Chapter 19: Glacial Modification of Terrain

CHAPTER OUTLINE

I. The Impact of Glaciers on the Landscape

A. Glacier occurs when there is a net year-to-year accumulation of snow over a period of years.

B. Glaciers have had overwhelming impact on landscape because moving ice grinds away almost anything in its path.

1. Significantly reshape the topography.

C. About 7 percent of all contemporary erosion is accomplished by glaciers.

II. Glaciations Past and Present

A. Record is incomplete and often approximate.

B. Pleistocene Glaciation

1. Only most recent ice age has influenced contemporary topography, as most changes made by previous ice ages have been eradicated by other geomorphic events.

a) Ice Age, when capitalized, refers to Pleistocene because of its impact.

(1) Many parts of the continental terrain have been imprinted by Pleistocene events.

b) Pleistocene Epoch began at least 1.5 million years ago, but almost every year find evidence that it began even earlier.

(1) Consisted of an alternation of glacial and interglacial periods.

(a) Glacial — times of ice accumulation.

(b) Interglacial — times of ice retreat.

(2) Evidence showing that the last major ice retreat took place more recently than 9,000 years ago.

(a) Ice Age may not have ended yet at all.

(i) Period since last glacial stage is known as the Holocene Epoch.

(a) May either be a postglacial epoch or just the latest in a series of interglacial interludes.

c) Refrigerated high-latitude and high-altitude areas.

(1) At its maximum, covered one-third of the total land area of Earth.

d) Pleistocene had indirect effects on landscape through periglacial processes, sea-level changes, crustal depression, and pluvial developments.

(1) Periglacial processes of erosion and deposition from glacier meltwater and of frost weathering affected more than 20 percent of Earth’s land area.

(a) Periglacial zone — an area of indefinite size beyond the outermost extent of ice advance that was indirectly influenced by glaciation.

(2) Worldwide lowering of sea level occurred during every episode of Pleistocene glacial advance.

(3) Weight of ice caused portion of Earth’s crust to sink, sometimes to depths of 4,000 feet. Portions of Canada and northern Europe are still in isostatic rebound.

(4) Pluvial developments increased rainfall, which in turn created lakes where they never existed.

C. Contemporary Glaciation

1. Ice covers about 10 percent of Earth now.

a) More than 96 percent of that is in Antarctica and Greenland. (Fig. 19–2 shows distribution of glacial ice).

III. Types of Glaciers

A. Glacier’s pattern of movement and its effect on topographic shaping can vary considerably depending on quantity of ice and particularly on the environment.

B. Two different types of glaciers: ice sheets and mountain glaciers.

C. Continental Ice Sheets

1. Ice sheet — an immense blanket of ice that completely inundates the underlying terrain to depths of hundreds or thousands of feet.

a) Formed in nonmountainous areas of continents.

2. Only two true ice sheets currently, in Antarctica and Greenland.

3. Outlet glacier — a tongue of ice around the margin of an ice sheet that extends between rimming hills to the sea.

a) Icebergs form from chunks of ice that break off ice shelves and outlet glaciers.

D. Mountain Glaciers

1. Two types of mountain glaciers: ice fields and alpine glaciers.

a) Icefield — An unconfined sheet of ice in high-mountain areas, and which can develop into valley glaciers and piedmont glaciers.

(1) Valley glacier — a long, narrow feature resembling a river of ice, which spills out of its originating basin and flows down-valley.

(2) Piedmont glacier — a valley glacier that extends to the mouth of the valley and spreads out broadly over the flat land beyond.

b) Alpine glacier — individual glacier that develops near a mountain crest line and normally moves down-valley for some distance.

(1) Cirque glacier — a small glacier confined to its cirque and not moving down-valley.

(2) An alpine glacier typically breaks out of its basin and forms a valley glacier, and can extend to mouth of valley to create a piedmont glacier.

IV. How Glaciers Form

A. Glaciers require certain circumstances to form and then depend on just the right combination of temperature and moisture to survive.

B. Balance of accumulation and ablation is critical for persistence of glacier.

1. Accumulation — addition of ice into a glacier by incorporation of snow.

2. Ablation — wastage of glacial ice through melting and sublimation.

C. Changing Snow to Ice

1. Firn (Névé) — snow granules that have become packed and begin to coalesce due to compression, achieving a density about half as great as that of water.

2. Equilibrium line — a theoretical line separating the ablation zone and accumulation zone of a glacier along which accumulation exactly balances ablation.

3. Every glacier can be dived into two portions.

a) Upper portion is the accumulation zone, because accumulation exceeds amount lost by melting and sublimation.

b) Lower portion is ablation zone, because more is lost than is added each year.

