Glacial Landforms

• Roche Moutonnée (Erosional)

  • a mass of more resistant rock; has smooth, rounded upvalley (stoss slope) facing the direction of ice flow formed by abrasion + steep, jagged downvalley (lee slope) formed by plucking
  • stoss slope; pressure meltpoint reached, forms meltwater leading to basal sliding then abrasion (smoothes the slope)
  • lee slope; extensional forces causes plucking and regelation (ice melting under pressure/refreezing when pressure is reduced)

  A) compressional flow

  B) extensional flow

  
• • Arêtes - when two adjacent corries erode backwards/sideways towards each other, the previously rounded landscape becomes a narrow, rocky, steep-sided ridge.
  • Pyramidal Peak - if 3 or more corries develop on all sides of a mountain, a pyramidal peak/horn may be formed; has steep sides and several arêtes radiating from central peak.
• Corries/Cirques/Cwms - amphitheatre or armchair shaped hollows with a steep back wall and rock basin.

  • frost shattering above glacier provides morainic debris + abrasion/grinding of angular material; if pmp reached, extra water supply reduces friction and increases velocity/erosion
  • plucking steepens back wall and adds to debris of supply + plucking takes place on rock lip + widening of joints by pressure release (exaggerates slope)
  • meltwater flows down crevasses to base of glacier + glacier experiences rotational movement deepening its base + uneven floor from extending/compressing flow

  
• Corries/Cirques/Cwms:

  • nivation processes cause underlying rocks to disintegrate (freeze thaw, solifluction, and chemical weathering) - resultant debris removed by summer meltwater streams to leave embryo cirque
  • overdeepening needs several periglacials/glacials to form - rock lip develops where erosion decreases (increases in height when morainic debris is deposited) - when climate warms, ice in the hollow melts to form a deep, rounded lake (tarn)
• Corries/Cirques/Cwms:

  • in Britian, majority of corries orientated north west (315$^o$) through north east to south east (135$^o$); northern slopes recieve less sunlight (glaciers remain there longer/less melting) + western slopes face the sea bringing relatively warmer winds to melt snow/ice (more accumulation on eastern slopes)
• • valley glaciers straigthen, widen and deepen pre-glacial (river V-shaped) valleys into U-shaped valleys + glacial troughs are steep-sided, flat-floored valleys
  • over-deepening of valleys; ice movement (aided by meltwater/subglacial debris) more erosive than rivers + extending and compressing flow overdeepens trough floor (later filled by long, narrow ribbon lakes or more resistant rock steps)
  • causes of over-deepening; more erosion following confluence of two glaciers + presence of weaker rocks + area of deeply weathered rock in periglacials times + zone of well-jointed rock

  
• • Truncated Spurs - abrasion by englacial/subglacial debris and plucking along the valley sides removes tips of pre-glacial interlocking spurs leaving cliff-like truncated spurs.
  • Hanging Valleys - result from differential erosion between a main and tributary glacier; floor of tributary glacier deepens at slower rate so when glaciers melt its left hanging high above the main valley (river descends by waterfalls).

  
• • Striations - as glacier moves across areas of exposed rock, larger fragments of angular debris embedded in ice leaves a series of parallel scratches and grooves.
  • Crag and Tail - larger mass of resistant rock (crag), protected the lee side from erosion, forming a gently sloping tail of deposited material.
  • Rock Drumlins are more streamlined bedrock which lack quarried lee face of roche moutonnée.
• Glacial Transportation:

  • large amount of debris/material (varying in size) carried by glacier
  • rock debris falls on top of glacier (from frost shattering) and carried on surface as supraglacial debris
  • debris buried in ice/fell into crevasses transported within glacier as englacial debris
  • subglacial debris is rock debris carried at base of glacier (most erosive)
• Moraine - material deposited by glacier as it melts.

  • medial - forms in middle of glacier
  • terminal - formed in front of glacier when it stops advancing
  • lateral - carried along/deposited at glacier edges
  • recessional - marks an interuption to a glacier's retreat
• Drumlin - hill of compact, unsorted glacial till with long axis showing former glacier's direction of movement.

  • formed of till; elongated features that can reach 1km+ in length, 500m in width and 50+ in height - common for several drumlins to be found together (swarm or 'basket of eggs')
  • stoss end is steeper and faces direction of ice flow + lee slope more gentle (becomes lower the further from ice source; highest point of drumlin at stoss end)

  
• Drumlin Formation:

  • formed when ice becomes overloaded with sediment; competence of glacier reduced (by ice melting/changes in velocity), material then deposited - small obstacle in ground acts as trigger (till builds up around it then reshaped by further ice movement after deposition)
  • form where material is deposited underneath a glacier as ground moraine; material then shaped into a drumlin as ice advances and retreats (+ meltwater under ice helps shape the drumlin)
• • Terminal Moraine - material deposited at front of glacier's snout when it stops advancing and starts to melt/retreat; aligned perpendicular to direction of ice movement/along front of glacier + mark maximum extent of glacier margin (up to 100m high/tens of km long).
  • Medial Moraine - found at centre of glacier/forms in middle of the glacier; form from supraglacial debris concentrated in thin ribbon in mid-glacier below confluence of two tributary glaciers.
• • Lateral Moraine - material deposited and carried along edges of a glacier; formed primarily from frost-shattered debris which had fallen on to edge of glacier from adjacent rock walls.
  • Recessional Moraine - mark an interruption to a glacier's retreat; parallel series of end moraines + formed when glacier remains stationary and deposits mound of material.
  • Erratic - large boulders/rocks carried by the glacier and eventually deposited as glacier started to melt/retreat.