Fluvio-glacial Landforms

• Role of Glacial Meltwater:

  • fluvioglacial landforms created by glacier meltwater, eg. subglacial streams flow beneath a glacier forming tunnels
  • warm-based glaciers produce lots of meltwater + surface meltwater filters through the glacier

  Importance of Meltwater; transports morainic debris in subglacial rivers, erodes channels/forms distinctive landforms, lubricates glacier base, helps enlarge nivations hollows, enables plucking through regelation and forms rivers on/beneath the glacier.
• Meltwater Streams (Erosional)

  • cause erosion at faster rate than rivers of the same size; confinement of ice means they flow quickly under pressure (can carry lots of material increasing erosive power; glacial streams have higher carrying capacity when confined)
  • streams form deep troughs in landscape called meltwater channels (wide/deep) + glacier meltwater has milky appearance from suspended fine sediment (increases erosive power) + ablation and runoff increases volume of meltwater
• Pro-glacial Lakes (Erosional)

  • lake developed immediately in front of glacier, commonly bordered by mounds of unconsolidated deposits that characterise terminal zone of a glacier (terminal moraine acts as barrier) - landscape/weight of ice (depresses landscape) forms lake
  • sudden release of pro-glacial lake carves out channels/valleys; eg. Yorkshire Moors created as Lake Wheeldale emptied in ice retreat
• Glacifluvial Depositional Landforms:

  • moulded by glacial meltwater (most meltwater from ablation); discharge of glacial streams (supraglacial/subglacial) high during summer/warmer months
  • water flowing under pressure has high velocity/very turbulent;able to transport large amounts of material (material can erode vertically to create subglacial valleys/large potholes
  • deposition occurs when there is a decrease in discharge
• Outwash Plains/Sandur (Depositional)

  • composed of gravels, sands and clays (uppermost/furthest from snout) + deposited by meltwater streams issuing from the ice either during summer/when glacier melts
  • material may originally been deposited by glacier/later picked up, sorted and dropped by running water beyond maximum extent of the ice sheets
  • outwash material also deposited on top of till after ice retreat

  
• Varves (Depositional) - Glacilacustrine Sediments

  • a distinct layer of silt lying on top of a sand layer, deposited annually in lakes found near glacial margins
  • coarser/lighter-coloured sand deposited during late spring when meltwater streams have peak discharge (carry maximum load)
  • discharge decreases in autumn when temperatures drop, finer/darker-coloured silt settles
  • each band of dark/light material represents a years's accumulation + variations in thickness indicate warmer/colder periods (greater melting casues increased deposition)
• Kames (Depositional)

  • undulating mounds of sand/gravel deposited unevenly by meltwater along the front of a stationary/slowly melting ice sheet
  • as ice retreats, unsupported kame often collaspes
• Kame Terraces (Depositional)

  • flat areas found along the sides of valleys; deposited by meltwater streams flowing in the trough between glacier/valley wall
  • troughs occur here as in summer, the valley side heats up faster than the glacier ice (so ice in contact with it melts)
  • kame terraces are sorted deposits so different to lateral moraines
• Eskers (Depositional)

  • very long, narrow, and sinous ridges composed of sorted coarse sands/gravels (may be fossilised courses of subglacial meltwater streams) + formed during deglaciation
  • as channel is restricted by ice walls, the hydrostatic pressure and transported load are considerable; as the bed of the channel builds up (no floodplain) material is left above the surrounding land following ice retreat
• Kettles (Depositional)

  • form from detached blocks of ice (left by glacier as it retreats) and then partially buried by glacifluvial deposits left by meltwater streams
  • when ice blocks melt, they leave enclosed depressions which often fill with water to form kettle-hole lakes and 'kame and kettle' topography
• Braided Streams (Depositional)

  • channels of meltwater rivers often become choked with coarse material as a result of the marked seasonal variations in discharge