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 agents of geomorphic change

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 — Why different glaciers produce different landforms

The landform assemblage a glacier produces depends on several independent controls, so no two glacial settings look alike. Thermal regime is the single most important control: a temperate (warm-based) glacier, at the pressure-melting point at its bed, is wet-based and erodes vigorously by plucking and abrasion, producing striated, polished bedrock, cirques and U-shaped valleys; a polar (cold-based) glacier is frozen to its bed, essentially non-erosive, and instead preserves or only lightly modifies the pre-existing landscape beneath it. Setting matters equally: a valley (alpine) glacier is topographically confined and produces linear erosional/depositional features (U-valleys, lateral/medial moraines, arêtes, horns), whereas a continental ice sheet is largely unconfined by topography and instead produces broad, areally extensive erosional (roche moutonnée fields, streamlined till plains) and depositional (ground moraine, drumlin fields) landscapes. Bedrock lithology and structure (resistant vs. erodible rock, joint spacing/orientation) controls how readily the substrate is plucked or abraded. Ice dynamics — velocity, whether the glacier is advancing, in equilibrium, or retreating, and its debris content/englacial sediment load — controls the balance between erosional and depositional landforms produced at a given point. Finally, climate (precipitation supply and ablation rate) sets the overall mass-balance regime and hence how much ice, and for how long, is available to do geomorphic work.

2.6 — Braided vs. meandering river deposits

Braided rivers divide into multiple, shifting channels separated by unstable mid-channel bars, producing a broad belt of interwoven, coarse-grained (sand-to-gravel), poorly-sorted, laterally and vertically stacked bar and channel-fill deposits with abundant internal scour surfaces and cross-bedding, and little fine-grained overbank material preserved. Meandering rivers flow in a single, sinuous, well-defined channel that migrates laterally by eroding the outer (cut) bank and depositing point bars on the inner (convex) bank, producing a fining-upward sequence from a coarse channel-lag base through cross-bedded point-bar sand to fine-grained, laminated overbank/floodplain silt and clay (with levee and crevasse-splay deposits), and preserving abandoned meander loops as oxbow lakes.

The controlling factors are: (1) channel slope/stream power — braided channels typically occupy steeper gradients with higher stream power than meandering channels of comparable discharge; (2) sediment load and calibre — braided rivers carry a high bedload of coarse, poorly-sorted sediment that exceeds the channel's capacity to move it in a single thread, forcing bar formation and channel division, whereas meandering rivers carry proportionally more suspended (fine) load; (3) bank material and cohesion — braided-river banks are typically non-cohesive (sand/gravel) and erode easily on both sides, preventing a single stable channel from forming, while meandering rivers have more cohesive, vegetated banks that resist erosion enough to maintain one stable, migrating channel; and (4) discharge variability — braided rivers are often associated with highly variable (e.g. glacially-fed or flashy) discharge regimes, while meandering rivers are more commonly associated with steadier discharge.

2.7 — 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.

2.8 — Why glaciers advance or retreat

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 (e.g. rapid, unstable retreat once the terminus retreats into deeper water).

2.9 — 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.

2.10 — Mud flow, grain flow and turbidite (turbidity current) deposits

All three are sediment-gravity-flow deposits, but they differ in the physical mechanism that keeps the sediment in motion (the flow's "support mechanism"), and this controls the resulting deposit's texture and structure. A mud flow (debris/mud flow) is a dense, cohesive slurry of fine sediment (mud/clay) and water in which the sediment's own MATRIX STRENGTH (cohesion, plus buoyant support from the dense, viscous fluid) suspends and carries even large clasts; flow is essentially laminar/plug-like, and the resulting deposit is a poorly-sorted, matrix-supported, massive (unstratified) diamicton with no internal grading. A grain flow is a dry-to-nearly-dry mass of cohesionless, coarse (typically sand-to-gravel) grains that moves downslope (often on the steep lee face of a dune, or on a submarine slope) supported by GRAIN-TO-GRAIN COLLISIONS (dispersive pressure, per Bagnold), producing thin, structureless to inversely-graded beds as larger clasts are driven upward and outward by collision. A turbidity current (turbidite) is a dilute, sediment-water density current in which sediment is kept in suspension by FLUID TURBULENCE, moving as an underflow because its sediment load makes it denser than the surrounding (typically marine) water; as the current decelerates, sediment settles out progressively coarse-to-fine, producing the classic normally-graded Bouma sequence (massive/graded sand base grading up through parallel-laminated, then ripple cross-laminated, then fine laminated and pelagic mud). In short: mud flow = matrix/cohesive strength support → unsorted, ungraded; grain flow = grain-collision support → structureless/inversely graded; turbidity current = fluid-turbulence support → normally graded (Bouma sequence).