Glaciers, Ice, and Permafrost—Yosemite, Glacier, and Bear Meadows

Although not quite as large as minivans, musk oxen have better cornering and can surely accelerate rapidly. This picture shows musk oxen thundering across the tundra of east Greenland. If you had been in central Pennsylvania's Bear Meadows with a camera 20,000 years ago, you might have taken this picture--tundra and musk oxen very similar to these existed in Pennsylvania and adjacent states back then, while an ice sheet even bigger than modern Greenland's loomed just to the north. How do we know that the ice came and went, and what caused the changes? Look both ways for musk oxen or minivans, depending on where and when you are, and let's go see.

When we try to pick out anything by itself, we find it hitched to everything else in the Universe.

Learning Objectives

• Recognize that glaciers behave in predictable ways because of physical processes that are shared with other materials, including pancake batter spreading on a waffle iron.
• Understand how glaciers uniquely make certain features that are found today in many places without glaciers, suggesting that ice ages with larger glaciers occurred in the past.
• Discuss how this ice-age hypothesis leads to many predictions that have been tested and found to be correct, giving very strong confidence that ice ages did occur, and helping us understand how and why the ice ages occurred.

What to do for Module 7?

You will have one week to complete Module 7. See the course calendar for specific due dates.

• Take the RockOn #7 Quiz
• Take the StudentsSpeak #8 Survey
• Submit Exercise #3
• Begin working on Exercise #4

Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.

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Overview of the main topics you will encounter in Module 7.

Ice Is Nice: Yosemite, Glacier, and Rocky Mountain National Parks, Bear Meadows, and Greenland

• All piles tend to spread under their weight.
• A glacier is a pile of ice and snow that forms where snowfall exceeds melting long enough to make a pile that is big enough to spread under its own weight.
• A glacier takes water (as ice) and sediment from the accumulation zone (where snow accumulates faster than it melts) to the ablation zone (where melting, also called ablation, exceeds snow accumulation) or to calve icebergs that float away in surrounding oceans or lakes to melt elsewhere.
• A glacier flows in the downhill direction of its upper surface (where ice meets air), even if that means the bottom flows uphill like pancake batter spreading across the bumps on a waffle iron.

Which Way Did It Flow?

• A glacier moves by deformation within the ice, and if the bed is warmed to the melting point, by sliding over the glacier's bed or deforming the sediment there.
• Most deformation in the ice of a glacier happens near the bottom where stresses are highest, but the top of a glacier moves fastest because it rides along on the deeper layers.
• The ice of a glacier deforms under stress because the ice is almost hot enough to melt, even though the ice feels cold to us.

Glacier Tracks

• Glaciers erode by plucking rocks loose, sand-papering (abrading) the bed, and through the actions of subglacial streams that occur beneath some glaciers.
• Glaciers with thawed beds, especially those with surface meltwater reaching the bed, change the landscape more rapidly than is typical for landscapes shaped by non-glacial streams, wind, or mass movement.
• Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock.
• Abrasion smooths the up-glacier sides of bedrock bumps, while the glacier plucks blocks loose from the down-glacier sides of bumps.
• Glaciers erode valleys to give them “U”-shaped cross-sections, often with the floors of side valleys flowing into the main valley left "hanging" above the floor of the main valley; streams make “V”-shaped valleys without hanging valleys.
• Glaciers gnaw bowls called cirques into mountains.
• Glaciers deposit till, which contains rock pieces of all different sizes.
• Melting glacier ice also feeds streams that deposit outwash (because it is washed out of the glacier); the pieces in outwash are sorted by size (mostly sand in some places and mostly gravel in other places). Till and outwash often form ridges called moraines that outline the glacier.

Evidence of Ice Ages

• Some of the features that glaciers erode and deposit are not produced in other ways, and such features from a time interval centered on roughly 20,000 years ago are observed now across broad areas of the Earth where glaciers do not now occur, suggesting that we have had an ice age or ice ages in the past.
• This hypothesis of past ice ages has led to many predictions that were then confirmed, including that the land now should be rising where the ice was, and sinking just beyond where the ice was, and that the return of water to the oceans as the ice melted would have flooded old river valleys and killed shallow-water corals on the sides of islands.
• The success of the ice-age hypothesis in predicting things that have since been observed, and the failure of other hypotheses to do so, give us high confidence that ice ages did occur.

How Many Ages of Ice? An Ocean of Clues

• The history of ice ages is clearer in ocean sediments than on land, especially in the isotopic composition of shells buried in mud on the sea floor.
• Isotopically lighter water evaporates more easily.
• An ice sheet is formed from water that evaporated, mainly from the ocean, and then snowed, so during an ice age the water remaining in the ocean is isotopically heavier than observed today, causing shells that grew then to be isotopically heavier.
• The history of the isotopic composition of shells recovered from cores of mud from the ocean floor shows that the ice grew and shrank, with the biggest ice every 100,000 years, and smaller wiggles in ice size about 41,000 and 19,000 years apart.
• These timings were predicted by the scientist Milankovitch decades before they were observed because they are the timings of features in Earth's orbit that control the distribution of sunshine on the planet.
• Ice has grown globally when the far north was getting relatively little sunshine, especially in midsummer.
• The rest of the world has cooled when ice grew in the north, even though parts of the world were getting extra sunshine, and the world has warmed when the ice melted in the north, even when some places were getting less sunshine.
• This seemingly bizarre behavior occurred because the changing ice and other features of the climate changed atmospheric CO₂, which rose when the ice melted and fell when ice grew, and the CO₂ controlled global temperatures.

Bear Meadows

• Ice sheets today cover about 10% of the land area; at the height of the ice age the ice sheets covered about 30% of modern land; central PA was just beyond the edge of ice that grew in Canada and flowed into the USA.
• The high parts of Rocky Mountain National Park and the coastal parts of the NE Greenland National Parks are among the places that have permafrost today—soil at some depth is frozen year-round.
• Rocks in permafrost regions are especially affected by freeze-thaw processes that break rocks, and by downhill soil creep (during the summer, the meltwater can't drain downward through the frozen soil beneath, so the soil gets very soggy and creeps easily).
• These characteristics of permafrost regions contribute to the formation of distinctive features, which are observed in places such as central Pennsylvania that were just south of the ice-age ice sheets but which are not forming there today.
• This shows that central Pennsylvania, and other such regions just south of the ice-age ice sheets, were very cold during the ice age.

What Glaciers Do, Erosion and Yosemite

Left: Map of the U.S. with Yosemite National Park Location marked in California. Right: Bridalveil Fall and Half Dome, Yosemite National Park, California

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

When your tour guide, Dr. Alley, was much younger (the year he graduated from high school, 1976), he traveled with his sister Sharon and cousin Chuck on a camping tour of the great national parks of the American West (in Chuck’s car, a 1962 Ford Galaxy 500). At Yosemite, they hiked the 10-mile round trip to Glacier Point, climbing almost 1 km (0.6 miles) in elevation. The trail switch-backs up the granite cliffs, opening increasingly spectacular panoramas across the great valley of the Merced River. The view from Glacier Point, across the side of Half Dome and the thundering Vernal and Nevada Falls, is truly spectacular. It was here that John Muir helped convince President Theodore Roosevelt of the need for a National Park Service to care for the National Parks, which were protected by law but not by rangers for some decades after the parks were established.

The hikers were a bit disheartened by the crowd at Glacier Point—the view is also accessible by Glacier Point Road. While they sat and lunched, a tour bus pulled in. Most of the passengers headed for the gift shop, but three settled at a picnic table while a fourth strolled over to the railing to see the scenery for a few moments before joining the others at the picnic table. One of the three asked, “Anything out there?” To which the ‘energetic’ fourth replied “Nah, just a bunch of rocks. Let’s go check out the gift shop.” It must be a sad person indeed who would not walk 50 feet to see the glory of Yosemite.

