·  Any sediment of glacial origin is called drift. The two distinct types of glacial drift are (1) till, which is unsorted sediment deposited directly by the ice; and (2) stratified drift, which is relatively well-sorted sediment laid down by glacial meltwater.

·  The most widespread features created by glacial deposition are layers or ridges of till, called moraines. Associated with valley glaciers are lateral moraines, formed along the sides of the valley, and medial moraines, formed between two valley glaciers that have joined. End moraines, which mark the former position of the front of a glacier, and ground moraines, undulating layers of till deposited as the ice front retreats, are common to both valley glaciers and ice sheets. An outwash plain is often associated with the end moraine of an ice sheet. A valley train may form when the glacier is confined to a valley. Other depositional features include drumlins (streamlined asymmetrical hills composed of till), eskers (sinuous ridges composed largely of sand and gravel deposited by streams flowing in tunnels beneath the ice, near the terminus of a glacier), and kames (steep-sided hills consisting of sand and gravel).

·  The Ice Age, which began about two million years ago, was a very complex period characterized by a number of advances and withdrawals of glacial ice. Most of the major glacial episodes occurred during a division of the geologic time scale called the Pleistocene epoch. Perhaps the most convincing evidence for the occurrence of several glacial advances during the Ice Age is the widespread existence of multiple layers of drift and an uninterrupted record of climate cycles preserved in seafloor sediments.

·   In addition to massive erosional and depositional work, other effects of Ice Age glaciers included the forced migration of organisms, changes in stream courses, adjustment of the crust by rebounding after the removal of the immense load of ice, and climate changes caused by the existence of the glaciers themselves. In the sea, the most far-reaching effect of the Ice Age was the worldwide change in sea level that accompanied each advance and retreat of the ice sheets.

·  Any theory that attempts to explain the causes of glacial ages must answer two basic questions: (1) what causes the onset of glacial conditions? and (2) what caused the alternating glacial and interglacial stages that have been documented for the Pleistocene epoch? Two of the many hypotheses for the cause of glacial ages involve (1) plate tectonics and (2) variations in Earth's orbit.

Chapter Outline

   I.     Glaciers: a part of two basic cycles

      A.     Glaciers are parts of the

• Hydrologic cycle, and the
• Rock cycle

      B.     Definition: A thick mass of ice that originates on land from the accumulation, compaction and recrystallization of snow.

     C.      Types of glaciers

• Valley (alpine) glaciers
• Exist in mountainous areas
• Flows down valley from an accumulation center at its head
• Ice sheets
• Exist on a larger scale than valley glaciers
• e.g., Over Greenland and the Antarctic Ice sheet
• Often called continental ice sheets
• Ice flows out in all directions from one or more snow-accumulation centers

• Other types of glaciers
• Ice caps
• Outlet  glaciers

•       Movement of glacial ice

      A.     Generally referred to as flow

• Two basic types
• Plastic flow
• Within the ice
• Under pressure, ice behaves as a plastic material
• Basal slip
• Entire ice mass slipping along the ground
• Most glaciers are thought to move by this process

            2.     Zone of fracture

• Uppermost 50 meters
• Tension causes crevasses to form in brittle ice

B.     Rates of glacial movement

• Average velocities vary considerably from one glacier to another
• Rates of up to several meters per day
• Some glaciers exhibit extremely rapid movements called surges
• Budget of a glacier
• Zone of accumulation - the area where a glacier forms
• Outer limits are defined by the snowline
• Elevation of the snowline varies greatly
• Zone of wastage - the area where there is a net loss to the glacier due to
• Melting
• Calving – the breaking off of large pieces of ice (icebergs where the glacier has reached the sea)
• Balance, or lack of balance, between accumulation at the upper end of the glacier, and loss at the lower end is referred to as the glacial budget
• If accumulation exceeds loss (called ablation), the glacial front advances

• If ablation increases and/or accumulation decreases, the ice front will retreat

•      Glacial Erosion
• Glaciers are capable of great erosion and, as a medium of sediment transport, have no equal
• Glaciers erode the land primarily in two ways
•             1.     Plucking - lifting of rock blocks

            2.     Abrasion

                    a.  Rocks within the ice acting like sandpaper to smooth and polish the surface below

•                     b.  Abrasion produces
• 1.   Rock flour (pulverized rock)
• 2.   Glacial striations (grooves in the bedrock)

