Geology Cafe

Introduction to Geology

Chapter 12 - Glaciers and Glaciation

Glaciers occur in regions with average annual temperatures are below freezing—such as in high mountain ranges and in polar regions. In the geologic past, great glaciers covered large portions of North America and Europe where forested landscapes and cities exist today. In contrast, Antarctica and Greenland are still experiencing "ice age" conditions today. However, Around the world glaciers, are melting and retreating, including the great ice caps in Antarctica and Greenland. This melting is a continuation of glacial retreat that began at the end of the peak of the last ice age (the "Wisconsin Stage" in North America. Since the end of the last ice age, sea level has risen hundreds of feet around the world, and sea level is still rising.

This chapter reviews landscape features associated with processes and landscape features of existing glaciers, and examines landscape features from past glaciation periods where continental glaciers once covered large regions of North America and Europe.
Water Cycle
Fig. 12-1. Glaciers and ice caps more freshwater that all steams, rivers, lakes, and groundwater combined.

What are glaciers and glaciation?

A glacier is a slowly moving mass or "river of ice" formed by the accumulation and compaction of snow over many years, forming on mountains or land masses near the Earth's poles. The difference between and ice field and a glacier is that glaciers move under the influence of gravity. Glacial ice forms from the accumulation of frozen precipitation (primarily snow) that compacts (degasses) into solid, clear blue, glacial ice. Although ice is a solid, under pressure it deforms and fractures easily. Ice on the surface of a moving glacier is highly fractured. However, at a depth of about 80 feet (25 meters) the weigh of overlying ice will cause it to deform plastically, and if there is a slope, the ice will begin to flow downslope. There are several types of glaciers.

An ice sheet is the layer of ice covering extensive regions of the world, notably Greenland (Figure 12-2) and Antarctica (Figure 12-3). The ice sheets form from the accumulation of thousands or millions of years of snowfall. With compaction, the snow turns to glacial ice.

An ice shelf is a thick floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface.
Greenland Map of Antarctica showing ice sheets, ice caps, and ice shelfs
Fig. 12-2. Greenland is covered with a great ice sheet. This satellite view shows much of it surrounded with sea ice. Fig. 12-3. Map of Antarctica (Earth's 5th largest continent) showing ice sheets, ice caps, and ice shelves.
Antarctica has several vast ice shelves in addition to its massive ice sheets and ice caps in its mountainous regions (Figure 12-3).

An ice cap an extensive dome-shaped or plate-like perennial cover of ice and snow that spreads out from a center and covers a large area, especially of land. Examples include upland glacial regions in Iceland and the Canadian and Alaska coastal mountain ice caps.

A valley glacier is a glacier confined by the walls of a valley in a mountainous region. Snow accumulating in mountainous regions turns to glacial ice, and once thick enough begins to flow down valleys. The ice carves the floor and walls of the valley eroding large quantities of rock and transporting downslope (an example in Figure 12-5).

Ice caps and mountain glaciers of British Columbia
Fig. 12-4. Ice caps (left) and valley glaciers (right) in British Columbia. In this region ice is flowing from accumulation sites in the mountainous uplands, flowing down valleys in to the ocean.
Gulkana Glacier, Alaska
Fig. 12-5. Moving ice (glaciers) erodes bedrock and carry large quantities of sediment. This view shows Gulkana Glacier, a valley glacier in Alaska.
A piedmont glacier is a thick, continuous ice sheet formed along the base of a mountain range formed by the spreading out and coalescing of valley glaciers supplying ices from higher mountain elevations. Figure 12-6 shows the Malaspina Piedmont Glacier in Alaska. An ice cap region in the Elias Mountains of southern Alaska and Yukon Territory provide glacial ice via valley glaciers to the Malapina Piedmont Glacier that spread out across the lowlands along the Gulf of Alaska. In the low country, the piedmont glacier gradually melts, leaving large amounts of rock material to accumulate along with remnants of the melting ice.

A tidewater glacier is a glacier that meets the ocean and its mouth is influenced by the daily rise and fall of tides, shedding sediment-laden ice into coastal waters, providing nutrients to the local ecosystem (Figure 12-7).
Malaspina Glacier (Google Earth view)
Glacier Bay National Park, Alaska
Fig. 12-6. Malaspina Glacier is a piedmont glacier at the southern end of the St. Elias Mountains along the Gulf of Alaska (Part of Wrangell-Elias National Park). Fig. 12-7. The Grand Pacific Glacier is an example of a tidewater glacier located in Glacier Bay National Park, Alaska. The glacier is constantly calving icebergs and silty sediment into Glacier Bay.