To anyone with open eyes, Yosemite Valley—the “Incomparable Valley”—is well worth inspection. It is carved into the granites and similar rocks of the high Sierra Nevada of California. Once, this granite was magma (melted rock below the surface), far beneath an earlier mountain range. The magma may have fed subduction-zone volcanoes much like those of the Cascades, which continue to the north of the Sierra. However, stratovolcanoes along this part of California have died as the East Pacific Rise spreading center ran into the trench along the west coast, forming the San Andreas Fault but ending subduction, as you learned earlier in the course. Such a fate eventually awaits the Cascades volcanoes, some millions of years in the future.

The Sierra Nevada was raised above Death Valley and the rest of the Great Basin by motion across great faults. Earthquakes that continue to occur, and breaks in recent sediments caused by earthquake faults, show that the mountain range is still being lifted above the still-dropping Great Basin.

The tough granite of the Sierra Nevada is more resistant to weathering and erosion than most rocks, however, granite does eventually break down, and some streams have managed to exploit weaknesses and cut deep channels through the range. These streams include the Tuolumne River, which carved the mighty Hetch Hetchy Valley, now dammed so that a valley that rivals Yosemite is lost underwater. The Merced River, which runs through Yosemite Valley, also cut into the range.

The stage was then set for the ice ages. Glaciers gathered on the high peaks, flowed into the valleys, and began to change the landscape. Later in the course, we will briefly discuss why the climate changed naturally to bring the ice ages, and why knowledge of these natural changes fully confirms our scientific understanding that human fossil-fuel burning is warming the climate today. For now, we’ll look at what a glacier is, what it does, and how we know glaciers were much more widespread during the ice ages than they are today.

Take a Tour of Yosemite National Park

Glaciers and Pancakes (4:26)

A glacier is a mass of snow or ice that deforms and moves. Glaciers form wherever snowfall exceeds melting over enough years to make a pile big enough to flow. In places with extremely high snowfall, this can occur where average temperatures are near or even slightly above freezing, such as on the mountains of the Olympic Peninsula. In dry places, glaciers may be absent even if average temperatures are well below freezing. Such places have frozen ground instead, called permafrost (because the frost is permanent).

A glacier is a mass of snow or ice that deforms and moves. Glaciers form wherever snowfall exceeds melting over enough years to make a pile big enough to flow. In places with extremely high snowfall, this can occur where average temperatures are near or even slightly above freezing, such as on the mountains of the Olympic Peninsula. In places with very little precipitation, glaciers may be absent even if average temperatures are well below freezing. Such places have frozen ground instead, called permafrost (because the frost is permanent). We will spend a little while looking at how glaciers flow, in part because glaciers are important, and in part, because they reveal processes that are important in many other settings—pancakes, landslides, swimming pools, huge cathedrals, and glaciers, and many other things have some behaviors in common.

If you pour water onto a tabletop to make a pile, the water will spread out across the table and eventually drip off onto the floor. Pour pancake batter onto a griddle, and the pile will spread, although the spreading will be slower and the pile will be thicker than for water. All piles tend to spread under their own weight, because the pressure at the bottom of the pile is higher than the pressure outside the pile, giving a net push outward. Spreading may be avoided if the pile is strong enough—a hot griddle cooks the pancake batter, making it stronger and stopping the spreading. Most buildings are strong enough to resist spreading, too. However, builders of early cathedrals faced the problem that the roofs tended to cave in as the walls bulged out because the “pile” of the cathedral was spreading, requiring the invention of flying buttresses to support the walls and prevent pile-spreading collapses.

The diagram illustrates the spreading of a pile, with water or ice or pancake batter moving away from where the upper surface is highest. This occurs because the pressure at point A (the weight of the material above point A) is larger than at point B because there is more ice above A than above B. The higher pressure at A gives a net push from A to B. Thicker or steeper piles give larger pushes, and tend to spread faster. Typically for glaciers, ice thicker than about 50 m (150 feet) will deform and flow fast enough to be easily measured, making a glacier.

If you make a pile of pancake batter on a waffle iron, rather than on a flat griddle, some of the batter may flow along the low grooves and then move up to cover the bumps, but the flow will still move away from the place where the upper surface of the pile is highest. In the same way, ice can flow up a hill in the bedrock if the flow is going in the “down” direction of the upper surface. (If you want more detail on this, just for fun or some other class, we recommend the text The Physics of Glaciers, by brilliant Penn State grad Kurt Cuffey, which is available at the Penn State library and in many other libraries) For example, farmers in northeastern Pennsylvania grow food in soils made of pieces that were brought across Lake Ontario and New York by the glaciers. The bottom of that glacier climbed out of the low spot that now is the lake basin, driven by the upper surface of the ice sloping down from Canada to the U.S.

If you have ever slipped on the ice, you know how slippery ice can be, and you won’t be surprised by the second part of the figure. Where glaciers are thawed at the bottom, they generally slide over the rocks or soil beneath (shown in the figure), and if the material is loose soil, it often deforms in a sort of slow landslide that lets the glacier go even faster. And, all glaciers deform internally, like your slow pancake batter spreading on the griddle. A vertical hole drilled in a glacier will deform as shown in the figure. The stresses are the largest, causing the most intense deformation (the permanent bending of the hole shown in the figure), in the deepest ice. The upper ice rides along on the deeper ice, so the velocity is the fastest at the top of the ice.

Recall that rivers adjust to move sediment and water from one place to another. So do glaciers. Frozen water is supplied where snowfall exceeds melting in the accumulation zone. The frozen water flows to where melting exceeds snowfall, called the ablation zone, or else flows to where icebergs break off (called calving) and drift away to melt elsewhere. For ice sheets covering continents or for smaller ice caps covering plateaus or mountain tops, the ice forms a dome and spreads out in all directions. For glaciers on the sides of mountains, the ice flows down the mountain.

When melting decreases or snowfall increases, a glacier generally thickens and advances—its terminus, where it ends on land or calves icebergs, moves over land or water that did not have ice before. When melting increases or snowfall decreases, the terminus retreats and the glacier gets smaller. Notice that ice almost always continues flowing in the same direction, from the accumulation zone through the ablation zone to the terminus, whether the glacier is advancing or retreating.

Some people find it strange that we can walk on glaciers (being careful not to fall into crevasses!) and even land airplanes on glaciers—which clearly are solid—yet the glaciers flow. We have met something similar before, though. Like the soft rock of the asthenosphere down in the mantle, or the soft chocolate bar in a hot pocket, or the red-hot horseshoe in the blacksmith’s shop, ice in all glaciers on Earth is nearly warm enough to melt, and so can flow slowly. As a general rule, materials heated more than halfway from the coldest possible temperature - absolute zero - to their melting point can flow slowly, and flow becomes easier as the temperature increases closer to the melting point. For ice, the coldest yearly average temperature on Earth is about eight-tenths of the way from absolute zero to the melting point, so ice at the Earth’s surface is always “hot” and can flow. For more on this, and on the occurrence of crevasses as well as flow, see the Enrichment.

A glacier frozen to the rock beneath does not erode much. However, thawed-bed glaciers, especially those with surface meltwater streams draining to their beds through holes (something like cave passages, although formed in different ways), can erode even more rapidly than streams or wind erode, creating features made only by glaciers. Consider for a moment the Great Lakes of the U.S. and Canada - these lakes were carved by glaciers. The bedrock beneath Lakes Superior and Michigan is well below sea level, and was carved by glaciers, not rivers! Today, rivers carry sediment into the Great Lakes, slowly filling them up. We will see later that over the last million years, times when the glaciers were eroding have alternated with times when streams were filling the lakes back up with sediment, and the streams have had more filling-up time than the glaciers had eroding time. And yet, there are the lakes, not the least bit full of sediment. The glaciers have been much better at their “job” than the streams. The same can be said for many other places. It is not too extreme to say that the regions that had glaciers 20,000 years ago and are free of ice today still preserve a glacial landscape.