D. Glacial Movement

1. Very little similarity between glacial movement, which is orderly, and liquid flow, which is disordered.

2. Because ice under glacier is under considerable pressure, it deforms rather than breaks.

3. Flow is often erratic and all parts of glaciers do not move at the same rate.

a) Fastest moving is at and near the surface.

b) If glacier is confined, as in valley glacier, the center moves faster than sides (as in streamflow).

E. Erosion by Glaciers

1. Volume and speed determine the effectiveness of glacial erosion.

2. Erode by plucking and abrasion.

a) Plucking — quarrying action in which rock particles beneath the ice are grasped by the freezing of meltwater in joints and fractures and pried out and dragged along in the general flow of a glacier.

(1) Probably accomplishes a glacier’s most significant erosive work.

(2) Particularly effective on leeward slopes (those facing away from the direction of movement).

b) Abrasion tends to polish when bedrock is highly resistant and dig striations and grooves in less resistant.

3. Glacial erosion effects are more notable in hilly areas; making entire landscape becomes more angular and rugged.

F. Transportation by Glaciers

1. Glaciers are extremely competent in their ability to transport rock debris.

2. Glacier flour — rock material that has been ground to the texture of very fine talcum powder by glacial action.

a) Perhaps most typical component of glacial load.

b) Most of load is picked up from bottom, and so carried along there in a narrow zone.

c) Alpine glaciers also carry some material on top of ice, where mass wasting from surrounding slopes placed debris.

d) Transportation occurs at variable speeds outward or down-valley.

(1) Rate depends on season, variations in ice accumulation, and gradient of underlying slopes.

e) Flowing water transports water to many glaciers.

G. Deposition by Glaciers

1. Transportation and deposition are probably the major roles of glaciers in landscape modification.

2. Gave U.S. midwest one of world’s the most productive soils (at expense of central Canada, where the soil, regolith, and even some bedrock was scoured, transported, and later deposited.

a) Drift — all material carried and deposited by glaciers.

(1) Comes from thought the material had drifted from biblical floods.

b) Till — rock debris that is deposited directly by moving or melting ice, with no meltwater flow or redeposition involved.

c) Glacial erratic — outsized boulder included in the glacial till, which may be very different from the local bedrock.

H. Deposition by Meltwater

1. Glaciofluvial deposition, through meltwater, occurs around margins of all glaciers and can continue far out into periglacial zones.

a) Meltwater actually deposits or redeposits much of the debris carried by glaciers.

b) Can occur by

(1) Subglacial streams issuing from ice, depositing debris.

(2) Meltwater from glaciers, picking up material already deposited and redepositing it elsewhere.

(a) Most of meltwater deposition actually involves redeposition.

V. Continental Ice Sheets

A. Most extensive features to appear on face of planet.

1. Pleistocene ice sheets reshaped the terrain and drainage of nearly one-fifth of Earth’s land surface.

B. Development and Flow

1. Pleistocene ice sheets, except for that in Antarctica, developed in subpolar and midlatitude locations then spread in all directions.

2. Preexisting terrain channeled the initial flow, but then ice accumulation overrode most of the preglacial topography.

a) Eventually, various ice sheets coalesced into one, two, or three massive sheets on each continent.

C. Erosion by Ice Sheets

1. Ice sheets generally result in a gently undulating surface: low relief but not absolute flatness.

a) Ice-scoured rock knobs and scooped-out depressions; bare rock and lakes dominate.

b) Erratic and inadequately developed stream patterns.

c) Creates most conspicuous features of U-shaped valley bottoms.

(1) Ice sheet in central New York reshaped parallel stream valleys into the long, narrow, deep Finger Lakes.

2. Roche moutonnée — a characteristic landform produced when a bedrock hill or knob is overridden by moving ice. The stoss side is smoothly rounded and streamlined by grinding abrasion as the ice rides up the slope but the lee side is shaped largely by plucking, which produces a steeper and more irregular slope.

3. Erosional effects, however, are modified by depositional debris.

D. Deposition by Ice Sheets

1. Till plain — an irregularly undulating surface of broad, low rises and shallow depressions produced by the uneven deposition of glacial till.

2. Moraine — the largest and generally most conspicuous landform feature produced by glacial deposition, which consists of irregular rolling topography that rises somewhat above the level of the surrounding terrain.

3. Kettle — an irregular depression in a morainal surface created when blocks of stagnant ice eventually melt.

4. Drumlin — a low, elongated hill formed by ice-sheet deposition. The long axis is aligned parallel with the direction of ice movement, and the end of the drumlin that faces the direction from which the ice came is blunt and slightly steeper than the narrower and more gently sloping end that faces in the opposite direction.

a) Depositional features subsequently shaped by erosion.

b) Usually occur in groups, sometimes in hundreds.

(1) Central New York and eastern Wisconsin have the greatest concentrations.