What are ice ages and glaciations?

An ice age is a period in Earth's history when the global temperatures cooled enough for glaciers (both alpine and continental glaciers) to form. Note that parts of the world (Greenland and Antarctica) are still experiencing ice-age-like conditions.

A "glaciation" is a period when ice or glaciers cover the land's surface. In the "not too distant" geologic past large continental ice sheets covered large portions of North America and Europe.

Earth has experienced ice ages many times. The current cycle of ice ages began about 3 million years ago during Pliocene Epoch (an epoch of the late Tertiary Period spanning about 5.3 to 1.8 million years ago). The cycles of ice-age glaciations and intervening warming periods began in Pliocene time. Major glaciation cycles occur many times throughout the following Quaternary Period. The Quaternary Period is subdivided into the Pleistocene Epoch and the Holocene Epoch. The Pleistocene Epoch represents the time span of about 1.8 million to about 11,000 years ago. There were many episodes of continental glaciation and intervening ice-free periods occurred within the Pleistocene Epoch. The Holocene Epoch began about 11,000 years ago, about the time that human population growth and distribution expanded worldwide.

Wisconsin Stage
—the last, most recent glaciation period within the current ice age occurring in the geologic time interval about 110,000 to 10,000 years ago, at the end of the Pleistocene Epoch. Note that Antarctica and Greenland are still enduring ice-age conditions, whereas continental glaciers on North America and northern Europe vanished at the end of the Wisconsin Stage.
Larentide Glacier
Fig. 12-8. Maximum extent of the Wisconsin Glaciation, the last glaciation cycle during the current ice age. Earth is now within an interglacial warming period called the Holocene Epoch.

What are some of the possible causes of glaciation?

Many factors come into play when considering the cause of glaciation periods. Studies of ice cores from Antarctica, Greenland, cores samples derived from ocean sediments, and studies of glacial deposits found on land indicate that there may have been as many as 16 glaciation periods during the late Pliocene through the Pleistocene Epochs (during the last 3 million years). The rise and fall in sea level is preserved in sediments deposited in restricted coastal and marine sediments. These sediments are well exposed in many places allowing paleoclimate scientist to evaluate the sedimentary record in many locations (such as at Ft. Funston sea cliffs near San Francisco, CA, Figure 12-10). Some of these cycles were more intense than others, and they impacted regions around the world differently. Each of the glaciation cycles was followed by an interglacial warming period in which the continental glaciers "retreated" (or melted back). We are currently in one of the interglacial warm periods (although it wouldn't feel like that in Greenland or Antarctica where glaciation is still occurring). The current arguments supporting of "global warming" due to "greenhouse gases" created by human activity is only part of the story.

The astronomical connection: Milankovitch Theory

Early in the 20th century, a Serbian geophysicist and astronomer name Milutin Milankovic worked out mathematically the subtle changes in Earth’s orbital cycles, involving cyclic changes in its rotational axis and and its revolution around the sun changes. Three “orbital forcing” cycles include the eccentricity of Earth's orbit, Earth' axial precession (41,000 years), precession of equinoxes (21,000 years)(illustrated in Figure 12-11). Eccentricity refers to the change of earth's orbit from being round to more elliptical in shape this cycle repeats every 95,000 years. When it is more elliptical the Earth has shorter, warmer summers and longer, colder winters. Axial precession refers to the "wobble" in the tilt of Earth's axis. The tilt of the axis changes from about 21.5 to 24.5 degrees on a cycle lasting about 41,000 years. The precession of equinoxes refers to which hemisphere is facing the sun when it is closest to the sun. Right now, the earth is closest to the sun during the winter in the Northern Hemisphere. These cycles impact how much incoming solar radiation that the regions of the earth receive over time, most important being where land is exposed in high latitude regions (where continental glaciation has taken place repeatedly). Milankovitic showed that these cycles combine or interfere with each other in the amount of energy the polar regions receive through time. Climate investigations in the last century have shown that Milankovitch Cycles closely correspond with the record of global temperature changes retrieved from ice cores, marine sediments, and other sources.