Ice moving over bedrock “plucks” smaller rocks free, then uses those rocks to abrade or “sandpaper” the bedrock, scratching and polishing it. As ice flows over a bedrock bump, the side the ice reaches first is abraded smooth while the other side is plucked rough, as you can see so clearly on slide 15 of the Yosemite VTrip earlier in this module. Subglacial streams sweep away the loose pieces and may cut into the rock.

Diagram showing a small V-shaped stream valley flowing into the side of a larger V-shaped stream valley. The "more sloping waterfall" is just rapids for white-water kayaking, perhaps, but not a waterfall.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Plucked and abraded rocks show clearly that glaciers were present, but so do big features, such as Yosemite Valley. The steep walls and nearly flat bottom of the valley make a characteristic “U” shape. A river without a glacier tends to cut downward and then mass movement processes remove material from the walls, giving a "V' shape. (Where a V-shaped stream enters an ocean to make a delta, the outward and upward growth of the delta over time may eventually fill the bottom of the V with mud to make a flood plain, as we saw with the Mississippi, but initially, when a stream is cutting down, it tends to make a V.) However, glaciers are quite wide and can erode across a broad region, giving the classic "U" shape.

Yosemite is famous for its waterfalls from “hanging valleys” far up in the cliffs, another sign of recent work by glaciers. With streams, steeper ones generally erode faster. If a main river cuts down rapidly, its tributary streams become steeper and cut downward faster. In this way, even a small side stream can “keep up” with the erosion by the main stream so stream erosion generally produces "rapids" rather than waterfalls. Glaciers are different. A main glacier often fills its valley, burying most or all of the rock. The ice from a side glacier then does not drop steeply down into the main glacier. For various reasons, the main glacier tends to erode faster than the side glaciers. When the ice melts, a “hanging valley” is left behind — a small stream that replaces the small side glacier must plunge over a glacially carved cliff and then flow across the bottom of the “U”-shaped valley to reach the main stream. Eventually, the side stream will wear away the waterfall. But today in Yosemite, numerous streams emerge from small “U”-shaped hanging valleys to cascade down the glacially carved cliffs—the landscape is pretty much what the glaciers left. (Piles of rocks at the bottoms of waterfalls show that the streams are indeed changing things, but slowly.)

Diagram showing the effect of glaciation on the river valleys illustrated in the previous figure. The glaciers broaden the V-shaped river valleys to a U-shape, and the tributary valley now ends high above the main valley floor, feeding a large waterfall.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Glaciers make many other erosional features. At the head of a glacier, freezing and thawing can break rocks, and the loose pieces can be hauled away by the glacier, carving a bowl into the side of a mountain. If bowls chew into a mountain from opposite sides until they meet, a knife-edged ridge is left—such as the Garden Wall of the continental divide in Glacier National Park, which we’ll meet soon. Where three or more bowls intersect from different sides, a pinnacle of rock is left, such as the Matterhorn of Switzerland. Mountaineers have dubbed the bowls cirques, the ridges aretes, and the pillars horns, and geologists continue to use these terms.

Bridalveil Falls, Yosemite National Park. The waterfall comes from a hanging valley into the main valley. Rocks are piling up at the bottom of the waterfall, making the rapids seen in the lower-left part of the picture.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Glaciers also leave distinctive deposits. Streams, waves and wind all sort rocks by size, leaving too-big ones behind and carrying away smaller ones, eventually depositing sediments dominated by a single grain size such as sand. Glaciers don’t care how big the rocks are that the ice carries, so a deposit put down directly from the ice may have the tiniest clay particles mixed in with house-sized or bigger boulders. Such a deposit is called a glacial till. Till plus glacial outwash (sediment washed out of a glacier by meltwater) may be piled up together in a ridge that outlines the glacier, called a moraine.

Pennsylvania has a Moraine State Park, which features glacial moraines. Cape Cod is a moraine, and a moraine is draped across Long Island, showing some of the places where glaciers from the ice age ended.

Left: Map of the U.S. with the location of Glacier National Park marked in Montana. Right: View of Hidden Lake from near Logan Pass, Glacier National Park, Montana

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Glacier National Park is the southern part of the Glacier-Waterton Lakes International Peace Park, extending north-south across the Canadian-U.S. border and east-west across the great Front Range of the Rockies. Glacier is home to wolves and grizzly bears, mountain goats balanced on cliffs, moose munching on water plants, and beautiful flowers such as beargrass and avalanche lilies. Going-to-the-Sun Road winds past Going-to-the-Sun Mountain, among the best-named features of the park system. The Garden Wall is a knife-edge ridge left as glaciers gnawed into the backbone of the continent from the east and the west, and in many places is the continental divide. Long, narrow lakes lie along the valleys, which sometimes host lines of lakes strung like beads on the string of the connecting river. (Such glacier-carved strings of lakes are called paternoster lakes, after a resemblance to the beads of a Catholic rosary.)

Glacier National Park had roughly 80 active glaciers in 1850, dropping to perhaps 25 in 2022. There is a little uncertainty related to exactly how big and active a mass of ice must be, to be called a glacier. Many of the former glaciers may be essentially dead now, as human-caused warming melts many away (see the changes shown by the older and more recent photos below; again, we will come back later to how we know that the changes are human-caused). Glacier National Park is now more noted for the tracks of past glaciers than for the activity of present ones. But, we suspect that “Ex-Glacier National Park” would not have made the Great Northern Railway happy when they were promoting tourism in Glacier National Park (via the Great Northern Railway, of course). When the last glacier has melted away, perhaps within a few decades, there are no plans to change the park's name.

Historical photos from the United States Geological Survey archives, show Grinnell Glacier as it looked in 1910 (top), and again in 1997 (bottom) after much of the ice had melted away. Technically, the bottom picture shows Salamander Glacier; the original Grinnell Glacier split into two parts as it melted, with the upper one visible as Salamander, and the lower one, still called Grinnell, no longer visible from this vantage point.

Credit: USGS

Sperry Glacier in about 1930 and 2008. Repeating this photo from the same photo point was impossible since the historic photo was shot from the elevated perspective of the glacier’s surface.

Credit: Sperry Glacier: circa 1930, MJ Elrod, U of M Library and 9/17/2008, L McKeon from United States Geological Survey (USGS) (Public Domain)

Evidence of Ice Ages

Today, permanent ice covers about one-tenth of the land on Earth, mostly in Antarctica and Greenland, with a little ice in mountainous regions. We saw at Yosemite that glacier erosion and deposition produce features that differ from those produced by mass movement, rivers, wind, or coasts. Geologically recent examples of those features, produced by glaciers, from roughly 20,000 years ago, are spread across almost one-third of the modern land surface—in places such as Wisconsin, northern Pennsylvania, Yosemite, and Glacier National Parks, and many others, the mark of the ice is unmistakable. The 10,000 lakes of Minnesota, the Great Lakes, the gentle moraines of Illinois, and many more features reveal a glacially-dominated landscape. Such features in Europe first motivated the hypothesis that ice ages have occurred.

This ice-age hypothesis makes many predictions, which allow testing. In times before modern geology, the glacial deposits were called “drift” because they were thought to have drifted into place in icebergs during Noah’s flood. Other people have suggested that the deposits were splashed into position by a giant meteorite that hit Hudson Bay, and other hypotheses have been advanced. But, the ice-age hypothesis makes predictions that differ from the Noah’s-flood hypothesis or the meteorite hypothesis in many ways, and the ice-age predictions are confirmed beautifully, while the others failed. (The biggest difference is that icebergs and meteorites do not make features that even vaguely resemble those actually observed, but let’s look at other differences.)

