Part 3 — Point-Form Interpretation: 18-Geol-B4 Geomorphology and Pleistocene Geology, December 2017, 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.

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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) + 2+2(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 specify exact counts and, for 3.4, a GLACIATED setting.

3.1 — Drainage patterns and the terrain conditions they indicate

(A) Rectangular (2 mks):

Main channels and their tributaries meet at consistent, sharp right angles, giving the drainage net a grid-like, rectangular geometry (distinct from trellis, which has one dominant strike direction with short perpendicular tributaries feeding parallel main channels).
Indicates structural control by two mutually perpendicular sets of joints, fractures or faults in the underlying bedrock (rather than by bedding/folding as in a trellis pattern) — typical of flat-lying or gently-dipping, uniformly jointed sedimentary or crystalline rock where streams preferentially erode along the weaker, more permeable joint planes in both orthogonal directions.

(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 — Sand/gravel landforms and basal-till properties

(A) Two glacigenic landforms of well-sorted sand and gravel (2 mks):

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); an outwash plain (sandur) or a kame terrace/glaciofluvial delta would be equally acceptable alternatives.

(B) Two characteristic properties of basal till (2 mks):

Poorly sorted and unstratified (massive) — basal till is a diamicton spanning clay to boulder sizes in a single, unlayered matrix-supported mass, because it is deposited directly by melting/lodging ice with no meltwater sorting, unlike the well-sorted glacifluvial sand and gravel of part (A).
Overconsolidated (dense) with striated/faceted clasts showing a preferred, ice-flow-parallel fabric — basal till is compacted and sheared beneath the full weight and forward motion of the overriding ice, so it is typically much denser and stiffer than its age/burial depth alone would predict, and its embedded clasts are commonly striated, faceted and statistically aligned parallel to the former ice-flow direction.

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 glaciated mountain valley bottom, and their airphoto indicators

Rockfall/rockslide from oversteepened, glacially-scoured valley walls (2 mks) — glacial oversteepening leaves valley walls at slope angles well beyond what fluvial processes alone would produce, and de-buttressing on deglaciation further destabilizes them; indicated by talus cones and aprons of angular debris at the base of steep, exposed rock faces, and by fresh scarps or disturbed, hummocky ground higher on the wall showing a source area still actively shedding material toward the valley bottom.
Debris flows from tributary gullies and moraine-mantled catchments (2 mks) — steep tributary catchments choked with loose glacial/colluvial debris readily mobilize as debris flows during heavy rain or rapid snowmelt; indicated by a debris-flow or alluvial fan built out from a tributary gully mouth onto the valley floor, its fan surface showing lobate, boulder-rich flow deposits and a channel scoured/leveed straight down the gully above it.
Moraine-dammed lake outburst flood (glacial lake outburst flood, GLOF) (2 mks) — a lake impounded up-valley behind a recessional or terminal moraine (or a stagnant/buried ice core within the moraine) can breach catastrophically, releasing a sudden high-magnitude flood/debris flow down the valley; indicated by a visible lake ponded directly against a moraine ridge on the airphoto, together with evidence of a formerly larger lake extent (relict shoreline terraces) or a hummocky, ice-cored moraine dam that is itself an unstable containment structure.

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

Key relations. Drainage pattern ↔ substrate structure (rectangular = orthogonal joint/fracture control; multibasinal = young, poorly-integrated glacial or karst terrain); landform texture/sorting ↔ depositional agent and energy (well-sorted sand/gravel = glacifluvial meltwater; unsorted, overconsolidated diamicton = direct subglacial lodgement); glacial valley hazard chain ↔ oversteepened walls (rockfall) + debris-charged tributaries (debris flows) + moraine-impounded water (GLOF).

Why this works. A drainage pattern or landform assemblage is the surface expression of the material properties, structural control 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, and the same forward-reading logic that flags glaciated-valley hazards specifically absent from an unglaciated fluvial valley.

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; and, for glaciated valleys specifically, screening only for landslide/rockfall scars on the valley walls while overlooking a moraine-impounded lake up-valley, which can threaten the entire valley bottom far beyond the footprint of any single slope failure.

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