Milankovitch Cycles alone don't explain the onset of the ice ages. Other large-scale factors may have influenced the global climatic cycles. For instance, during the last several million years, the Himalayan Mountains and Tibetan Plateau have risen, impeding atmospheric circulation in the Northern Hemisphere. In addition, the formation of the Isthmus of Panama (connecting South America to North America) changed the patterns in the circulation of equatorial ocean waters around the globe. In addition, the amount of energy the sun releases may be cyclic as well. The interactions of global plant cover, global cloud cover (and precipitation), polar sea ice, and impacts of massive volcanic eruptions may all have possible influence on glaciation cycles.
Glacial periods
Fig. 12-9. Glaciations of the late Pleistocene Epoch
Thornton Beach and cliffs near San Francisco, CA
Fig. 12-10. The "Sedimentary Record" preserves evidence of numerous glaciation cycles (Ft. Funston Beach, CA).
Milankovitch Cyles
Fig. 12-11. Milankovitch cycles

How do glaciers move?

Gravity is the driving force for glacier movement. However, the rate of accumulation and melting of ice are factors that influence glacial activity. Figure 12-12 shows a profile of a simple glacier, showing its motion and impact on erosion of bedrock.

The zone of accumulation is the upper part of a glacier where snow accumulation and ice formation takes place faster that it melts, forming the glacier. The zone of ablation is the downslope end of a mountain glacier where melting outpaces the accumulation of new ice and snow. (Ablation is the removal of snow and ice by melting or evaporation, typically from a glacier or icefield.) A theoretical line called the snowline is the location on a mountain glacier where the rate of accumulation is equivalent to the rate of melting. Snowline rises and falls with seasons and climate changes from one year to another. When the rate of accumulation increases a supply of ice to a glacier, surging may occur; surging is the sudden movement of a glacier downslope.

As glaciers move downslope they may meet resistant obstacles of hard bedrock, or they my encounter a change in slope. These cause changes in ice motion causing fractures to form and open in the ice flow. A crevasse is a deep open crack, especially one in a glacier.

Where glaciers meet a body of water (ocean or lake), large blocks of ice will split of fall away from the toe of the glacier. Calving is the process of blocks of ice falling off the face of a parent glacier, ice shelf, or iceberg.
Glacier profile
Fig. 12-12. Profile of a glacier
Ice claving from the face of a glacier, forming icebergs
Fig. 12-13. "Big slash!" Ice calving from the face of a glacier, forming icebergs.

Cold Climate Environments

High Elevation and high latitude regions are colder because the atmosphere is colder. Timberline is the line or altitude above which no trees grow because the annual temperature is too cold. Timberline is about 16,000 at the equator and near sea level north of the Arctic Circle. In the continental United States, timberline ranges roughly between 8,000 and 11,000 feet (Figure 12-14). In southern Alaska, timberline is as low as 2,000 feet. Timberline approaches sea level north of the Arctic Circle.

Large regions of the northern and southern polar regions have permafrost, meaning the soil on and near the surface is permanently frozen. The word tundra applies to a vast, flat, treeless Arctic region of Europe, Asia, and North America in which the subsoil is permanently frozen. Mountain regions above timberline also have tundra-like characteristics.
Wheeler Peak, Nevada rises above timberline.
Fig. 12-14. Wheeler Peak (Nevada) rises above timberline (about 9,000 feet).

Landforms created by glacial activity in mountainous regions

The effect of glaciation is most obvious in areas where glaciers are currently active. These same landscape feature are still visible in regions that experience glaciation during the Pleistocene Epoch. For instance, Glacier National Park's landscape was heavily sculpted by mountain glaciers, but there is very little ice remaining in the region today. At one time all the valleys in the park were filled with large valley glaciers. Examples of glacial landforms in mountainous regions include:

horn—a pointy mountain peak having concave faces carved by glaciation (Figures 12-15 to 12-17).

arete—a narrow, jagged mountain ridge that divides two cirques or glaciated valleys.

col—a saddle in a glacially carved mountain ridge (a gap in an arete).

cirque—a bowl-shaped, steep sided hollow at the head of a valley or on a mountainside, formed by glacial erosion (Figure 12-17).

tarn—a small mountain lake, especially one formed by glaciers; typically found within the basin of a cirque.

hanging valley—a U-shaped canyon (carved by a mountain glacier) that intersects with a larger, deeper U-shaped valley. The mouth of a hanging valley is a typical locations where large waterfalls occur (such as in Yosemite National Park, Figure 12-18).