If huge ice existed, its great weight must have pushed down the land beneath—recall that the deep rocks are hot and soft, with a “water-bed” cover of stiffer rocks on top. If the ice age peaked only about 20,000 years ago, the slow flow of the soft, deep rocks should mean that the land where the ice once sat would still be rising after the melting, while the land around the former ice would be sinking as the soft, deep rocks return to their pre-ice-age positions. The global flood hypothesis and the meteorite hypothesis do not predict such a bulls-eye pattern of rising and sinking centered on the regions with features known to be made by glaciers today—the flood would have spread evenly across the land, and would not have concentrated its weight in one place. The sudden blast of the meteorite would not have left its weight long enough to push the slow-flowing deep rocks far. Measurements by GPS and other techniques show just the pattern expected from the ice-age hypothesis, a pattern not predicted and not explained by the other hypotheses. (We will show you some of the evidence for this, and for the flooded river valleys described just below, when we visit Acadia in our discussion of coasts in Module 8.)

The water for huge ice sheets would have been supplied by ocean evaporation, with the water getting stuck in the ice rather than returning rapidly to the sea in streams. Hence, if ice ages occurred recently, there should be evidence of lower sea levels when the ice was big. No such prediction comes from the meteorite or big-flood hypotheses (the meteorite might have made a wave but otherwise would not have affected sea level; the big flood would have raised sea level). Again, the ice-age prediction is borne out by the evidence, and the predictions of the other hypotheses are wrong. For example, some corals grow only in shallow waters with much sunlight. Dead samples of such corals from about 20,000 years ago can be found where they grew, down the sides of islands and now under more than 300 feet of ocean water. Other evidence also points to lower sea level in the recent past—geologic evidence shows that the Chesapeake Bay, for example, is a river valley drowned by rising ocean waters.

Take a Tour of Glacier National Park

So, we have an immense amount of evidence that ice ages really occurred. But, on land, one advancing glacier might erode the record of an older one. A pile of four tills deposited by glaciers and separated by soils from warmer times may record four advances, or forty, with most of the record having been eroded away and only a few deposits remaining. To learn how many ice ages there were, we go to places in the deep oceans, where sediment has been piling up without erosion for millions of years. We can identify glaciations and warmer times using characteristics of the shells in those sediments, as we discuss next, and we can learn their ages.

Water in the oceans is not all the same—roughly one molecule in 500 has an extra neutron or two in one or more of the oxygen or hydrogen atoms. Such “heavy” water is still water but weighs a little extra. (If you don’t remember isotopes, go back and look at the introduction to chemistry near the start of the course.) Not surprisingly, light molecules evaporate more easily than heavy molecules. Water vapor, rain, and snow thus are slightly “lighter” than the ocean; that is, the ratio of light water molecules to heavy ones is larger in vapor, rain, and snow than in the ocean from which the vapor, rain, and snow came.

During an ice age, roughly 300 feet of this slightly light water evaporates from the ocean and piles up on land as gigantic ice sheets, leaving the oceans a bit heavier isotopically. When the ice age ends and the ice melts, that light water from the ice sheets is returned to the ocean, making the ocean isotopically lighter.

These changes are small - the ocean is water at all times! Rounding just a little, in the modern ocean 1 of each 500 water molecules is heavy, which is the same as saying that 1000 of each 500,000 water molecules are heavy. When the ice sheets were big, roughly 1001 of each 500,000 water molecules were heavy. This difference does not affect the water, but it is easy to measure with modern instruments.

Many plants and animals that grow in the ocean build shells of calcium carbonate (the stuff of limestone) or silica, both of which contain oxygen obtained from the water, and record the isotopic composition of the water. During times with more ice, the shells that grow and then fall to the sea floor have slightly heavier oxygen isotopes. As the shells pile up, they record a history of the ice volume on Earth with the youngest layers on top. With enough care, knowledge, and instrumentation, dedicated workers can obtain consistent, reproducible data that tell a wonderful, clear story. (There are a few additional details, but the main story is this simple.)

Amazingly, the story was predicted correctly decades before scientists gained the ability to test it, as we’ll see in the next section! The story is that, over the most recent 800,000 years, ice has generally grown for about 90,000 years, shrunk for 10,000 years, grown for 90,000 years, shrunk for 10,000 years, etc. Superimposed on this are smaller wiggles, with a spacing of about 19,000 years and 41,000 years. (As described in the Enrichment, the ice was growing and shrinking more than 800,000 years ago, and there have been times in Earth’s history with no ice on Earth, and other times when the Earth was completely ice-covered. We will revisit some of these issues later.)

Are Glaciers Really Changing?

During the 1920s and 1930s, a Serbian mathematician named Milutin Milankovitch, building on work by earlier scientists, calculated how the sunshine received at different places and seasons on the Earth has changed over a long time in response to features of Earth's orbit. As the sun, moon, Jupiter and other planets tug on the Earth, the orbit changes a bit. Earth wobbles with a 19,000-year periodicity, the pole tilts a little more and then a little less with a 41,000-year periodicity, and the orbit changes from nearly round to more squashed or elliptical and back with a 100,000-year periodicity. NASA animations of these are shown below. With modern computers, these changes are relatively easy to calculate for many millions of years; for Milankovitch, the calculation was the labor of a lifetime. (He did it very well, though, even correctly noting that the 19,000-year periodicity goes from 19,000 to 23,000 years and back, a pattern that is indeed observed in the data testing his prediction! Also note that the NASA animation for precession says that it is a 26,000-year cycle, which is correct, but it interacts with other cycles to cause variation in sunshine with periodicities of 19,000 to 23,000 years. The ability to calculate these variations accurately really is amazing.)

Video: Changes in Eccentricity (Orbit Shape) (0:07 seconds)

Video: Changes in Obliquity (Tilt) (0:01 second)

Video: Axial Precession (Wobble) (0:04 seconds)

These orbital wiggles have little effect on the total sunshine received by the planet, but they do move the sunshine from north to south and back, poles to the equator and back, or summer to winter and back in various ways. For example, today the Northern Hemisphere is farther from the sun in northern summer than in northern winter. (Remember that summer is controlled by the tilt of the planet’s spin axis relative to the plane in which the planet orbits, not by the distance from the sun!) In the few millennia centered 9000 years ago, the Northern Hemisphere had slightly warmer summers and cooler winters than recently because the Earth was closer to the sun during northern summers and farther from the sun during northern winters than today. (Note that this was reversed in the south.) The drop in summer sunshine in the north over the last 9000 years allowed mountain glaciers to slowly expand a little, culminating in the so-called "Little Ice Age" of the 1600s to 1800s; the strong melting of glaciers since then is mostly the result of human-caused warming. (We will discuss this later in the course.)

Summer in the Northern Hemisphere is most important in controlling ice ages, because the Northern Hemisphere is mostly land and can grow big ice sheets, but the Southern Hemisphere is mostly water, and already has ice on Antarctica, so can’t change its land ice much more. In the north, even during warm winters, the highlands around Hudson Bay are cold enough to have snow rather than rain. During times when features of the Earth's orbit gave reduced sunshine in the north, ice has survived summers and grown; increasing summer sunshine has melted the ice. The way the various cycles interacted led to larger or smaller changes, and thus to the ice ages we know.

You may guess that this is slightly oversimplified so far. For example, during times when Canada has received more summer sunshine, allowing its ice to melt, the Southern Hemisphere or the tropics often received less sunshine, yet they warmed too! How Canada told the glaciers of Patagonia and Antarctica to shrink was a great puzzle for a long time. The answer involves global warming from atmospheric carbon dioxide. The growth and shrinkage of the vast ice sheets, the changes in sea level, and other changes had the effect of shifting some carbon dioxide (CO₂) from the air into the deep ocean during ice ages and bringing the CO₂ back out into the air during times when the northern ice was melting. The orbits affected the ice, which affected currents and sea level, and plants and other things, which affected CO₂. But, as we will discuss later in the semester, CO₂ in the air warms the Earth's surface no matter how CO₂ gets into the air. And, the changing CO₂ explains why, when the ice was growing, places getting more sunshine still got colder, and why, when the ice was shrinking, places getting less sun still got warmer.

