Unit 47


Unit Overview

This unit explores the landforms and landscapes of mountain (alpine) glaciers. The main sections are as follows:

  • Mountain glaciers today
  • The mass balance
  • Degradational landforms of mountain glaciers
  • Aggradational landforms of mountain glaciers

Since mountain glaciers rely on altitude, they occur on landmasses across the globe. These glaciers produce aggradational and degradational landforms. One of the most characteristic glacial landforms is the V-shaped glacial valley.

Different landforms are associated with different portions of a mountain glacier. In the upper portion of the glacier's zone of accumulation, landforms known as cirques are formed. A cirque is a bowl-shaped, steep-sided depression with a gently sloping floor. Other related glacial landforms are horns and aretes. Lakes, such as tarns and finger lakes, are associated with continental glaciers. Fjords - narrow, steep-sided, elongated estuaries - are among the most spectacular landforms associated with glacial erosion. Aggradational landforms associated with glaciers include moraines.

Unit Objectives

  • To examine the current distribution of mountain glaciers and to comment on the Late Cenozoic extent of these glaciers
  • To discuss the landforms produced by mountain glacier erosion and deposition

Glossary of Key Terms

Arete A knife-like, jagged ridge that separates two adjacent glaciers or glacial valleys.
Cirque An amphitheatre-like basin, high up on a mountain, that is the source area of a mountain glacier.
Finger lake An elongated lake that fills much of an even longer, fairly narrow glacial trough.
Fjord A narrow, steep-sided, elongated estuary formed from a glacial trough inundated by seawater.
Glacial trough A valley that has been eroded by a glacier; distinctively U-shaped in cross-sectional profile.
Ground moraine Blanket of unsorted glacial till that was laid down at the base of a melting glacier.
Hanging valley A valley formed by the intersection of a tributary glacier with a trunk glacier; when the ice melts away, the tributary valley floor usually is at a higher elevation and thus "hangs" above the main valley's floor.
Horn The sharp-pointed, Matterhorn-like mountain peak that remains when several cirques attain their maximum growth by headward erosion and intersect.
Lateral moraine Moraine situated along the edge of a mountain glacier, consisting of debris that fell from the adjacent valley wall.
Medial moraine A moraine - situated well away from a glacier's edges - formed by the intersection of two lateral moraines when a substantial tributary glacier meets and joins a tributary glacier.
Moraine A ridge or mound of glacial debris deposited during the melting phase of a glacier.
Rock flour The very finely-ground up debris carried downslope by a mountain glacier; when deposited, often blown away by the wind.
Rock step The step-like mountainside profile (in the postglacial landscape) often created as an eroding alpine glacier moved downslope.
Tarn Small circular lake on the floor of a cirque basin.
Truncated spur Spurs of hillsides that have been cut off by a glacier, thereby straightening the glacially eroded valley.
Valley train Meltwater-deposited alluvium in a glacial trough; derived from the morainal material left behind by a receding mountain glacier.

Unit Outline

  • Mountain glaciers today
    • Global distribution
      • every landmass except Australia has alpine glaciers
      • may be as many as 100,000 glaciers at present
      • North America
        • Alaska
        • Canada's Yukon Territory and British Columbia
      • South America
        • southern Andes
      • Africa
        • Mount Kilimanjaro and Mount Kenya
      • Australia-New Zealand
        • Southern Alps, South Island of New Zealand
      • Europe
        • Alps
      • Asia
        • Mount Everest and other high Himalayan peaks, Nepal
        • certain ranges across south-central Asia
  • The Mass Balance
    • The accumulation zone
      • Sometimes called neve
    • Losses
    • The Victoria Glacier
      • net accumulation
      • net ablation
  • Degradational landforms of mountain glaciers
    • Glacial valleys
      • typically a U-shaped valley, widened into a U-shaped trough by glacier (Fig. 47.6)
      • truncated spurs are mountain edges that have been sheared off by glaciers
      • hanging valleys, formed by large and small glaciers joining together that have different base levels
    • High-mountain landforms (Fig. 47.8)
      • initial landscape has ridges and peaks
      • glacial erosion begins, and ice hollows out cirques, steep-sided depressions
      • as cirques intersect over time, their edges form knifelike ridges (aretes) that eventually are eroded away to form a single jagged peak (horn)
      • rock steps show local effects of frost wedging and jointing
    • Lakes (Fig. 47.8)
      • in warmer areas, cirque basins filled with water during interglacials; called tams
      • lakes also form in glacial troughs; many are elongated finger lakes
    • Fjords
      • a narrow, steep-sided estuary formed by a glacial trough that filled with ocean water
  • Aggradational landforms of mountain glaciers (Fig. 47.11)
    • Finely ground particles of glacial debris called rock flour
    • Moraines
      • larger fragments deposited in terminal moraines, the stalled edge of a moving glacier
      • recessional moraines and associated debris formed during glacial retreat
      • both terminal and recessional moraines form dams to create lakes
      • material that falls first from valley wall becomes lateral moraine
      • when a trunk glacier is jointed by a tributary one, their lateral moraines join to become a medial moraine
      • erosion at the base of the valley forms a ground moraine, which is thickened by deposits when the glacier recedes
    • Postglacial landscape change
      • river action modifies glacial action when glaciers retreat
      • a valley train is left when meltwater fills the valley floor
      • glacial lakes often drained when glaciers recede, and deposits redistributed
      • areas modified by glaciers at risk for mass movements

Review Questions

  1. Describe how lateral, medial, and terminal moraines form.
  2. Name and discuss the main features of three degradational landforms of mountain glaciers.
  3. List four features of a mountain landscape that has been transformed by alpine glaciation, using Fig. 47.8 as a guide.