      C.     Landforms created by glacial erosion

•             1.     Erosional features of glaciated valleys
• Glacial trough
• Truncated spurs
• Hanging valleys
• Pater noster lakes
• Cirques
• Tarns
• Fiords
• Artes
• Horns

            2.     Roches Mountonnées – most frequently where ice sheets have modified the terrain

IV.     Glacial deposits

• Glacial drift
• All sediments of glacial origin
• Types of glacial drift
• Till - material that is deposited directly by the ice
• Stratified drift – sediments laid down by glacial meltwater

            3.     Glacial erratics – boulders of different than the bedrock that are found in till or lying free on the surface

• Landforms made of glacial deposits
• Moraines
• Layers or ridges of till
• Types produced by alpine glaciers
• Lateral moraine
• Medial moraine
• Other types of moraines associated with both alpine glaciers and ice sheets
• End moraine
• Terminal moraine
• Recessional moraine
• Ground moraine

2.     Landforms made of stratified drift

• Outwash plains (with ice sheets) and
• valley trains (when in a valley)

     1.   Broad ramp-like surface composed of stratified drift deposited by meltwater leaving a glacier found adjacent to the downstream edge of most end moraines

     2.   Often pockmarked with basins or depressions called kettles

3.        Ice-contact deposits

                  a.   Drumlins

•                         1.  Smooth, elongated, parallel hills

                        2.  Steep side faces the direction from which the ice advance

• 3.   Occur in clusters called drumlin fields

4.  Formation not fully understood

                  b.   Deposited by meltwater flowing over, within, and at the base of motionless ice

• 1.   Eskers – sinuous ridges of sand and gravel

                        2.            Kames – originate when glacial meltwater washes sediment into openings and depressions in stagnant ice

V.     Glaciers of the Ice Age

• Ice Age

• Several glacial advances, each separated by extended warmer periods
• Ice covered 30% of Earth's land area
• Began between two million and three million years ago
• Most of the major glacial stages occurred during a division of geologic time called the Pleistocene epoch
•  Indirect effects of Ice-Age glaciers
• Forced migration of animals and plants
• Changes in stream courses
• Rebounding upward of the crust in former centers of ice accumulation
• Worldwide change in sea level
• Climatic changes

VI.     Causes of glaciation

• Any successful theory must account for
• What causes the onset of glacial conditions, as well as
• What caused the alteration of glacial and interglacial stages that have been documented for the Pleistocene epoch
• Some possible causes
• Plate tectonics
• Continents were arranged differently in the past
• Changes in oceanic circulation
• Variations in Earth's orbit
• Milankovitch hypothesis
• Shape (eccentricity) of Earth's orbit varies
• Angle of Earth's axis (obliquity) changes
• Earth’s axis wobbles (precession)
• Changes in climate over the past several hundred thousand years are closely associated with variations in the geometry of Earth's orbit

Answers to the Review Questions

  1. Today, glaciers cover about 10 percent of the land area. Valley glaciers are found in high, mountainous regions at all latitudes. Ice sheets and ice caps are found only at high latitudes in such areas as Iceland, Greenland, and Antarctica; the Antarctic ice sheet is by far the largest. During the height of the Pleistocene glaciations, about 30 percent of the land area was ice covered. Due to lowered sea level, the Antarctic ice sheet was slightly larger than at the present time; most of the other ice-covered lands were beneath the continental ice sheets of North America and Europe.

  2. The water in ice sheets and glaciers can be viewed as removed from the oceans and temporarily stored on land. Glacial ice, like groundwater, does eventually return to the sea, but the recycling time is hundreds to thousands of years compared to months or a few years for surface water runoff from rainfall events and melting snow. By a large margin, glacial ice represents the largest freshwater reservoir in the hydrologic cycle.

As powerful agents of erosion, transport, and deposition, glaciers contribute to the accumulation of sediments directly, till for example, and indirectly, sand and gravel from meltwater streams and loess from windblown glacial rock flour. Given the proper conditions for preservation and burial, these sediments can eventually lithify to rock as shown by occurrences of tillites, clastic rocks of glaciofluvial origin, and loessite in the rock record. Thus glaciers play an important role in the rock cycle as well as in the hydrologic cycle.

  3. (a)   The term continental is often used to describe this type of glacier. - This refers to continental glaciers or continental ice sheets like the ones that cover most of Greenland and Antarctica today.

      (b)   This type of glacier is also called an alpine glacier. - This would be a valley glacier, a long ice stream that flows downslope along a valley in a mountainous region.