Climate records show many other types of changes. Very large, rapid changes have been caused by sudden surges of ice sheets, and by jumps in the way the ocean circulates. We do not understand these faster changes well enough to know whether they could happen again. We're cautiously optimistic that we will avoid crazy climate jumps, but we're more worried about Antarctic ice-sheet surges raising sea level. Naturally, the Earth’s orbit right now is in an intermediate state, orbital changes are causing almost no change in the climate, and we should be looking forward to another 50,000 years or more with little change in the climate from orbits before we begin the natural slide into a new ice age. (See the Enrichment for a little more on this.) However, humans almost certainly are now more important to the climate than are such slow changes, as we will see later, and we probably have already stopped the next natural ice age.

Permafrost and Periglaciation

Talus Slope, Trail Ridge Road, Rocky Mountain National Park (left) and Talus Slope, outside of State College, PA (right)

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Trial Ridge Road and Seven Mountains Locations

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Meanwhile, when the ice age had covered such places as today's New York, Chicago, Minneapolis, Seattle, and much of Europe, with thick ice, what was happening in central Pennsylvania and other areas around that ice? The evidence is clear that the ice ages cooled all or almost all of the Earth. And, some of that evidence comes from central Pennsylvania, which was a real frozen tundra.

If you climb the ridges of Central Pennsylvania, perhaps up in the Seven Mountains just southeast of Penn State's University Park campus - go up Bear Meadows Road past the ski area, for a start - you may notice several interesting things geologically. Beneath the Pennsylvania forest, the soils, streams, and hillslopes have more in common with the high meadows of Trail Ridge Road in Rocky Mountain National Park, or with the regions around the ice sheet in Greenland, than with the modern climate of University Park. Trail Ridge Road crosses tundra, where small, hardy plants grow atop permafrost. Although the uppermost soil along Trail Ridge Road thaws during the brief summers, and the deep Earth is thawed by the heat of the Earth, the materials between are frozen year-round in permanent frost. (Such areas are sometimes called “periglacial,” because they may occur around the perimeter of a glacier, but sometimes they are far from glaciers, so "permafrost" is the better name.)

The Bear Meadows National Natural Landmark, just over the ridge from Penn State’s University Park campus, was recognized by the National Park Service in 1966 as a site that “possesses exceptional value as an illustration of the nation’s natural heritage.” Although many guidebooks somehow have decided that Bear Meadows is 10,000 years old, the Meadows are much older, having formed during the most recent ice age, roughly 20,000 years ago (and possibly earlier). Here, take a walk just above the Meadows, and learn why Pennsylvania hikers, like those in the high country of the Rocky Mountains, are wise to wear sturdy shoes. Then, see what this has to do with the Formation of the Meadows — they really are related.

Video: The Formation of Bear Meadows (1:56 minutes)

Back in Module 5, we saw how freeze-thaw cycles can break rocks. We also saw how ice crystals can grow under rocks that have been broken free and push them upward, allowing smaller rocks to fall underneath and thus raising the bigger rocks toward the surface. This behavior is especially common in permafrost regions, causing them to have broken-up big rocks sitting on top of soil or smaller rocks.

Ice under rocks very near the surface melts in summer. In warm places, as we saw in Module 5, the water drains down through spaces in the ground, with some of the water eventually reaching rivers. But, in permafrost regions, the deeper spaces are clogged with still-frozen ice. The upper layers then become saturated with water, which can lubricate the downhill motion of the rocks on top of the soil. Permafrost regions thus commonly have blockfields extending downhill from ridges, with big rocks on top of soil, often with the blocks tipped up on edge and pointing downhill. Such a moving mass that reaches a valley may dam a small stream to make a wetland or small lake, and then the mass may turn and move downhill, filling the former stream valley with rocks much too big for the stream to wash away.

Features such as this are still developing today in parts of Alaska, around the ice sheet in Greenland, and on top of Trail Ridge Road in Rocky Mountain National Park, where the role of permafrost can be documented in detail. And, features such as this are spread across the mountains of central Pennsylvania and into surrounding states. Hikers on the Appalachian Trail, Mid-State Trail, and other trails in Pennsylvania are wise to wear sturdy shoes, and often complain bitterly about “Rocksylvania”, where hiking boots go to die.

The resistant sandstone layers that form the backbones of many ridges in Pennsylvania were broken by freeze-thaw cycles and produced blockfields that crept downhill and into the valleys during permafrost times of the ice ages, damming a small stream to make Bear Meadows, and giving the large blocks that fill other stream valleys. It takes a little investigation to find a road cut or other excavation cutting through one of these block fields, but if you do, you will see that the big sandstone blocks are on top, with smaller pieces of soil below, and then a different type of bedrock (commonly shale) below that. Note that the trees growing on top of the blockfields stand straight and tall, and roads bulldozed through the blockfields are not being buried or carried downhill—the motion largely or completely stopped when the ice age ended, waiting for the next occurrence of permafrost.

Permafrost produces many other features that are easily recognized. In nearly flat places, wintertime cooling often causes the ground to contract so much that it breaks into patterns, with summertime meltwater flowing into the cracks and filling them. Such ice wedges are still present in modern permafrost, and ancient ones can be recognized where they cut across other layers and then filled with things washed in as the ice melted. Such former ice wedges have been found in parts of Pennsylvania.

Patterned ground from Svalbard, Norway. Similar features have been found in Pennsylvania, but are not so easy to see.

Credit: Photo by Grzegorz Rachlewicz, Uniwersytet im, Adama Mickiewicza, Poznan, Poland, and reproduced from Williams, R.S., Jr., and Ferrigno, J.G., eds., 2012, State of the Earth’s cryosphere at the beginning of the 21st century–Glaciers, global snow cover, floating ice, and permafrost and periglacial environments: U.S. Geological Survey Professional Paper 1386–A via US Geological Survey (Public Domain).

Also on nearly flat places in permafrost regions, the freeze-thaw processes combined with such ice wedging may sort the larger and smaller rocks into patterns, and such patterns have been found in Pennsylvania. (Note that many of these features, such as those on Big Flat near Bear Meadows, were described by geologists during times when logging and fire had removed the thick vegetation; the features are hard to see and almost impossible to photograph today but can be found during careful bush-whacking.) But if you're not an expert walking in thick vegetation on uneven blockfields, we recommend you just take our word for it.

Bear Meadows even records the history of warming from the ice age. A core collected from the sediment in the bog has silt with little evidence of vegetation in the oldest layer at the bottom. Above that, pollen and other remains of cold-weather plants appear, dating to the first bit of warming from the ice age, followed by a progression to warmer-weather types and on to the modern, productive bog with its blueberries and bears and other interesting plants and animals. A nearly barren tundra of the Trail Ridge Road type, with a creeping permafrost lobe that dammed a stream, followed by warming, would have produced the sediments we see.

The conclusion is nearly inescapable—Trail Ridge Road in Rocky Mountain today is an excellent picture of what Pennsylvania looked like during the ice age. Permafrost is common across much of northern Alaska, Canada, and Siberia, around the coast of Greenland, and in high-altitude regions. Permafrost poses grave problems for construction — the heat of a building can melt permafrost beneath, causing uneven settling that breaks the building. Permafrost also records the climate changes that have come to central Pennsylvania and other regions.

Video: Rivers of Rocks and Permafrost (2:30 minutes)

Take a Tour of Bear Meadows

Much of the science we have covered so far in this course is based on measurements taken today or very recently. The slow motion of drifting continents really can be measured with various techniques such as GPS, landslides are obvious to people whose houses are carried away, and the silt on the teeth of anyone who foolishly drinks untreated water from many rivers will prove that sediment is being moved.

But, much of our science involves history. For example, humans were not around with GPS receivers and satellites when Africa crashed into North America to raise the Appalachian Mountains. We are increasingly moving into subjects that involve “historical” geology, and reconstructing the events before modern humans were measuring the motions and writing down the results. We have presented a little of the evidence documenting past ice ages in this Module, in part to get ready for historical parts in future Modules.

