Part 3 — Point-Form Interpretation: 18-Geol-B4 Geomorphology and Pleistocene Geology, May 2016, Q3

Question 3: Part 3 — Point-Form Interpretation (20 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.

Check

The source prints only sub-items (A) and (C) for question 3.1 (no (B)); this matches the paper's own mark arithmetic — 2(A) + 2(C) + 4(3.2) + 6(3.3) + 6(3.4) = 20 marks, matching the "20 mks total" stated for Part 3, so only two drainage patterns are examined in this sitting. Items 3.3 and 3.4 ask for a specific COUNT (three origins / three hazards), so a count-matched selection is given below.

3.1 — Drainage patterns and the terrain conditions they indicate

(A) Trellis (2 mks):

Main channels flow parallel to one another with short, right-angle tributaries joining them — a rectangular, "vine-trellis" geometry.
Indicates strong structural/lithologic control: alternating bands of resistant and non-resistant rock (typically folded or tilted sedimentary strata), where main streams follow strike valleys in weak rock and tributaries cut directly down-dip/across-strike through resistant ridges — classic of a folded sedimentary (e.g. Appalachian-style) terrain.

(C) Multibasinal (2 mks):

Numerous separate, closed drainage basins (ponds, lakes, wetlands) with poorly integrated, often disrupted or absent through-going channels.
Indicates young, immature, poorly-integrated drainage typical of recently glaciated terrain — irregular hummocky ground moraine, kettled outwash, or a stagnant-ice topography where original surface irregularities (kettles, closed depressions between hummocks) have not yet been graded into a connected channel network; also occurs on karst terrain with numerous sinkholes, or on very flat, poorly-drained ground with little relief to organize flow.

3.2 — Four landforms of well-sorted sand and gravel

Eskers — sinuous ridges of stratified sand and gravel deposited by meltwater flowing in a subglacial or englacial tunnel.
Kames — irregular mounds/hummocks of stratified sand and gravel deposited by meltwater in contact with stagnant or slowly-moving ice (in crevasses, ice-marginal ponds, or against the ice margin).
Outwash plains (sandar) — broad, gently sloping plains of stratified, well-sorted (though often coarsening proximally) sand and gravel deposited by braided meltwater streams beyond the ice margin.
Kame terraces / glaciofluvial deltas — benches of stratified sand and gravel deposited by meltwater flowing between an ice margin and a valley wall (kame terrace), or where a meltwater stream entered a proglacial lake and built a Gilbert-type delta of well-sorted sand and gravel.

3.3 — Three possible origins of a wide, flat topographic plain

Glaciolacustrine plain (2 mks) — the flat former bed of a proglacial or post-glacial lake, mantled in fine-grained sediment. Distinctive characteristic: uniform, smooth, fine (silty/clayey), often varved photo tone with sparse, poorly-integrated drainage, and relict shoreline features (raised beach ridges, wave-cut scarps) marking the former lake margin.
Glaciofluvial outwash plain (2 mks) — a braided-stream sandur built of coarser, stratified sand and gravel beyond a former ice margin. Distinctive characteristic: mottled, coarser photo texture with a braided (multi-channel, bar-and-channel) drainage pattern and good surface drainage (few standing wetlands), fanning out from an identifiable former ice-margin position.
Alluvial floodplain (2 mks) — the flat valley-bottom surface built by a modern or recent meandering/braided river. Distinctive characteristic: relict meander scars, point-bar scroll-bar topography, oxbow lakes and natural levees, all absent from the other two origins, plus a clear, ongoing connection to a present-day river channel.

3.4 — Three hazards along a mountainous valley bottom, and their airphoto indicators

Slope instability / landslides on the valley walls (2 mks) — indicated by hummocky, disturbed ground, back-tilted (slumped) blocks, arcuate head scarps, or lobate toe deposits reaching toward the valley bottom, signalling past mass movement and the potential for reactivation or new failures onto the road.
Debris flows / flash floods from tributary gullies (2 mks) — indicated by an alluvial or debris-flow fan built out from a tributary gully mouth onto the valley floor; a fan crossing or narrowing the valley bottom shows a history of debris-flow or flash-flood activity capable of burying or washing out a road.
Rockfall from steep valley walls (2 mks) — indicated by talus cones or aprons of angular rock debris at the base of steep, exposed rock faces above the valley bottom, showing an active source of falling rock that can reach a road built along the toe of the slope.

Topic. Airphoto/terrain interpretation — drainage-pattern diagnosis, glacial/fluvial landform recognition, and geomorphic hazard indicators for valley-bottom development.

Key relations. Drainage pattern ↔ substrate homogeneity/structure (trellis = structural/lithologic banding; multibasinal = young, poorly-integrated glacial or karst terrain); landform texture/sorting ↔ depositional agent and energy (well-sorted sand/gravel = glacifluvial meltwater; unsorted diamicton = direct glacial or mass-wasting deposition).

Why this works. A drainage pattern or landform assemblage is the surface expression of the material properties and process history beneath it, so a trained interpreter reads terrain conditions (material type, structure, age, stability) FORWARD from the pattern visible on an airphoto, without any subsurface investigation — the basis of the airphoto-terrain-analysis method used throughout preliminary engineering geology reconnaissance.

Common pitfall. Reading a flat plain as automatically "safe, uniform ground" without checking which origin (lacustrine clay vs. outwash gravel vs. alluvial) it actually is — the engineering implications (bearing capacity, drainage, seismic/liquefaction susceptibility, flood risk) differ sharply between these superficially similar-looking flat surfaces; likewise, screening only for landslide scars along a valley wall and overlooking debris-fan or talus evidence misses hazards that strike the valley bottom itself rather than the slope.

Source. Mollard & Janes, Airphoto Interpretation and the Canadian Landscape (drainage-pattern and terrain-condition interpretation, slope-movement morphology); Fulton (ed.), Quaternary Geology of Canada and Greenland, GSC (glacial/glaciofluvial landform recognition).