Unit 2 — Weathering, soils and hillslope dynamics

How does rock become sediment, and when does material move downslope?

Hillslopes receive and produce material through chemical alteration, physical breakdown, bioturbation, rockfall and mass movement. Soil thickness is a state variable governed by production from bedrock, surface transport, deposition and removal. A thick soil layer need not mean a stable old surface: rapid production and supply may coexist with rapid erosion. Conversely, a bare rock slope can persist because weathering is slow, transport is rapid or rockfalls expose fresh surfaces. The manner in which material moves depends on slope geometry, grain size, vegetation, water pressure, cohesion and time. Slow hillslope diffusion is a useful long-term approximation where many small disturbances accumulate. It cannot capture a large debris flow, which may initiate abruptly and move far beyond the source slope. We must keep continuum descriptions distinct from event-driven failures. Even a textbook factor-of-safety calculation is an idealized balance, not an operational landslide prediction. For conservation, housing and road design, geology must be supplemented by site measurements, engineering assessment and an explicit treatment of uncertainty.

Learning outcomes and scale discipline

By the end of this unit, students should be able to describe at least five independent landform or sediment observations, demonstrate one dimensionally consistent quantitative calculation, design a comparative measurement, and distinguish an interpretation from the observations used to support it. Practice viewing the same landform in a map, a section and a material description. Keep an explicit column for phenomena that are absent, unobserved or only assumed. At the finest scale, a few grain contacts may govern transport; at larger scales, inherited valley geometry and event histories control the response. Switching scale is legitimate only after specifying which variables remain meaningful. At the start of the field exercise, make a drawing of soil depth along one crest-to-hollow traverse and timed tracer distances; list which parts would be measured directly and which reconstructed from other evidence.

The unit's measurement choices should include bedrock exposure, soil composition, slope angle and water conditions; the strongest report also checks parent-rock identity and direct soil contacts. Even a carefully derived rate does not establish its formative mechanism unless the sedimentary or stratigraphic evidence agrees. Interpret changes in the context of the process and observation interval, and use at least two sources of independent evidence. Most useful scientific uncertainties are not just numerical instrument errors: researchers must also consider incorrect boundaries, missing events, biased sample sites, mistaken ages and incomplete constitutive laws.

Process mechanism and scale

Mineral dissolution and hydrolysis alter primary silicates and carbonates, releasing solutes and producing secondary phases; reaction progress depends on water availability and flow paths. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Color changes, clay mineral assemblages, corestone surfaces and pore-water chemistry indicate alteration histories. Record bedrock exposure, soil composition, slope angle and water conditions. In the diagram of soil depth along one crest-to-hollow traverse and timed tracer distances, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

Iron staining may reflect later groundwater flow rather than progressive weathering of all minerals. Sample an intact bedrock-to-soil profile, compare weathering indices with immobile element ratios. Consider transported colluvium mistaken for in-place weathering. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Catchment solute export

Given. 15 metric tonnes per hectare per year from a 35 ha catchment. Calculation: 15 × 35. Result: 525 tonnes/yr. Work through the units before using the output to discuss Earth-surface history. If runoff chemistry changes seasonally, a single water sample is unrepresentative. Solute yield also includes atmospheric and non-weathering sources. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document parent-material discrimination; independently check water-pressure monitoring. Assess sensitivity to transported versus residual layers and verify rare-event documentation. Record each raw observation, units, uncertainty and assumptions before model fitting.