roche moutonnee—an elongate mound of bedrock worn smooth and rounded by glacial abrasion, typically with a steep slope of cliff on the downhill side formed by the plucking away of blocks of bedrock by the moving glacial ice (Figure 12-19).

pater noster lakes—a series of moraine-dammed lakes formed by the intermittent retreat of a valley glacier in a mountainous region.

fjord—a long, narrow, deep inlet of the sea between high cliffs typically formed by submergence of a glaciated U-shaped valley (Figure 12-21).
Alpine Glacial Features arete
Fig. 12-15. Landscape features associated with alpine glaciation in mountainous regions. Fig. 12-16. An arete with horns and cols in North Cascades National Park, Washington
cirque Yosemite
Fig. 12-17. An arete with a large cirque with snow fields in Great Basin National Park, Nevada Fig. 12-18. U-shaped valleys (with steep walls) and a hanging valley in Yosemite National Park, CA
Roche moutonee Pader noster lakes, Torrey Canyon, Wind River Mountains, Wyoming
Fig. 12-19. A roche moutonnee in Harriman State Park, New York Fig. 12-20. Pater noster lakes in Torrey Canyon, Wind River Mountains, Wyoming

Erosional or depositional evidence of past glaciation

Glaciers both scour the bedrock and deposit large quantities of sediment. All kinds of glaciers leave traces when the ice melts. Figure 12-22 shows landforms left behind by a retreating continental glacier. The "toe"region of a continental glacier (where it is melting faster than advancing) is where large amounts of sediments accumulate). Both erosional and depositional features are summarized below:

Erosion features:

striations—parallel grooves such as: the scratches left by a glacier on rocks (Figure 12-23). Striations reveal the direction that glacial ice moved as rock imbedded in the base of the moving glacial ice scrape and erode the bedrock.

glacial grooves—trench-like striations carved in bedrock caused by rock-bearing moving ice at the bottom of a glacier. Grooves can range from small to large striations up to many meters wide (Figure 12-23).

Depositional features include both sediment deposits and ice-sculpted landforms:

erratic—a rock or boulder that differs from the surrounding bedrock and is believed to have been transported from a distant location by glacial action (Figure 12-24).

till—unsorted material deposited directly by glacial ice and consisting of rock fragments ranging from large boulders to sand, fine silt, and clay (Figure 12-25).

moraine—accumulations of rocks and sediment deposited by a glacier, typically as ridges at its edges or its terminal boundary of flow and zone of wastage.

drumlin—a low oval-shaped mound or small hill, typically one of a group, consisting of compacted glacial till shaped by flowing ice in a region that experienced glaciation.

esker—a long, typically winding, ridge composed of gravel and other sediment deposited by meltwater from a retreating glacier or ice sheet.

kettle—a shallow, sediment-filled body of water formed by retreating glaciers.

outwash—sediment deposited by streams draining from a melting glacier. Valleys downstream of a melting glacier typically have an outwash plain formed by sediment-choked meltwaters depositing sediments along migrating braided-stream channels.

loess—a tan, buff to gray windblown deposit of fine-grained, loamy, calcareous silt or clay; fine-grained deposits typically derived from glacial outwash plains or dust derived from arid regions.
Hudson River fjord glacial landscape features
Fig. 12-21. Hudson River fjord at Bear Mountain, New York Fig. 12-22. Features associated with continental glaciation deposits.
Glacial striations glacial erratic
Fig. 12-23. Glacial striations and grooves in bedrock,
New York City, New York
Fig. 12-24. Glacial erratic
Jenny Jump State Park, New Jersey
Glacial till and outwash Glacial moraine
Fig. 12-25. Glacial till and outwash exposed at Caumsett State Park
Long Island, New York
Fig. 12-26. Glacial moraine at Montauk Point is part of the terminal moraine on
Long Island, New York

Sea level changes caused my melting continental glaciers since the end of the Pleistocene

When continental glaciers form, vast quantities of water migrate from the ocean basins to become ice on land. These continental ice sheets (such on Greenland and Antarctica) can "create their own weather" piling ice up to many thousands to several miles thick in some places. The formation of continental glaciers causes sea level to fall, exposing large regions that are now submerged on the continental shelves. Likewise, when all the ice melts, sea level rises, flooding all the worlds low-lying regions and coastal plains. After the peak of the last glaciation cycle about 18,000 years ago (the Wisconsin glaciation) sea level has risen about 350 to 400 feet (Figure 12-23). The rate of sea-level rise drastically slowed down about 3,000 years ago, but is starting to rise again largely due to the impacts of human activity on the global environment. Greenland's ice is melting rapidly. If the world became ice-free (meaning all the ice in Greenland and Antarctica were to melt) sea level would rise as much as 200 feet above it's current level, but hopefully, that won't happen! Sea level has risen about 1 foot in the last century, but the rate is increasing.

