of primary silicate minerals commonly yields two broad classes of secondary product. The first is clay minerals, formed by the incongruent dissolution/hydrolysis of feldspars and other silicates (e.g. K-feldspar or plagioclase breaking down under acidic, hydrolyzing conditions); under the most intense, prolonged tropical weathering and leaching, clay minerals themselves break down further, and the most stable end-member clay commonly cited is kaolinite (with even more intense leaching eventually removing silica entirely, discussed next). The second is oxides and hydroxides of iron and aluminum, formed by the release and oxidation/hydration of Fe and Al liberated from ferromagnesian silicates (pyroxene, amphibole, biotite) and feldspars; under intense, well-drained tropical weathering (laterization) these accumulate as residual concentrations because Fe and Al are among the LEAST mobile elements released by weathering, while silica and base cations are progressively leached away. The most stable secondary compounds in this class are hematite/goethite (iron oxide/hydroxide, giving lateritic soils their characteristic red-brown colour) and gibbsite (aluminum hydroxide, the principal mineral of bauxite ore) -- both are essentially insoluble under normal surface weathering conditions and represent the geochemical end point of intense chemical weathering.