A weathering index integrates parent composition and drainage as well as time. State the assumed parent rock before assigning any alteration gradient to climate. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for chemical weathering and reaction fronts without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Thermal stress, frost cracking, salt crystallization and root growth can fragment rock, but the dominant mechanism depends on climate and material. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Fracture patterns, grain-size distributions, moisture histories and fresh fragment textures constrain mechanisms. Record bedrock exposure, soil composition, slope angle and water conditions. In the diagram of soil depth along one crest-to-hollow traverse and timed tracer distances, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

Angular debris at a cliff base may result from seismic shaking rather than freeze–thaw. Log fracture orientation and seasonal temperature-moisture cycles; compare sheltered and exposed microenvironments. Consider transported colluvium mistaken for in-place weathering. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Rock-fracture frequency

Given. 18 joints intersect a 6 m scanline. Calculation: 18 / 6. Result: 3 joints/m. Work through the units before using the output to discuss Earth-surface history. A scanline oblique to a preferred joint set produces a different apparent frequency. Frequency is not volumetric fracture density. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document parent-material discrimination; independently check water-pressure monitoring. Assess sensitivity to transported versus residual layers and verify rare-event documentation. Record each raw observation, units, uncertainty and assumptions before model fitting.

Temperature oscillations alone do not establish ice-driven cracking without available liquid water. Link each proposed mechanism to a measurable environmental precondition. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for physical breakdown and regolith production without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Soil thickness responds to bedrock conversion and downslope flux divergence; a steady thickness can coexist with active production and export. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Auger transects, soil-bedrock contact roughness, tracer distributions and clast weathering show storage and transport. Record bedrock exposure, soil composition, slope angle and water conditions. In the diagram of soil depth along one crest-to-hollow traverse and timed tracer distances, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A deep colluvial hollow may reflect deposition rather than high in-place weathering rates. Measure across ridge, midslope and hollow while separating transported from residual regolith. Consider transported colluvium mistaken for in-place weathering. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Soil storage rate

Given. Regolith production of 0.70 mm/yr and local net export of 0.45 mm/yr. Calculation: 0.70 − 0.45. Result: 0.25 mm/yr. Work through the units before using the output to discuss Earth-surface history. If export exceeds production, regolith thins in this simplified point balance. Deposition and compaction are neglected. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document parent-material discrimination; independently check water-pressure monitoring. Assess sensitivity to transported versus residual layers and verify rare-event documentation. Record each raw observation, units, uncertainty and assumptions before model fitting.

Bioturbation and mixed parent materials compromise simple soil-age estimates. Treat soil depth as a balance, not a direct duration counter. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for soil thickness and residence time without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

For gentle slopes, sediment flux is often approximated as proportional to local surface gradient, producing systematic smoothing of topography over time. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Convex soil-mantled crests, bent posts and tracer displacement can support slow redistribution. Record bedrock exposure, soil composition, slope angle and water conditions. In the diagram of soil depth along one crest-to-hollow traverse and timed tracer distances, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A long convex ridge may also reflect lithologic inheritance rather than active diffusion. Use an assumed transport coefficient to predict flux changes along a synthetic hillslope and compare with mapped soil depth. Consider transported colluvium mistaken for in-place weathering. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Linear hillslope sediment flux

Given. An assumed D=0.030 m²/yr and downslope slope magnitude 0.28. Calculation: 0.030 × 0.28. Result: 0.0084 m²/yr. Work through the units before using the output to discuss Earth-surface history. Flux doubles if the transport coefficient doubles at fixed slope. The model is a per-unit-contour-length idealization, not a debris-flow flux. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document parent-material discrimination; independently check water-pressure monitoring. Assess sensitivity to transported versus residual layers and verify rare-event documentation. Record each raw observation, units, uncertainty and assumptions before model fitting.

Linear diffusion is inappropriate where slopes approach threshold conditions or landslides dominate. Give a coefficient calibration method and the range of slopes for which the model is defensible. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for hillslope creep and diffusive models without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Slope failure depends on material strength, pore pressure, loading and geometry; failure can transform a slide into a rapidly channelized debris flow. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Headscarps, displaced blocks, shear surfaces, runout deposits and rainfall records distinguish failure stages. Record bedrock exposure, soil composition, slope angle and water conditions. In the diagram of soil depth along one crest-to-hollow traverse and timed tracer distances, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A fresh debris fan can be a flood deposit rather than a hillslope failure if no source scar is documented. Map the source, transport and deposition zones and compare sorting, matrix content and levee geometry. Consider transported colluvium mistaken for in-place weathering. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Idealized stability ratio

