CENOZOIC GLACIATION AND ITS MARK ON THE LANDSCAPE

Contents

I. Development of Cenozoic Ice Sheets

A. Antarctica

B. Northern Hemisphere

III. Glacial Influence on Sea Level

IV. Pliocene-Pleistocene Drainage Changes

A. General pre- and post-glacial drainage of North America

B. Glacial Lake Agassiz

C. Formation of the modern Great Lakes

D. Changing pathways for de-glacial meltwaters

E. Catastrophic floods

F. Finger Lakes

G. Pluvial Lakes

A. U-shaped valleys and fjords

B. Hanging Valleys

C. Cirques, Aretes and Horns

A. Till
1. Glacial erratics

2. Moraines

a. end moraines
1a. terminal moraines

1b. recessional moraines

b. ground moraine

c. lateral moraine

d. medial moraine

3. Drumlins
B. Stratified Drift
1. Outwash plains

2. Valley train

3. Kettles

4. Eskers

I. Development of Cenozoic Ice Sheets

A. Antarctica

1. The extinctions at the end of the Cretaceous, including the dinosaurs, marked the end of the Mesozoic Era and the beginning of the Cenozoic. Throughout the previous Mesozoic, Antarctica remained relatively ice free, except for mountain glaciers and perhaps a small ice sheet near the pole. Coastal areas remained ice-free and fossils indicate heavily forested in many regions.
2. Relatively ice-free conditions continued during the early Cenozoic (65-40 Ma). South America was still connected to Antarctica and Australia was beginning a slow separation from Antarctica. Since Antarctica was not yet isolated by continuous ocean around it, larger subtropical gyres in the Atlantic and Indo-Pacific still carried relatively warm surface waters to its margins.
3. By 40 million years ago, Australia was moving more rapidly to the north and a more continuous ring of cooler water began separating Antarctica from the world's oceans. Ice sheets began forming in the interior of the continent but not in coastal regions.
4. By 23 million years ago, South America separated from Antarctica. From this time forward, Antarctica became decoupled from the warmer waters of the world. A cold circum-Antarctic continent encircled it, cutting off warmer waters from the subtropical gyres, and an ice cap covered most of the continent.
5. By 6-7 million years ago, the continent of Antarctica looked much as it does today, however, glaciation was only beginning in the Northern Hemisphere.

B. Northern Hemisphere

1. Large scale glaciation began in the Northern Hemisphere much later than in the Southern Hemisphere.
2. Some small scale glaciation occurred in the Northern Hemisphere by about 5-6 Ma.
3. Widespread ice sheets, though smaller than more recent ones, began waxing and waning by 3.5 Ma.
4. During the past 1 million years, the largest of all ice sheets covered vast areas of North America.

What caused the large scale glacial-interglacial variations in the Northern Hemisphere?

Clues to this question come from the more complete deep-sea sediment record. It is from this record that the most detailed record of glacial-interglacial variations is revealed from oxygen isotopes of fossil planktonic foraminifera. These records indicate the most important cycles of climate change have frequencies of 400,000, 100,000, 41,000, and 23,000 years.

Additional insight became evident with the discovery that these climate frequencies corresponded to changes in orbital parameters of the Earth as noted by the Yugoslavian scientist Milankovitch.

This discovery of matching frequencies of climate with the Milankovitch orbital cycles, led to publication in the late 1960s of the theory "Orbital Pacemaker to the Ice Ages". Yes, glacial-interglacial variations appear to be related to changes in these parameters (text page 265): MILANKOVITCH CYCLES

1. Axial Tilt (period = 41,000 yr.) 21.5-24.5o range tilt shallow=greater polar snow because less summer melt
2. Eccentricity of Earth orbit (period= 400,000 & 100,000 yr.) 0.017-0.053 eccentricity, greater eccentricity= colder
3. Precession of the Equinoxes Wobble of the axis (period=23,000 yrs)
• causes the position of the equinoxes and solstices to shift in position around the Earth's elliptical orbit
• today closest approach of Earth to Sun is in Jan. so N. Hemisphere winter warmer
• 10,000 yr. ago in July so colder winter
• closest approach changes by 1 day per 60 yrs

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II. Extent of Glaciation in North America and elsewhere.

• The largest continental ice sheets to ever cover the Northern Hemisphere occurred during the past 1 million years.
• In Europe and Asia, a continental ice sheet covered vast tracks of land, spreading to the SW, S, and SE from the Scandinavian Ice Sheet. From its center of accumulation over Scandinavia, the continental ice sheet covered the North Sea, nearly all of Great Britain and Ireland, the Baltic, northern and portions of central Europe, and northern Russia. Among other countries covered by ice were: the Netherlands, Denmark, Norway, Sweden, Finland, northern Germany, most of Poland, Ukraine, and much of Russia. Ice caps covered the Pyrenees and Alps.
• On the other side of the Atlantic, ice sheets spread from centers in Greenland, Labrador, and west central Canada. These ice sheets covered virtually all of Canada, portions of Alaska, and spread south into the mid-west of the United States.
• Within the United States, ice sheets once covered portions of North and South Dakota, Iowa, Illinois, Pennsylvania, and Alaska; most of Minnesota, Indiana, and Ohio; all of Michigan, New York, Connecticut, Massachusetts, Vermont, New Hampshire, and Maine.
• The Ohio River marks the southernmost extent of the ice.
• As much as 70 million cubic kilometers of ice once covered the continents.
• The last glacial maximum occurred 18,000 years ago.

