Question 3 of 3: Part 3 — Point-Form Interpretation

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.

3.1 — Drainage patterns and the terrain conditions they indicate

(A) Dendritic (2 mks):

• A tree-branching pattern of tributaries joining at acute angles, with no preferred orientation.
• Indicates a homogeneous substrate with uniform resistance to erosion in all directions — flat-lying, unfractured or uniformly-textured bedrock, or a thick, texturally uniform surficial deposit (e.g. till plain, uniform sand/silt) with no significant structural or lithologic control on channel orientation.

(B) 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 — Possible origins of a wide, flat topographic plain

Possible origins:

• Glaciolacustrine plain — the flat former bed of a proglacial or post-glacial lake, mantled in fine-grained (silt/clay, often varved) sediment.
• Glaciofluvial outwash plain — a braided-stream sandur built of coarser, stratified sand and gravel beyond a former ice margin.
• Alluvial floodplain — the flat valley-bottom surface built by a modern or recent meandering/braided river.
• Emerged marine plain — a former shallow seabed or shoreline terrace exposed by isostatic rebound or eustatic sea-level fall.
• Till plain (ground moraine) — a broad, subdued surface of unsorted till left by a retreating ice sheet.

Differentiating characteristics to look for on the air photograph:

• Photo tone/texture: uniform, smooth, fine-grained (silty/clayey) tone suggests glaciolacustrine or marine; mottled, coarser texture suggests outwash or till.
• Drainage pattern and density: dendritic, well-integrated drainage on a floodplain vs. sparse/multibasinal, poorly-integrated drainage on a till or lacustrine plain.
• Presence of relict channel/meander scars, point-bar scroll-bar topography, oxbows and natural levees — diagnostic of an alluvial floodplain, absent from lacustrine/marine/outwash plains.
• Presence of shoreline features — raised beach ridges, wave-cut terraces/scarps, deltaic fan shape at the plain's margin — diagnostic of a lacustrine or marine origin.
• Elevation and position relative to present-day water bodies and valley walls: a plain perched well above the modern floodplain and bounded by a distinct terrace scarp suggests a former (higher) lake or sea level rather than the active river system.
• Vegetation and drainage condition: poorly-drained, wetland/muskeg vegetation is common on flat lacustrine clay or till plains with low permeability, versus better-drained vegetation on coarse outwash sand and gravel.

3.4 — Hazard-indicating geomorphic features along a valley bottom with high relief walls

• Landslide/slope-failure scars and deposits — hummocky, disturbed ground, back-tilted (slumped) blocks, or lobate deposits on the valley walls indicate past mass movement and the potential for reactivation or for new failures onto the valley bottom.
• Alluvial/debris-flow fans at tributary (gully) mouths — a fan built out onto the valley floor indicates a history of debris-flow or flash-flood activity from the tributary catchment, a direct hazard to anything sited on or near the fan.
• Talus cones and rockfall debris at the base of steep valley walls, indicating an active source of falling rock above.
• River terraces at various heights above the present channel — indicate the range of past flood/channel levels and, combined with the modern floodplain width, help bound the flood hazard zone.
• Cutbank erosion / actively migrating meanders undercutting the valley wall or terrace edge — indicates ongoing lateral channel erosion that threatens infrastructure near the bank.
• Avalanche tracks/chutes (linear, vegetation-free or shrub-only swaths running down the valley wall) — indicate a snow-avalanche hazard reaching the valley bottom in winter/spring.
• Evidence of past flooding — driftwood lines, sediment/silt drapes, or vegetation-age discontinuities on the floodplain surface, indicating the extent of past high-water events beyond the normal channel.
• Tension cracks or scarps at the crest of the valley walls — indicate an incipient or actively developing slope failure not yet fully mobilized.