Given. c=6 kPa, soil unit weight 19 kN/m³, z=2 m, angle 30°, friction angle 28°, water ratio 0.40. Calculation: [6 + (19×2−0.40×9.81×2)cos²30° tan28°] / [(19×2)sin30°cos30°]. Result: 1.09539 ratio. Work through the units before using the output to discuss Earth-surface history. Larger pore pressure lowers resisting effective stress in this simplified calculation. Never use this classroom number for landslide forecasts or engineering design. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document parent-material discrimination; independently check water-pressure monitoring. Assess sensitivity to transported versus residual layers and verify rare-event documentation. Record each raw observation, units, uncertainty and assumptions before model fitting.

Ideal infinite-slope calculations omit progressive failure, three-dimensional geometry and debris-flow rheology. Never use a classroom stability index to certify slope safety or predict exact failure timing. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for landslides and debris flows without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

An imaginary hillside has three auger transects, three vegetation cover classes and one dated debris-flow deposit. Construct a source-to-storage diagram. Compute a simplified diffusive flux where slopes are gentle, then identify exactly where the approximation fails. Use rainfall and headscarp notes to distinguish storm-related failures from long-term creep without pretending that a classroom stability ratio is an engineering assessment.

Protocol and evidence allocation

Write a project proposal that names the study question, a defined sampling region, hypotheses H1 and H2 and three observations that could discriminate them. Build an input table with at least twelve synthetic measurements; record a unit, identifier, uncertainty class and data source for every row. Design one transect, one temporal repeat survey and one test using a different kind of evidence. State how the field team would avoid confirmation bias: preregister predicted outcomes, assign a second mapper to identify landforms independently and blind the initial data processing to the favored model. Any unmeasured quantity must be marked as an assumption. Base the arithmetic on the numerical cases in the unit, but do not present the invented data as actual measurements.

In the analysis, start with a mass or elevation balance rather than a purely visual explanation. If the balance does not close, label the residual and list processes that may lie outside the boundary, including sediment stored in a channel, transport at an unmonitored exit and error in estimated density or surface area. Separate interpretation of rate from reconstruction of age and chronology. An apparently consistent average rate can conceal long inactive periods punctuated by large storms, landslides or channel switching. Explore a parameter range wide enough to expose this sensitivity. Explain what observation would tell you whether the residual is an actual physical loss or a measurement artifact.

Reporting template and peer review

The final report should contain: a title that describes an inquiry rather than a conclusion; data source and acquisition notes; a map or section with scale; a table of raw observations; a short derivation with units; alternative interpretations; uncertainty and detection limits; and an honest conclusion identifying what the exercise cannot show. A peer reviewer should trace one plotted value back to an original input without assistance. Report uncertainty in both physical data and causal attribution. Describe how sampling locations could bias the result, and identify one independent validation datum that was not used when choosing the model. Treat a good fit as a starting point for further testing, not proof of a unique historical trajectory.

Assessment prompts and rubric

Interpretation. Provide two competing process histories consistent with the invented evidence, predict one differentiating field measurement, and identify which uncertainties could reverse the preferred interpretation. Quantitative reasoning. Recompute the selected unit case in SI units, vary two inputs independently, and interpret signs, magnitudes and nonlinearities. Evidence ethics. Identify all fabricated datasets and explain why they must never be described as real hazard, glacial or coastal monitoring records. Review standard. Full credit requires correct units, reproducible methods, a mass or geometry balance, explicit chronology limits and an alternative interpretation; numerical correctness alone is insufficient.

Return to the guiding question: How does rock become sediment, and when does material move downslope? Make a process diagram contrasting an observable pattern with a process that could produce it. Now add a second process capable of leaving a similar pattern. Underline measurements that would distinguish the two. Ask whether the lesson's calculations predict the type of change measured by those instruments or only provide a scale for discussing it. The emphasis throughout geomorphology is not on naming spectacular landforms but on reconstructing defensible histories, with uncertainties, from repeatable measurements and physical consistency.