Periglaciation (+Landforms)

• Freeze Thaw/Frost Shattering

  • forms scree slopes at foot of valley sides; in flatter areas, large areas of boulders can be left ('blockfield'/felsenmess) + frost induced shattering (-4 to -15$^oc$)

  Nivation

  • freeze thaw/weathering occurs under patches of snow left in landscape depressions + rocks break down/summer meltwater washes away material (nivation hollows form)
• Solifluction

  • in summer months, active layer becomes saturated with water as it thaws; this layer becomes unstable, and there can be considerable downward movement on shallow slopes - material forms solifluction lobes (tongue-like features found on valley sides)

  Frost Heave

  • freezing water within soil expands pushing up ground surface above; growth of ice crystals can raise soil particles to create spiky surface and patterned ground
• Frost Heave:

  • fine grained silts/soils/clays expand to form small domes or individual stones with soil to move to the surface; results from formation of ice as crystals/lenses
  • thermal conductivity of stones greater than soil so area under stone colder than surrounding soil so ice crystals form; ice expansion allows moisture to rise and freeze
  • crystals/larger ice lenses force stones above to rise to the surface; during thaw, meltwater leaves fine material under uplifted stones preventing them falling back
• • in areas of repeated freezing/thawing, frost heave lifts and sorts material to form patterned ground on the surface
  • larger stones (extra weight) more outwards to form stone circles/polygons in flat areas + if this occurs on slopes >2$^o$ gradient, stones will gradually move downhill under gravity to form elongated stone stripes

  
• Ice Wedges (Ground Contraction):

  • refreezing of active layer during severe winter cold causes soil to contract/cracks open up; in next summer, cracks open/close or fill with meltwater (+water/wind blown deposits)
  • in following winter, water refreezes and cracks widen/deepen to form ice wedges; process repeated annually until wedges grow to 1m wide/3m deep
  • fossil ice wedges (cracks filled with sand/silt from meltwater) are signs of past periglacial condition

  
• • frost heaving results in small dome-shaped polygons with larger stones found outisde the circles
  • ice contraction results in larger polygons with centre of the circles depressed in height/with bigger stones
• Pingos - dome shaped, isolated hills which interrupt flat tundra plain; can have diameter up to 500m/may rise 50m in height to a summit (sometimes ruptured to expose icy core) + mainly occur in sand so not susceptible to frost heaving + can have open/closed pingos (or polygenetic/mixed pingos).
• Open System (hydraulic) Pingos - East Greenland Type

  • occur in valley bottoms/areas of thin or discontinous permafrost - surface water infiltrates into upper layers of ground where it circulates in unfrozen sediments before freezing
  • water freezes/expands to form localised masses of ice; ice forces overlying sediment upwards into dome-shaped features (summitt ruptures/opens)
  • artesian pressure forces groundwater to surface pushing up overlying material then freezing in lens shape

  
• Closed System (hydrostatic) Pingo - Mackenzie Type

  • characteristic of flat, low lying areas of continous permafrost + form on sites of small lakes as water trapped/enclosed by freezing from above/permafrost advance inwards from lake margins
  • as water freezes it expands, forcing ground above to rise upwards into dome shape; results from downward growth of permafrost
  • frozen lake with sediment floor insulates ground beneath creating talik + as water freezes/expands, frozen lake breaks up/slides away as ground heaves + ice core expands upwards from increase in hydrostatic pressure

  
• Ruptured Pingo:

  • as surface of pingo stretches, summitt may rupture/crack; as surface ruptures, heat enters and melts the ice core
  • where ice core melts, hill may collaspe leaving meltwater filled hollow - new pingo may form on same site (repeated cycle of formation/collaspe)
• Frost Weathering (freeze thaw/frost shattering):

  • on relatively flat, upland surfaces, extensive spreads of large, angular boulders (formed in situ by frost action) are called blockfields/felsenmeer
  • scree forms at foot of steep slopes (of well-jointed rocks prone to frost action); freeze thaw turns well-jointed rocks into tors (weathered debris later removed by solifluction)
• Solifluction Lobes:

  • during thaw periods, upper active zone of permafrost melts and becomes saturated; if on slope, it begins to move downhill under gravity through solifluction
  • material flows into lobe shaped structure (solifluction lobe); will flow until they reach a natural barrier or melting permafrost allows water to percolate from the lobe
  • solifluction leader to infilling of valleys/hollows by sands/clays to form solifluction sheets
• Loess - sedimentary deposit composed of silt-size grains loosely cemented by calcium carbonate.

  • lack of vegetation/plentiful supply of fine, loose material found in glacial environments enabled strong, cold, and out-blowing winds to pick up large amounts of dust and redeposit it as loess in areas beyond its source
• • Brickearth - superficial windblown deposits found in southern england; brickearths are periglacial loess (windblown dust deposited under cold/dry, periglacial conditions).
  • Terracettes - series of small ridges on a sloped piece of land; formed when active layer thaws/gravity allows material to slump down underneath vegetation (frost creep enlarges terracettes as material lifted by ice then deposited).
• Asymmetrical Valley - valley with steeper slows on one side; formed by south facing valley side recieving more sunlight than north facing side (warmer slope has more thawing/solifluction pushing debris downslope) + debris pushes summer meltwater against north facing slope undercutting it.