      (c)   This is a glacier formed when one or more valley glaciers spread out at the base of a steep mountain front. - a piedmont glacier

      (d)   Greenland is the only example in the Northern Hemisphere.- an ice sheet or continental ice sheet

      (e)   This is a stream of ice leading from the margin of an ice sheet through the mountains to the sea. - The statement describes an outlet glacier.

  4. Glacial flow involves two mechanisms. One is basal sliding, in which the entire glacier moves forward by sliding or slipping along the bedrock at the base of the glacier; the other flow mechanism involves internal deformation and plastic flow. Ice in the interior parts of the glacier slowly deforms and recrystallizes, producing a net, downslope movement of all the ice above the zone of deformation. Ice higher in the zone of deformation moves faster than the ice beneath it, and surface ice in the center of the glacier moves faster than ice near the margins. Glaciers move very slowly on average; velocities may be less than a meter per year. Periods of unusually rapid glacial movements are called surges. As the temperature at the base of a glacier reaches the melting point, quantities of liquid water accumulate at the ice-bedrock contact. Frictional resistance is greatly reduced and the ice tends to “float” on the meltwater. Thus basal slip is accelerated. Temperatures below freezing at depth in the glacier result in low basal slip and internal flowage velocities.

  5. Crevasses are transverse, open cracks or fissures in a glacier that extend from the surface to depths of about 50 meters. The cracks are widest at the surface and taper downward. The bottom tip of the crevasse marks the base of the brittle ice zone and the top of the plastic flow zone. The brittle, surface ice layer, while being carried passively downslope by flowage in the plastic zone, responds to stresses by cracking and fracturing.

  6. Excluding surges and other, unsteady flow movements, the glacier will advance (its snout will move downslope) when the snow and ice accumulated during many, consecutive years exceed that lost by melting and other forms of ablation. The glacier will retreat (the snout melts back to higher elevations)

when the reverse is true, ablation exceeds accumulation. The snout will be stationary if accumulation and ablation are exactly balanced year after year.

  7. Glaciers are powerful agents of erosion. The melting and refreezing of water at the base of the glacier can dislodge large blocks of rock (plucking); rock particles entrained in the ice at the bottom and sides of the glacier scratch and gouge the bedrock. Areas of unconsolidated materials and soft bedrock can be deeply eroded, the debris being carried away by the glacier. Rock and soil particles of all sizes, entrained in the glacier through plucking, erosion, abrasion, and mass wasting, are carried along and eventually deposited at the snout of the glacier.

  8. Nonglacial valleys were cut by streams and widened by mass wasting; they have V-shaped cross-sections; sinuous, longitudinal profiles; and lots of sharp ridges that extend downslope to the valley floor and stream. Glacial valleys were strongly scoured by the moving ice. They have U-shaped cross-sections with wide, relatively flat floors; very steep walls; and straight, longitudinal profiles. Truncated spurs are blunt facets eroded from ridges that extended to the valley floor before the valley was glaciated.

  9. Large, open, bowl-shaped, erosional basins (cirques) are present at the heads of the larger valleys. The highest mountains are horn peaks, and sharp, knife-edged ridges (arêtes) form common boundaries between neighboring cirques. Valleys are fairly straight with U-shaped, cross-valley profiles and numerous, truncated ridge spurs; hanging valleys and waterfalls may be evident where tributary canyons were left dangling high above the floor of the main valley.

10. Glacial drift denotes any sedimentary material deposited from melting ice or meltwater streams. Till is the unsorted, unstratified drift deposited directly as the ice melts. Stratified drift (also called outwash) denotes sand and gravel beds deposited from glacial meltwater streams. Many glaciated landscapes exhibit low, irregularly shaped hills, mounds, and ridges of till standing above lower, marshy areas. Outwash plains are typically flat and like moraines they may be pitted by kettles (depressions formed by collapse of drift into voids formed by melting of buried ice blocks).

Glaciers can excavate deep valleys and lake basins, obliterate pre-glacial drainage systems, deposit moraines and outwash plains, carve mountain regions into alpine peaks and valleys, and leave behind the flat, silt covered floors of once immense lakes. Glaciation profoundly alters the morphology and appearance of landscapes.

11. The four types of moraines are end, lateral, medial, and ground. All are composed of till and with the exception of ground moraine, they all form prominent, irregularly shaped mounds and ridges. End moraines form around snouts of glaciers. A recessional moraine is any end moraine left by a retreating

Glaciers and Glaciation summary and notes