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) 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.

(B) Deranged (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.