We have focused on the scientifically accepted answers, but consider how scientists got those answers. If you go up to Bear Meadows and look around carefully, you will see the blockfields of big rocks extending down from ridges, sitting on soil, and then shale bedrock.

Many hypotheses could explain this observation—space aliens dropped the big rocks, or bulldozers pushed the rocks into place; or, the rocks slid down in a catastrophic, fast-moving giant landslide; or, they came creeping down slowly in permafrost; or, … you could probably think of others. Each hypothesis leads to predictions. If a bulldozer pushed the big rocks into place, we should find the bulldozer tracks, and we should be able to trace back in historical records to who was driving the bulldozer, and why. The first settlers, who arrived before bulldozers were invented, should have found hillslopes free of big rocks. Landslides start with big falls or slumps from particular places, so a landslide should have a big scar at its head where the rocks started, whereas creep slowly collects rocks as they are worked loose and carries them along, lining them up as they go.

So, scientists have looked for evidence that supports or refutes each hypothesis. The early settlers complained about the big rocks, and old cabins were built on the big rocks, so the bulldozer hypothesis wouldn’t work. There is no evidence of a landslide scar anywhere at the heads of these features, despite evidence for lots of different “stripes” of big rocks extending downhill from a ridgetop source where the bedrock of the same type as the big rocks sticks out.

You could follow the earlier scientists and quickly come to the realization that the rocks look like soil-creep deposits extending down from the ridge crests; the predictions from the other hypotheses fail, but each of the predictions from the soil-creep hypothesis is supported by additional data collected for testing purposes.

You can also note that the material is not now creeping—roads and trails are not being slowly buried by big rocks today, the trees are not knocked over, etc. Tree roots hold many of the rocks in place and prevent motion. So geologists looked for a time in the past when tree roots were not holding the rocks in place. The geologists collected the additional information given above (and much more!) - the big rocks are on top of smaller rocks and soil, not on the bottom, the big rocks are often standing on the edge, the rocks show patterning of coarse and fine, etc. Other geologists were scanning the whole planet, laboring over centuries, and learning the conditions of creeping hillslopes in the tropics, the deserts, the temperate zones, and the poles. By talking to other geologists, reading the literature, and devoting careers to careful study, they learned that the things you can see today on the slopes of Central Pennsylvania resemble features of permafrost and not features of any other modern setting.

But, if you are correct and these are permafrost features, there should be other evidence of cold conditions in the past, at the time that these features were active. Taking a core from the bog showed that the bog started in a very cold time (the deepest pollen is from plants that today are found only on the tundra), and the bog seems to be dammed by one of the soil-flow lobes, linking the soil-flow lobes to the time of the tundra cold. (It's true, no one has used a backhoe to take the dam apart to look for buried space aliens, but we'll stick to vaguely plausible things here).

Next, scientists ask whether this makes sense. Scientists have tentatively concluded that the hillslopes of Pennsylvania recorded cold conditions at a particular time in the past. Is there a reason why cold should have been here at that time? Well, just to the north, glaciers were pushing up moraines at the same time. Astronomers making orbital calculations find that the high northern latitudes were receiving about 10% less sunshine than today during that glacial age. Climate modelers who test whether such a drop in sunshine would be sufficient to grow glaciers and make conditions very cold find that cold indeed makes sense, especially when the modelers include the effects of the drop in atmospheric CO₂ levels that was triggered by the change in the sunshine and that is recorded in ice-core bubbles from the time.

Now, a modern geologist who tells this “story”—Pennsylvania hikers risk twisting their ankles on permafrost deposits—actually has a lot more evidence than the little sketch provided here. For example, hypotheses often suggest new measurements that have never been made before but that can be used in further testing. Penn State researchers have even measured the concentrations of rare isotopes that are produced in the rocks by cosmic rays only very near the surface where abundant cosmic rays penetrate, showing the increase in erosion and transport caused by the onset of ice-age conditions. A vast amount of information collected by centuries of Earth scientists is combined in our modern understanding.

The most recent ice age may have ended, but there is still a lot of ice remaining in Antarctica and Greenland. Here’s a little more about the Antarctic ice, who lives around it, how it behaves, and why we might care. We’ll explore the warming effect of rising CO₂ in Module 12; for now, just note that we are raising CO₂ in the air, and it does have a warming influence, based on fundamental physics discovered in part by the Air Force for military applications.

Video: Snowflake: An ode to traditional folk song, "A Fox Went Out On A Chilly Night" (4:08 minutes)

Optional Enrichment Article - A little more about glaciers

Types of Glaciers

Glaciers occur in different forms and sizes, and you might occasionally find knowledgeable scientists who disagree about what to call a particular glacier. An ice sheet is huge - the size of a continent, or at least the world's largest island (Greenland) - and spreads in all directions. An ice cap or ice dome is a smaller version of an ice sheet, sitting on a mountain top or high plateau, and also spreading in all directions (or at least in several directions). Many glaciers flow down from some mountain peak and may be called mountain glaciers, or valley glaciers, or just glaciers. An outlet glacier is a fast-moving part near the edge of an ice sheet or ice cap, especially if it flows between rock walls; fast-flowing parts near the edges of an ice sheet or ice cap flowing between slower-flowing ice are called ice streams. And yes, there are cases where an ice sheet is drained by a fast flow with ice on one side and rock on the other. Classifications such as this help us talk about things but are not precise.

Flowing Solids and Hot Ice

Dr. Alley has spent months of his life living on the great ice sheets of Greenland and Antarctica. (And Dr. Anandakrishnan has spent a lot more time on the ice sheets than Dr. Alley has!) Eating and sleeping and working at -30º, it is hard to think of ice as being a hot material, but that is exactly what it is, as noted earlier in this module.

Recall that heat is the vibration of atoms or molecules in a material and that in most solids including ice, the atoms or molecules are arranged in regular, repeating patterns. Melting of ice occurs when the typical molecule vibrates fast and hard enough to break free from the bonds that tie it to its neighbors and escape from that regular arrangement. When a material is almost hot enough to melt, the atoms vibrate almost hard enough to break free from their neighbors and move around, so it is relatively easy with a little extra push to move a few molecules at a time past their neighbors. The gravitational stresses caused by the surface slope of a glacier supply that little extra push and the ice deforms. (This deformation is primarily by dislocation glide - something like moving a carpet by making a rumple on one side of the room and then slowly shoving that rumple to the other side of the room.) When a material is not nearly hot enough to melt, the molecules are not even close to vibrating hard enough to break free from their neighbors, a whole lot of extra push is required to move molecules, and moving even a few molecules at a time is very difficult. The material then deforms elastically, or it breaks, but it does not creep and deform permanently in the way that a glacier does.

Most people measure temperature on a scale that gives “nice” numbers (something between 0 and 100) for typical daytime temperatures, so talking about the temperature is easy for us. But, other temperature scales make more sense in physics. If you slow the vibrations of molecules by cooling them, you can imagine that there must be some temperature at which vibration stops because all the heat has been removed. We call that temperature “absolute zero” or just zero on an absolute temperature scale. (Yes, in a quantum world, the Heisenberg uncertainty principle means that the last tiny bit of vibration can’t be removed, but vibration is almost completely stopped at absolute zero.) If we set the zero on our temperature scale to this “absolute zero,” and then use degrees that have the same size as in the commonly used Celsius or Centigrade scale, we get the Kelvin scale. Ice melts at 273ºK and water boils at 373ºK; there are 100 degrees between melting and boiling in Kelvin, just as in Celsius. (The Rankine scale uses Fahrenheit-sized degrees and absolute zero as zero, with ice melting at 460ºR and water boiling at 640ºR, but almost nobody uses Rankine anymore, so you are welcome to forget you ever heard about it.)