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III. Glacial Influence on Sea Level

• The source of moisture stored in the ice of continental glaciers was the oceans. The storage of as much as 70 million cubic kilometers of snow and ice on the continents resulted in enormous lowerings of sea level during glacial periods.
• During the most severe glaciations, sea level was lowered by more than 400 feet.
• The continental margins, above sea level, expanded great distances seaward as continental became exposed.
• The Bering Straits, between Alaska and Siberia, became a land bridge. Across this bridge, man first migrated to North America and animals migrated east and west.
• The British Isles were connected to Europe during some glacials.
• The base level of rivers was lowered, causing them to cut down their valleys out onto the exposed continental shelf. The subsequent rise of sea level has flooded portions of these valleys, creating many of our current coastal estuaries.
• Today, most land-based ice is in Antarctica and Greenland. If this ice were to melt, perhaps due to continued global warming, sea level would rise by some 230 feet (70m). Such a rise would put all of Florida under water!

IV. Pliocene-Pleistocene Drainage Changes

A. General pre- and post-glacial drainage of North America- class illustration

B. Glacial Lake Agassiz- 4 TIMES AS LARGE AS LAKE SUPERIOR

1. INTRODUCTION

• Largest of North American glacial lakes.
• Formed when ice began retreating because the former northern drainage was blocked by the ice sheet.
• Lake formed numerous times with discharge occurring to the Mississippi-Minnesota Rivers, Lake Superior, and Hudson Bay.
• Long thought to have last drained south into Gulf of Mexico.
• Southern discharge inconsistent with Gulf of Mexico planktic foraminiferal record and oxygen isotope record.

2. NEW INTERPRETATION OF DISCHARGE

• Discharge was to the NW and into the Arctic (see Figure)
• Part of drawn down of the lake level was catastrophic and occurred when the lake overflowed a drainage divide separating it from the Clearwater River.
• Incision of the drainage divide near the Alberta- Saskatchewan border caused it to drain.
• Catastrophic flood at ~9900 yr. B.P.
- rapid draw down of the lake by 46 meters in ~ 78 days

- flood mean velocity of 13.2 meters/second

- discharge of 2.4 x 106 cubic meters per second or 8.6 cubic kilometers per hour

- enlarged Athabasca delta by 4200 square kilometers

C. Formation of the modern Great Lakes

• Prior to Pleistocene glaciation (<1.8 Ma), the Great Lakes (Superior, Michigan, Huron, Erie, and Ontario) on our northern border did not exist. The area was a flat lowland, underlain by soft sedimentary rocks, with stream valleys draining to the north.
• Pleistocene lobes of the North American ice sheet pushed southward into the pre-existing stream valleys, deepening them by glacial erosion. These valleys became the Great Lakes basins. Upon retreat of the ice these basins filled with meltwater to become the Great Lakes.
• Today the Great Lakes contain 18% of water in all freshwater lakes of the world.
• Four of the five Great Lakes were eroded to below sea level.

D. Changing pathways for de-glacial meltwaters

• major shifts occurred in the region of meltwater discharge with the northward retreat of the ice margin

1. Minnesota River once the mighty river of N. America. Hugh volumes of melt water cut the narrows of St. Paul in Minnesota

2. As Lake Agassiz drained, major drainage through Mississippi, Illinois, Wisconsin, Wabash and Susquehanna rivers.

3. The Susquehanna River valley is the most significant crossing the Appalachians, from New York to the Chesapeake Bay. Deepened when it was a major discharge route prior to eastern ice retreat.

4. With eastern ice retreat the Hudson River became a major discharge route.

5. When ice cleared the Ottawa River valley it was left below sea level until the crust rebounded to 600' above sea level. The upper great lakes drained (later to refill) and the Champlain Sea flooded in (~12,000 yr. B.P.; Whale and seal skeletons found in sands and clays of this former inland sea)

E. Catastrophic Floods.

• ice dams-lake formation- breaching of ice dam
• flood Lake Missoula-Channelled Scablands.
• lakes breaching drainage divides: Lake Bonneville and American Falls lakes breaching lava dams.
• Lake Agassiz
• Subglacial ice sheet megafloods
• Alberta
• NOT ALL CATASTROPHIC FLOODING OCCURRED DURING DE-GLACIAL PERIODS (e.g. LATE WISCONSIN SUBGLACIAL MEGAFLOODS)
• Multiple subglacial discharges transmitting Laurentide water from NW Territories & N. Saskatchewan through Alberta to Montana and the Mississippi drainage system.
• southern flood aided by discharge from piedmont glaciers in region where Cordillearan Ice Sheet met Laurentide ice sheet
• individual flood events with discharges greater than 84,000 km3
• Peak deglacial river floods Many examples; Mississippi, Hudson, Minnesota, etc.