E. Glaciofluvial Features

1. Meltwater is incapable of moving larger material, so glaciofluvial features are composed largely or entirely of gravel, sand, and silt.

a) Stratified drift — drift that was sorted as it was carried along by the flowing glacial meltwater.

2. Outwash plain — relatively smooth, flattish alluvial apron deposited beyond recessional or terminal moraines by streams issuing from ice.

a) Most extensive glaciofluvial features.

3. Valley train — a lengthy deposit of glaciofluvial alluvium confined to a valley bottom beyond the outwash plain.

4. Esker — long, sinuous ridges of stratified drift composed largely of glaciofluvial gravel and formed by the choking of subglacial streams during a time of glacial stagnation.

5. Kame — a relatively steep-sided mound or conical hill composed of stratified drift found in areas of ice-sheet deposition and associated with meltwater deposition in close association with stagnant ice.

VI. Mountain Glaciers

A. Don’t reshape the terrain as much as ice sheets did.

1. Mountains protrude above mountain glacier ice.

2. Mountains channel the movement of mountain glaciers.

3. Produce a rugged landscape as opposed to the smoothing and rounding of terrain accomplished by ice sheets.

B. Development and Flow

1. Both highland icefields and alpine glaciers advance downslope, usually finding path of least resistance down preexisting stream valley.

a) Highland icefields can extend broadly, submerge all but uppermost peaks, and extend into a series of lobes that move down adjacent channels.

b) Alpine glaciers usually form in sheltered depressions near heads of stream valleys.

C. Erosion by Mountain Glaciers

1. Highland icefields and alpine glaciers can dramatically reshape topography.

a) Cirque — a broad amphitheater hollowed out at the head of a glacial valley by ice erosion.

(1) Basic landform in glaciated mountains, marking place where alpine glacier originated, being quarried out of mountainside, though precise mechanics of formation are unknown.

b) Arête — a narrow, jagged, serrated spine of rock; remainder of a ridge crest after several cirques have been cut back into an interfluve from opposite sides of a divide.

c) Col — a pass or saddle through a ridge produced when two adjacent cirques on opposite sides of a divide are cut back enough to remove part of the arête between them.

d) Horn — a steep-sided, pyramidal rock pinnacle formed by expansive quarrying of the headwalls where three or more cirques intersect.

e) Tarn — small lake in the shallow excavated depression of rock benches of a glacial trough or cirque.

f) Glacial trough — a valley reshaped by an alpine glacier, usually with a relatively straight course with a fluctuating gradient.

g) Paternoster lakes — a sequence of small lakes found in the shallow excavated depressions of a glacial trough.

h) Hanging valley — a tributary glacial trough, the bottom of which is considerably higher than the bottom of the principal trough that it joins.

(1) Typically, streams that drain from tributary valleys must plunge over waterfalls to reach the floor of the main trough.

D. Deposition by Mountain Glaciers

1. Drift occurs in the middle and lower courses of glacial valleys, only rarely in high country.

2. Moraines are the principal depositional landform, but are smaller and less conspicuous than those produced by ice sheets.

a) Lateral moraines are the largest depositional features, being well-defined ridges of unsorted debris built up along the sides of valley glaciers.

VII.The Periglacial Environment

A. More than 20 percent of world’s land area is presently periglacial, but mostly from Pleistocene Epoch.

B. Periglacial lands are either in high latitudes or high altitudes.

1. Patterned ground — various geometric patterns that repeatedly appear over large areas in the Arctic, with unknown origins.

a) Most unique and eye-catching periglacial terrain.

b) Widely accepted hypothesis is that frost action is instrumental in formation.

c) Emphasizes role of soil ice in producing geomorphic activities that usually don’t occur in warmer regions.

2. Proglacial lake — a lake formed when ice flows across or against the general slope of the land and the natural drainage is impeded or completely blocked so that meltwater from the ice becomes impounded against the ice front.

a) Most are small and temporary, but some are large and long-lived.

VIII.Are We Still in an Ice Age?

A. Scientists have postulated many theories and constructed many models to try to explain the sporadic glaciation and deglaciation of Earth.

1. Some theories are based on variations in intensity of solar radiation Earth has received.

2. Some look at shifting of Earth’s axis or variation in eccentricity of Earth’s orbit.

3. Some focus on changes in the amount of carbon dioxide in atmosphere.

4. Some founded on changes in the position of continents and ocean circulation patterns.

5. Some rooted in increased altitude occurring after period of tectonic upheaval.

6. Some combine elements of the above.

a) None of the theories are widely accepted — still looking for a convincing explanation.

B. Still question if living in postglacial period or interglacial period.

1. Many glaciers with a history of nothing by retreat in human record keeping began to readvance in 1960s and 1970s.

2. More recently, weather changes have been unfavorable for glacier growth.