As a general rule, little or no permanent deformation (creep) occurs when the temperature (in Kelvin or Rankine!) is less than about half the melting temperature, and creep occurs rather easily when temperatures exceed about three-quarters of the melting temperature. The coldest mean-annual temperature on Earth today is about eight-tenths of the melting temperature of ice (that is 217ºK, which is also -56ºC or -69ºF, in case you still like old-fashioned thermometers). Most ice is as close to melting as red-hot or even white-hot iron being worked by a blacksmith. This is why glaciers usually flow rather than break—although breaking is still possible where deformation is very fast and where the pressure is very low, producing crevasses. So, you may find wintertime ice to be uncomfortably cold, but as a material, it is still hot!

Glacier Erosion

Ice can be well below freezing, or at the freezing point. Anyone who has ever defrosted an old-style freezer knows that subfreezing ice built up on the walls of the freezer is VERY hard to remove, but warming it until it reaches the melting point allows the ice to suddenly move easily and slide off. If you are using a chisel or screwdriver to chip the ice away, the sudden motion when the contact with the freezer thaws may cause you to scratch or gouge the freezer. Glaciers that are frozen to rocks beneath them don’t slip over those rocks rapidly and don’t erode those rocks rapidly, but if enough geothermal heat or other heat is supplied to thaw the contact between glacier and rock, the ice slides and can erode rapidly.

Erosion by thawed-bed glaciers occurs mostly in one of three ways: plucking, abrasion, and subglacial streams. We'll describe them a little more here.

First, recall that ice is an unusual material—higher pressure lowers its melting point rather than raising it, opposite to most materials. Ice has a sort of tinker-toy or construction-set structure with a lot of empty space between the molecules, and squeezing ice tends to force molecules to move closer together, making denser water. Most materials have less space in the solid than ice does, and melting requires knocking molecules out of orderly arrangements in ways that take up more space, a change that is opposed by higher pressure.

If a glacier is sliding across a bump in its bed, ice will tend to melt on the up-glacier side of the bump where the pressure is higher. The meltwater will flow around the bump to the down-glacier side, where the lower pressure will allow the water to refreeze. The heat given up by the refreezing will be conducted back through the bump, to allow more melting. But, you may remember that melting and freezing can open cracks in a rock. So, a glacier sliding over its bed can work rocks loose, and then freeze those rocks onto its base, in a process known as plucking. (When water spreads over the bed of a glacier in the spring as melting on the surface starts to feed water downward, the friction with rock that holds the ice back becomes concentrated on smaller regions of the bed not lubricated by the water, and this stress concentration breaks rocks, helping to cause plucking.) And, sometimes basal ice picks up rocks, and those rocks get stuck for a while against the rock beneath and then break free in a little earthquake. The quake pulls ice away from the downstream sides of bumps, lowering the water pressure there, while high-pressure water persists in cracks and spaces in the thawed-bed rock beneath a glacier, allowing a sort of hydrofracking that breaks rocks. )

Once glacier ice contains rocks at the bottom, it is like sandpaper—it drags those rocks over other rocks, scratching and polishing and knocking loose smaller rocks. This process is called abrasion. If you examine rocks on the walls of Yosemite, many still retain a polished appearance with parallel scratches or striations, showing where abrasion was active. Bumps are smoothed and even polished on one side—the up-glacier side—but may be rough and jagged on the down-glacier side where rocks were plucked off of them.

The melting of glaciers can produce a lot of water. The toe of a fast-melting glacier may supply more water to streams than does a similar-sized region in the rainiest place on Earth. The glacier acts to collect snowfall from a big area and take the snow to melt in a much smaller area. Trees and grass do not grow on glaciers to use the melt water but they do grow on the ground to use rainfall. Glacier melt usually flows down holes in the glacier, called moulins, that often form at the bottoms of crevasses. (Some brave or foolhardy people like to go caving in moulins after they drain during the winter.) The moulins eventually reach the glacier bed, where they feed large, steep, fast-moving streams. These erode in the same ways as streams outside of glaciers. Glaciers with much meltwater usually cause erosion to be faster than in non-glaciated regions. Fluctuations in water pressure, as moulins fill with water during daytime melting and drain as melting slows at night, contribute to cracking rocks for plucking.

More on the History and Future of Ice Ages

As we will see later in the course, the climate has changed naturally. Some times far in the past were very hot—too hot for people to live in large parts of the Earth—with the heat primarily from naturally higher concentrations of atmospheric carbon dioxide. (Humans are raising carbon dioxide in the atmosphere now primarily by burning fossil fuels. We are raising carbon dioxide faster than almost all of the natural changes, and human decisions will control how much we raise carbon dioxide and thus how hot we make the climate). Times of very high temperatures had no ice even near the poles, and are sometimes called “hothouse climates”. (Melting all the ice on Earth would raise global sea levels a bit more than 200 feet (60 m).) Natural processes including the formation of fossil fuels have caused cooler times to occur as well, when ice existed near the poles; such times are sometimes called “icehouse climates”, even though most of the world did not have ice. During a few special times, ice spread to cover the whole Earth; these are called “Snowball Earth” events.

The ice has not been constant during icehouse climates, but has gotten bigger and smaller—the ice-age cycles discussed earlier. Recall that these have been paced by features of Earth’s orbit rearranging sunshine by location and season, with the effects made global by changing atmospheric carbon dioxide levels. We are still in an ice-house climate, with ice in Antarctica and Greenland. When this icehouse was established millions of years ago, the ice grew and shrank fairly rapidly, with spacings of 41,000 years between big-ice times especially common. Then, about 800,000 years ago, the behavior shifted (for reasons that are not fully understood, although we have some good hypotheses) to cooling and ice growth for roughly 90,000 years, followed by warming and ice melt for roughly 10,000 years, then repeating. The rate of cooling initially has been slow, so you may read about 10,000 years of warmth followed by cooling. Today, the northern hemisphere has been in the not-much-change/slight-cooling phase for almost 10,000 years already, and you might expect that we are ready to begin sliding into the next ice age. But, it isn't quite that simple.

The 100,000-year pacing of a 90,000-year-cooling/10,000-year-warming world is linked to the interaction of the different orbital cycles, but the 100,000-year cycle in the out-of-roundness of the orbit is important. The orbit goes from nearly round to more squashed and back in about 100,000 years, largely controlled by the tiny tug from the gravity of the planet Jupiter as we pass it in our orbits. And, there is a slower modulation of the out-of-roundness that takes about 400,000 years. More or less, the orbit goes from nearly round to a little squashed, to nearly round, to more squashed, to nearly round, to even more squashed, to nearly round, to not as squashed, to nearly round, to barely squashed, and then this whole thing repeats, with the nearly-rounds spaced roughly 100,000 years apart. We are in the barely-squashed part now, and the last time that the orbit was in the barely-squashed mode, the warm time of the ice-age cycle lasted 30,000 years rather than 10,000 years. Climate models have confirmed that this points to our natural future; actually with roughly 50,000 more years of warmth before the next ice age starts. However, human burning of fossil fuels has already released enough carbon dioxide to warm the climate more than 50,000 years into the future, likely stopping that next ice age. (If we were truly interested in stopping that next ice age, we would wait until the cooling was due and then release the carbon dioxide.)

Also, note that the 19,000-year cycle noted in the text is an oversimplification. There is instead a “quasi” periodicity ranging from 19,000 to 23,000 years, as we mentioned briefly, and this was calculated by Milankovitch and is observed in the data collected to test Milankovitch's calculations, beautifully confirming his predictions. The whole story is a little more complicated than we can fit into a short Enrichment section here, but the basics are clear—orbits pace the ice ages by moving sunshine around on the planet, and this causes environmental changes that shift carbon dioxide between the deep ocean and the atmosphere, globalizing the changes.