What does this mean? The extensive rise in sea level had largely erased or hidden perhaps most of the prehistoric knowledge of our species, homo sapiens. For instance, in the United States today, roughly 40% of the population lives in coastal areas (within about 50 miles of the ocean). For the rest of the world, the 40% number also applies. It is easy to assume that in ancient times people liked living near the beach just as much as they do today. This means, that areas that were once habitable on the coastal plain thousands of years ago, are now submerged on the continental shelves. If the trend continues, in the future, the populations that currently live in low-lying coastal regions will also be displaced, and large regions now used for agriculture and wildlife habitat will be submerged under rising sea level (an eventual "Goodbye" to places coastal places currently like South Florida, Houston, etc.!).
Sea level rise during the Holocene Epoch
Fig. 12-27. Sea level change following the end of the last glaciation. Sea level has risen as much as 120 meters (400 feet).
Flandrian Transgression—the name applied to the global rise of sea level of about 400 feet (120 m) caused by the melting of continental glaciers since the peak of the last Ice Age (Wisconsin Stage) about 18,000 years ago (Figure 12-27). Areas that were coastal plains are now submerged on the continental shelves around the world (Figures 12-28 to 12-30). Coastal plain and continental shelf Continental shelves Bearing Land Bridge
glacial rebound—ongoing regional isostatic uplift caused by the melting of continental glaciers (unloading as much as 5 miles of ice from the continental landmass). Regions like New York City and Boston are actually rising due to glacial rebound. Fig. 12-28. Coastal plain and continental shelf are exposed or submerged with sea level changes. Fig. 12-29. Continental shelves were exposed during the peak of the ice ages. Lower sea level allowed species to spread to other locations. Fig. 12-30. The Bering Straight was exposed during the peak of the ice ages allowing humans to migrate from Siberia to Alaska and throughout the Americas.
Elevated marine terraces on the California coastline.

California preserves much evidence of geologic, geographic, and climatic changes caused by ice ages. During the last ice age, alpine glaciers and ice caps covered upland regions in the Sierra Nevada Range and Cascades volcanoes, but lower elevations were ice free (Figure 12-31). The formation of continental glaciers in North America and Europe caused sea level to fall almost 400 feet, causing the shoreline to migrate seaward as much as 10 to 70 miles westward of the current coastline in some locations. This rise and fall of sea level happened with each glaciation cycle (of which there were many through the ice ages of the Pleistocene Epoch). In places where the California coastline is slowly rising, each of the major glaciation cycles is preserved as a step-like bench, called a marine terrace. The formation of marine terraces is illustrated in Figure 12-32. Examples in northern and southern California re illustrated in Figures 12-33 and 12-34.
California glaciation
Fig. 12-31. California at the peak of the last ice age. Glaciers covered the higher mountains, lakes filled inland valleys, and a coastal plain was extended offshore.
Formation of marine terraces
Fig. 12-32. Formation of marine terraces. This example shows the formation of two terraces. At least seven major terrace levels are preserved in some areas along the California coast.
Davenport marine terraces
Fig. 12-33. Marine terraces at Davenport, California
Marine terraces on San Clement Island, CaliforniaFig. 12-34. Step-like marine terraces on San Clement Island located offshore in southern California

Great Lakes in North America (Present & Past)

As the great continental glaciers advanced southward out of Canada, they soured the landscape, eroding away soft rock formation under the moving ice. After the glaciers melted scoured areas filled with water forming the Great Lakes in the mid continent region (Figure 12-35) and the Finger Lakes of upstate New York (Figure 12-36).

During the glaciation periods the western United States was much colder and wetter than it is today. In California, the increased regional precipitation and cooler weather conditions allowed great lakes to form filled lowland valleys regions. Ancient Lake Manix and other lakes covered parts of In the Mojave desert region near Barstow) and Ancient Lake Manly filled Death Valley. Forests (oak and mixed conifers) and shrublands probably covered many areas that are now deserts and glasslands.