F. Finger Lakes

• The Finger Lakes are located in western New York state.
• They are long, deep, narrow, and arranged in a radiating pattern south of Lake Ontario.
• The lake basins were carved by an ice sheet and later filled to form these beautiful lakes.

G. Pluvial Lakes

• 1. Common in the Basin and Range region during glacial episodes.
• 2. Formed as a result of cooler (vs. more arid now) and more moist conditions. Less evaporation, more rainfall and runoff formed many lakes in the southwest of the United States. Remnants of some of these lakes exist today but most have long since dried up, leaving behind wave-cut terraces, beaches, and deltas.
• 3. Largest (>50,000 sq. Km, >300 m deep) were:
• Lake Bonneville ( remnant is now Great Salt Lake, Utah) - (30,000 yr. B.P.) Overflowed to cause catastrophic floods down the Snake River
• Lake Lahontan ( remnant is now Pyramid Lake, Nevada)
• 4. Death Valley, California (now the hottest and driest place in North America) once the site of a pluvial lake. Lake Manly occupied this site, obtaining a length of 145 km and a depth of 178m. It has evaporated leaving evaporites such as borax.

V. Erosional Landforms of Alpine and Valley Glaciers (locations in parenthesis refer to sides of these features shown in class)

A. U-shaped glacial troughs and fjords (Yosemite, Norway, Antarctica, Alps)

• During the course of time, thousands of miles of ice can move through valleys in the form of glaciers. By bulldozing, plucking, and abrasion glaciers may severely change the profile of valleys from their more typical stream profile (V-shaped) to a glacial profile (U-shaped). Glacial erosion deepens, widens, and straightens their valleys often creating nearly vertical valley walls. With the much lower sea level during glaciations, glaciers with coastal access eroded their valleys much below current sea level. As sea level rose and glaciers melt during interglacials, these deeply eroded coastal valleys became flooded creating fjords. Fjords are often many hundreds to over a thousand feet deep.

B. Hanging Valleys (Yosemite, Norway, Alps)

Hanging valleys are tributary valleys to adjacent larger valleys once occupied by a valley glacier. The hanging valley was occupied by a smaller glacier which did not as deeply erode the valley floor. Once the glaciers melt the tributary valley outlet into the main valley is left hanging high on the main valley wall. Some of the largest waterfalls plunge from hanging valleys.

C. Cirques, Aretes and Horns

Alpine glaciers erode mountain peaks to create spectacular angular mountain landscapes. The source region of alpine glaciers is high on mountain walls where they create steep-walled, bowl-shaped depressions (cirques) which open down-slope to the glacial trough. Steep-walled, pyramidal peaks called horns (e.g. Matterhorn, Switzerland) are formed by three or more cirques eroding headward into the sides of mountains. Arêtes (Antarctica and Alps) are razor-sharp ridges, usually formed by cirque erosion on both sides of a ridge so that only a thin portion of rock remains.

VI. GLACIAL DEPOSITION (locations in parenthesis refer to sides of these features shown in class)

A. Till- materials deposited directly by glaciers

1. Glacial erratics- boulders derived from an outside area and transported by ice.

2. Moraines- a variety of land forms comprised of till, mostly ridge-like.
a. End moraines- ridges of till which form at the terminus of the glacier. Commonly forms crescent-shaped ridges. Moraines sometimes form natural dams so that lakes form upstream after the ice recedes. Common throughout Ohio, Indiana, and Wisconsin.

1a. Terminal moraine- outermost end moraine which marks the greatest limit of ice advance. Forms from a stationary ice front.

1b. Recessional moraine- an end moraine that forms as the ice front stabilizes during retreat.

b. Ground moraine- gently rolling layer of till laid down by a continuously retreating glacier. Till is deposited from the melting base of the glacier (Norway and Alps).

c. Lateral moraine- ridges of till deposited at sides of the glacier against valley walls (Alps). Composed of eroded sediment and rock carried on the sides of the glacier against the valley walls.

d. Medial moraine- created when two alpine glaciers merge to form a single ice stream. Two of the lateral moraines of the merging glaciers coalesce into the center of the ice stream they merge into (Alps).

3. Drumlins- smooth, elongate, parallel hills. Asymmetrical, steep-sided in up-stream direction, found in drumlin fields. Common in upstate New York and Wisconsin. Formed under continental ice sheets and aligned in the direction of ice movement.

B. Stratified drift- sediment laid down by glacial meltwater

1. Outwash plains- plains of sediment deposited by glacial outwash in front of an ice sheet.

2. Valley train- similar to above but in front of an alpine glacier or confined by valley walls.

3. Kettles- basins on outwash plain formed by sedimentation around isolated ice blocks which later melt leaving depressions.

4. Eskers- long, narrow, sinuous ridges of sand and gravel deposited by glacial meltwater flowing in confined channels within, on top of, and beneath a mass of stagnant ice. Most form in meltwater tunnels under continental ice sheets.