Central Pennsylvania and Glaciation

During at least one old glaciation (probably over 1 million years ago), ice flowing south from Canada dammed the West Branch of the Susquehanna River and formed a lake in the Lock Haven area of Pennsylvania. If that lake filled to the next lowest bedrock outlet (into the Juniata River along the Bald Eagle Valley at Dix), then the water would have lapped at the steps of Old Main on Penn State’s University Park campus. There is no evidence of such a large lake, and before the lake filled all the way, it probably drained through the failure of the ice dam, but we’re not sure. With ice so close, however, central Pennsylvania was cold during the ice ages.

Isotopic Ratios of Dead-Bug Shells

In the main text, you learned how the changes in ice volume control the isotopic composition of water in the ocean, and how we can reconstruct the ice-age cycle from the history of shell isotopic compositions in a sediment core because the shells record the water isotopic composition. As usual, things are a bit more complicated than that. Shell isotopic composition also is affected by temperature. When there is more ice on land, the ocean has heavier isotopic ratios in its water, and this gives heavier isotopic ratios in shells growing in the water, but colder temperatures also give heavier isotopic ratios in shells. (At high temperatures, both heavy and light atoms have plenty of energy to jump out of a shell; at low temperatures, the heavy ones tend to get stuck in shells while the light ones can jump out.) Because both colder water and ice favor isotopically heavier shells, measurement of shell isotopic composition cannot tell you the relative importance of temperature versus ice volume.

One way around this is to go to a place that is cold today; the water was above freezing during the ice age (shells were living in it…), so there the signal must be primarily one of ice volume. Other approaches include finding additional paleo-thermometers, such as estimating the temperature from the species living in a place and leaving their shells, or using changes in other “contaminant” ratios in shells that depend on temperature. Yet another way is that there is water in spaces in mud, and the water in some sediments is from the ice age, so just measure the isotopic composition of that water.

The result of this is that isotopic ratios did change because there was much more ice during the ice age than today and because most places were much colder during the ice age than today.

The directions for this exercise are very simple:

1. Watch a movie and read a short article about determining basic geological ordering and layering below.
2. Answer 6 questions to see if you'd pass the "Careful Geologist" test.

This exercise is not terribly difficult and is graded out of 12 points. There is no time limit (except to submit by the due date) to worry about for this exercise, and it will be graded automatically. You have one chance to submit ...so take care to proof your answers carefully in order to get all of your points.

Onwards and Upwards!

Please note that this is similar in format to the "RockOn" quizzes so the same technical "cautions" still apply.

1. Once you access a quiz, do NOT open a new browser window to look something up elsewhere in Canvas. The system gets very confused when two active sessions are going on in the same Web browser, typically resulting in an "access denied" message once you try to submit the quiz.
2. Be sure to double-check your answers before you submit! Materials submitted online are graded in the same way as any other course submissions--we grade what you submit. If the answer is D but you chose C, the answer is wrong...whether you accidentally wrote C by hand, or you accidentally colored in the C circle on the scantron sheet with your #2 pencil, or you accidentally clicked on C with your mouse.
3. Do not use the mouse wheel when taking the quiz as it can cause your answers to change indiscriminately. Instead, use the scrollbar or arrows to view and review the page.

Now, if you think you understand everything, you may go directly to the exercise. If everything isn’t crystal-clear to you, read the following article about determining basic geological ordering and layering, and everything should clear up.

Determining Basic Geological Ordering and Layering

When rivers, wind, and other processes make layers of sediment on the Earth’s surface, the layers are almost always nearly horizontal. Try to make a steep pile of mud, and you will see that gravity quickly pulls the mud down into a nearly flat layer. You can make a steep sand castle at the beach, but watch what happens to the castle after the first waves hit it. The new mud layer, or the remains of your sand castle, won’t end up perfectly flat, but they will be pretty close to flat. However, after sediment is turned to sedimentary rock by squeezing or hard-water deposits or other processes, the layers may be tipped up on end or even turned upside-down by mountain-building processes without being flattened by gravity (see the pictures of folds just below). When geologists become forensic scientists, figuring out the history at the scene of the “crime”, the geologists need to learn which side of a layer was on top when the layer formed. Knowing which way was up originally lets the geologists put the rock layers in order from oldest to youngest, because younger layers are deposited on top of older layers.

Folded rocks seen in Waterton Lakes N.P., Canada. Rocks were deposited in beds before being folded into this huge “S”-shaped curve

Folded layered rocks from Grand Canyon National Park. The red item in the lower left is a pocket knife.

Mud Layers

Mud layers often are disturbed from above before they are hardened, and this happened in the past as well. A dinosaur may have walked by and made a footprint. Dinosaurs didn’t wriggle under the mud, lie on their backs, and make footprints in the mud above; the dinosaurs walked on the mud. (We do see traces in mud where worms wriggled through. We also see places where critters burrowed down into undersea mud to avoid being eaten by hungry fish cruising by.) So, a footprint goes down into mud. Mud often cracks as it dries in the sun, with the crack going down into the mud, and thick mud layers often have cracks that end within the mud. Raindrops may pock-mark the upper surface of mud. Mud cracks, dinosaur footprints, raindrop imprints and hiding-from-fish burrows are among the features that help us put rocks in order. To see examples of this, use the slider below.

Sand Dunes

Sand dunes often give us “up” indicators. As we told you earlier, layers of loose sediment tend to be more-or-less horizontal, but not exactly horizontal. Some of the steeper natural layers are in sand dunes. As shown in the slides below, these layers usually curve, being steeper near the top of the sand dune and flattening out at the bottom where the sand grains bounce across the old surface. When a new dune is deposited on top of an old dune, the nearly horizontal layers (also called beds) at the bottom of the new dune will cross the steeper layers in the old dune, giving cross beds that tell us which way was up when the rocks were deposited.

Shells

Nature gives us many other “up” indicators. A living clam orients itself the way it wants to be oriented. But, after the clam dies, and the shells on the two sides of the clam come apart, those shells tend to be flipped by waves or currents to the stable frowny-face orientation (hollow-side down, or rocker-side up). Use the slider below for more details.

Lava

A thin frozen layer will form on top of a lava flow where cooled by the air, while the lava remains liquid beneath. Bubbles rise through the liquid lava, but are trapped by the frozen layer. Thus, one often finds more and bigger bubbles near the top of a lava flow, but not right at the top. It sometimes takes a little experience to recognize how thick the frozen layer was, but a good geologist can use this to learn which way was up when the lava was flowing.

Shadows, Hills and Holes

A word about shadows, hills and holes. Sometimes, it is hard to tell whether a picture is showing a hill or a hole. But, if you can see shadows, and if someone tells you where the main light source was when the picture was taken, you can always tell a hill from a hole in a picture. Take a look at the pictures of the moon snail shell by clicking on the slides just below. This shell was found on the Outer Beach of Nauset Marsh, Cape Cod National Seashore. After the snail died, some of the shell broke off in the surf. In A, you can probably tell that the shell sticks up towards you. But, if we tell you that the light was shining in the direction of the arrow, you can be sure. If the shell had been in the hollow-side-up configuration, it would have looked like B (A and B are the same shell). Notice the regions labeled “shadow” (which are in shadow) and “lighted” (which were in the direct light), which, together with knowing where the light was, tell you the orientation of the shell, as shown in the diagrams.

Review

Here's one more look at learning which way was "up" when a rock layer was deposited. Check out this extra Review Video and dance on down with the dinosaur. We’ll revisit these topics in Module 9. Everything you need to ace the exercise is in the instructions just above, but in case you really want to learn all the background and read more about these topics, you can take a sneak peek ahead to Module 9.

Now begin the exercise. Exercise #4 consists of 6 multiple choice questions in which your job is to identify which way was up when the rocks in the pictures were formed.

Review Module Requirements

You have reached the end of Module 7! Double-check the list of requirements on the Welcome to Module 7 page and the Course Calendar to be sure you have completed all the activities required for this module.

Supplemental Materials

Following are some supplementary materials for Module 7. While you are not required to review these, you may find them interesting and helpful in preparing for the quiz!

• Website: Glacier National Park
• Website: Yosemite National Park

Comments or Questions?

If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.