Unit 46


Unit Overview

This unit examines continental glaciers. The main sections are as follows:

  • The Antarctic Icecap
  • The Greenland Icecap
  • Age of the present ice sheets
  • Other Late Cenozoic ice sheets
  • North America's glaciation: the final four
  • Landscapes of continental glaciers
  • Aggradational landforms of ice sheets

Continental glaciers are important sculptors of the Earth's surface, and they tend to lower the overall relief. The two present-day ice sheets are located over Antarctica and Greenland. Those ice sheets are the two Holocene survivors of a larger group of Pleistocene ice sheets.

Ice sheets are thickest in their interior regions and thinner at their margins. Extensions of ice sheets over the ocean are known as ice shelves. Continental glaciers can produce degradational and aggradational landforms. The dominant aggradational landscape of continental glaciation is a flat to undulating plain underlain by heterogeneous material. Common glacial landforms shaped by ice sheets include moraines, drumlins, and eskers.

Unit Objectives

  • To delineate contemporary continental glaciers and define their former extent during the Late Cenozoic Ice Age
  • To identify typical landforms produced by continental glaciers

Glossary of Key Terms

Calving When an icesheet enters the sea, the repeated breaking away of the leading edge of that glacier into huge, flat-topped, tabular icebergs.
Drumlin A smooth elliptical mound created when an icesheet overrides and reshapes pre-existing glacial till; long axis lies parallel to direction of ice movement.
Esker A glacial outwash landform that appears as a long ribbon-like ridge in the landscape because it was formed by the clogging of a river course within a glacier, the debris from which remains after the ice melts.
Flow regime A discrete region of outward ice flow in a continental glacier; possesses its own rates of snow accumulation, ice formation, and velocity.
Glacial drift Comprised of unsorted till and stratified drift.
Glacial outwash Meltwater-deposited sand and gravel that are sorted into layers.
Ground moraine Blanket of unsorted glacial till that was laid down at the base of a melting glacier.
Ice cap A regional mass of ice smaller than a continent-sized icesheet. While the Laurentide Icesheet covered much of North America east of the Rocky Mountains, an ice cap covered the Rockies themselves.
Ice shelf Smaller ice sheet that is floating, seaward extension of a continental glacier, such as Antarctica's Ross Ice Shelf.
Kame A ridge or mound of glacial debris or stratified drift at the edge of a glacier; often found as deltaic deposits where meltwater streams flowed into temporary lakes near the receding glacier.
Kettle Steep-sided depression formed in glacial till that is the result of the melting of a buried block of ice.
Moraine A ridge or mound of glacial debris deposited during the melting phase of a glacier.
Nunatak A mountain peak of a buried landscape that protrudes through the overlying glacier.
Outwash plain Plain formed ahead of a receding ice sheet by the removal of material carried in the glacier by meltwater; exhibits both erosional and depositional features.
Pack ice Floating sea ice that forms from the freezing of ocean water.
Pluvial lake A lake that developed in a presently dry area during times of heavier precipitation associated with glaciations; Glacial Lake Bonneville, the (much larger) forerunner of Utah's Great Salt Lake, is a classic example.
Recessional moraine A moraine ridge marking a place where glacial retreat was temporarily halted.
Stratified drift One of the two types of glacial drift; the material transported by glaciers and later sorted and deposited by the action of running water, either within the glacier or as it melts.
Terminal moraine The rock debris, carried in and just ahead of the leading front of a glacier, that is deposited as an irregular ridge when the ice's forward progress stops; these ridges are important to Earth scientists because they mark the farthest extent of an ice lobe.
Till One of the two types of glacial drift; the solid material (ranging in size from boulders to clay particles) carried at the base of a glacier that is deposited as an unsorted mass when the ice melts back.

Unit Outline

  • The Antarctic Icecap
    • Continental landmass lies below glacier, with mountain ranges and a huge plateau
    • Nunataks are exposed mountain peaks that protrude through the ice and snow
    • Volume and weight of the ice sheet
      • about 65 per cent of fresh water on Earth is locked in this ice sheet
      • landmass below is depressed isostatically from weight of ice
    • Features of the Antarctic Ice Sheet
      • ice dome is divided into flow regimes
      • ice tongues are glaciers that let out into the sea
      • ice shelves are floating extensions of the main glacier, attached to continent
      • ice shelves break off into icebergs at their outer edges, a process called calving
      • icebergs float because they have lower density than the cold seawater
      • zone of floating pack ice surrounds zone of icebergs
  • The Greenland Icecap
    • Much smaller than Antarctic Ice Sheet
    • Greenland is only about 11 per cent free of ice cover
    • Northern hemisphere has no polar ice sheets
  • Age of the present ice sheets
    • Analyze microorganisms of sediment to measure age of ice sheets
    • There have been more than 30 glaciations since the Late Cenozoic began
    • Antarctic Ice Sheet appears older, possibly 15 million years old
  • Other late Cenozoic ice sheets
    • Former Northern Hemisphere ice sheets
      • Laurentide Ice Sheet
      • Scandanavian (Fennoscandian) Ice Sheet
      • Siberian (Barents) Ice Sheet
    • Former Southern Hemisphere ice sheets
    • smaller ice caps formed, in southern South America and South Island of New Zealand
  • North America's glaciation: the final four (Table 46.1)
    • Last four glaciations actually represent the last dozen of the Late Cenozoic
      • Nebraskan (most distant)
      • Illinoisan
      • Kansan Complex
      • Wisconsinan (most recent)
  • Landscapes of continental glaciers
    • Sheet glaciers create massive, erosional landscapes
    • Glacial lakes
      • lake bottoms contain bands of sediment from each year's deposition
      • water trapped when glaciers recede, leaving a huge deposit of soil and rock, from what used to be their leading edges
      • pluvial lakes develop in areas that receive a lot of precipitation during glaciations
        • Glacial Lake Bonneville, forerunner of Utah's Great Salt Lake
      • The Great Lakes were also created by receding Late Cenozoic glaciers
  • Aggradational landforms of ice sheets
    • Glacial drift
      • when glaciers recede, deposition of material begins as an unsorted till
      • material sorted by water to from stratified drift
      • glacial drift refers to till and stratified drift together
    • Moraines (Fig. 46.12)
      • a terminal moraine is the outermost ridge of debris left by a glacier
      • a recessional moraine is differentiated as a ridge where an already receding glacier temporarily stops and deposits additional material
    • Drumlins (whalebacks)
      • smooth, elliptical hills that rise from the till plain, may be from ice sheets that override preexisting till
    • Glacial meltwater deposits (Fig. 46.15)
      • meltwater flows in channels in several directions away from a glacier
        • glacial outwash is unsorted debris left by meltwater
        • eskers are former tunnels that formed when water flowed out of a glacier; debris eventually clogs them, and the landform remains
        • an outwash plain forms in front of a melting glacier
        • kettles are created when large blocks of ice are initially buried in the outwash plain, and create a depression as they melt
        • a kame is the remains of a glacial outwash delta

Review Questions

  1. Name five landforms associated with a receding ice sheet, using Fig. 46.15 as a reference.
  2. What were the most recent glaciations in North America?
  3. How were the Great Lakes formed?