The Pleistocene megafauna of North America included saber-toothed cats, the American lion, elephant-like mammoths and mastodons, giant sloths, short-faced bears, dire wolves, varieties of horses, camels, lamas, bison, moose, musk ox, giant tortoises, giant beaver, giant armadillo, and many other species of mammals and birds that are now extinct. Today, the last remaining large land from the Pleistocene is the American bison. These large animals survived the climate changes of the earlier glaciation periods of the Pleistocene Epoch were all driven to extinction at the close of the last ice age with the arrival and proliferation of our species in the region—Homo sapiens (and the vermin and diseases that followed them).

Great Lakes
Fig. 12-35. The Great Lakes were carved by the Laurentide Glaciers.
Finger Lakes, New York
Fig. 12-36. Satellite view of the Finger Lakes, northern New York.
Great Basin Lakes Fig. 12-37. Lakes filled the valleys of the Great Basin during the ice ages. The modern Great Salt Lake of Utah is a tiny remnant of the extensive network of lakes that existed during the peak of the wet glaciation periods.

Great Floods of the Pleistocene

As great glaciers moved across the landscape the ice and moraine sediments blocked valleys and great lakes (larger than any that exist today) formed along the margins of the advancing ice sheets. When the glaciers started to melt, blocked drainages opened up, and massive floods occurred along with spikes in sea level rise.

Before the continental glaciation cycles began, an ancient large river called the Teays River drained the Midwestern United States. It drained across the region of northern Ohio, Indiana, and Illinois before draining into the Mississippi Embayment (near Cairo, Illinois). None of the Great Lakes existed yet. As the glaciers advanced southward, river drainages were blocked, large lakes formed and eventually flooded over into adjacent valleys, forming the regional drainage system that exists today. The modern Ohio and Missouri Rivers basically follow the southern boundary where the continental glaciers ended.

Ancient Lake Missoula was an glacier-dammed lake that filled, broke out, and flooded portions of the Columbia Plateau several times between 15,000 and 13,000 years ago (near the end of the Wisconsin Stage). The "Spokane Floods" resulted in the formation of the Channeled Scablands. The Channeled Scablands are a water-scoured network of ancient flood-carved channels that cover large areas in central Washington. The the channels are passages of old flow channels essentially barren of soil and regolith.
Teays River system Ohio River system after glaciation (today)
Fig. 12-38. Ancient Teays River system before glaciation began. Fig. 12-39. The modern Ohio and Mississippi Rivers system after the Pleistocene.
Channeled Scablands Great Falls of central Washington
Fig. 12-40. The Channeled Scablands of central Washington are areas where soil was stripped away by the massive Spokane Floods. Fig. 12-41. Great Falls, central Washington is an ancient massive waterfall system formed by the Spokane Floods.

Changing landscapes of the
New York City Region

During the peak of the last glaciation cycle, all of New York State (including NYC) was under glacial ice. Two final glacial advances and retreats left large quantities of glacial deposits that make up the surficial deposits of Long Island (Figure 12-42). At the peak of the last glacial advances, sea level was much lower and the glaciers ended along the exposed continental shelf. As the glaciers retreated (melted) sea level rose, and large lakes formed in the drainage regions blocked by terminal glacial moraines.

The ice in NYC may have been a thousand feet thick and much thicker farther to the north. The weight of the ice caused the North American continental crust to sink under its weigh. When the ice melted, the land rose due to isostatic adjustments to the crust. Isostatic uplift (called glacial rebound) is still happening throughout the region. Sea level is also rising, causing major changes to the coastline over time.

See more about NYC regional geology

NYC glacial features New York City glaciation NYC Lakes
Fig. 12-42. Pleistocene glacial deposits in New York City region Fig. 12-43. Glaciation and retreat in New York City area Fig. 12-44. Glacial lakes at the end of the last ice age in New York City
NYC Wisconsin Stage New York City, Early Holocene New York City today
Fig. 12-45. New York City region near the end of the last glaciation Fig. 12-46. Early Holocene landscape in New York City region Fig. 12-47. Modern bathymetry and topography of the New York City region
Glossary of Glacier Terminology:
A glossary providing the vocabulary necessary to understand the modern glacier environment.

Chapter 12 Quiz Questions