(no subject)
. A glacier almost always flows:
C. From where the glacier’s upper surface is high to where the glacier’s upper surface is low.
The great ice sheet of Greenland spreads from its central dome, so the ice on the south side is moving south, the ice on the north side is moving north, the east-side ice moves east and the west-side ice moves west. Ice flows down many mountains, such as Mount Rainier, but ice came across the Great Lakes and up into the US. Thus, ice flows from where its upper surface is high to where its upper surface is low.
Points Earned: 1/1
Correct Answer: C
Your Response: C
In the picture above, the ice that modified the rock moved:
B. From left to right, striating the surfaces the ice reached first and plucking blocks loose from the far sides of bumps.
Indeed, ice sandpapers and striates the rocks it hits first, and then plucks blocks loose from the other side. And the striae go in the direction that the ice moved.
Points Earned: 1/1
Correct Answer: B
Your Response: B
3. Evidence that glaciers were much bigger about 20,000 years ago than they are now includes:
A. 20,000-year-old deceased shallow-water corals occur in growth position far below the surface on the sides of oceanic islands.
Correct Answer: A
In a glacier, the ice moves fastest:
B. At the upper surface, where ice meets air.
The ice at the surface rides along on that beneath but deforms a bit on its own, and so goes fastest. The fast-food ketchup-packet model in which the mid-depth ice goes fastest would require that the upper and lower pieces be especially strong and rigid (which they aren’t; and, it might require someone huge stomping on the glacier). The bed is held back by friction with the rock. And ice lacks the sentience needed to attempt to avoid commercials.
Points Earned: 1/1
Correct Answer: B
Your Response: B
5. Which of these materials is “hottest” in the sense that it is most likely to flow rather than to break (note that K stands for Kelvin, an absolute temperature scale in which zero is absolute zero and higher numbers mean warmer temperatures):
A. A material at 1000oK that melts at 2000oK.
B. A material at 100oK that melts at 200oK.
C. A material at 250oK that melts at 300oK.
Correct Answer: C
Ice Is Nice: Yosemite, Glacier, Rocky Mountain, Bear Meadows, and NE Greenland
• Glacier=pile of ice and snow that flows
• Forms if snow exceeds melt enough to make a pile
• Takes water (as ice) and sediment from accumulation zone (snow exceeds melt) to ablation zone (melt exceeds snow) or to calve icebergs
• Flows in downhill direction of the upper surface (where ice meets air), even if that means the bottom flows uphill
• Think of pancake batter flowing on a waffle iron
Slip Sliding Away
• Glacier moves by deformation within ice, and if bed warmed to freezing point, by sliding over substrate or deforming sediment there
• Most deformation deep, but top fastest because rides along on deeper layers
• Ice deforms because almost hot enough to melt
• Glaciers erode by plucking rocks loose, sand-papering bed, and by subglacial streams
• Thawed-bed glaciers, especially those with surface meltwater reaching the bed, change landscape more rapidly than streams, etc.
Ages of Ice
• Recent (about 20,000-year-old), unique glacier tracks across broad areas now far from ice suggest past ice age(s)
• Ice-age hypothesis predicts land rising where ice was, sinking around, and that is indeed observed
• Ice-age hypothesis predicts sea level was lower when ice big, and indeed observe dead shallow-water corals of that age in growth position deep, flooded river valleys, etc.
Ice-Age Records
• Isotopically lighter water evaporates more easily
• Bigger ice-->isotopically heavier ocean and shells
• Shell-isotopic history from ocean-mud cores shows biggest ice every 100,000 years, smaller wiggles about 41,000 and 19,000 years apart
• Predicted by Milankovitch before observed--these are wiggle-spacings in Earth’s orbit
• Ice grows globally when little northern sunshine
• Orbitally changing sun controls northern ice, which affects CO2, which controls southern ice
Bear Meadows
• Ice sheets today about 10% of land area; at height of ice age covered about 30% of modern land; central PA just beyond edge of Canadian ice
• Rocky Mountain, coastal NE Greenland National Parks have permafrost--soil at some depth frozen year-round
• Permafrost freeze-thaw and enhanced creep (summer melt can’t drain down, so soil soggy and creep easy) make distinctive features
• Those features exist but are not forming in central PA
• So, we were really cold in the ice age
Glacier Tracks
• Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock
• Smooths upglacier, plucks downglacier sides of bumps
• Glaciers make valleys with “U”-shaped cross-sections, often with side-valley floors hanging above main-valley floor; streams make “V” shape without hanging valleys
• Glaciers gnaw bowls called cirques into mountains
• Glaciers deposit all-different-size-pieces till and washed-by-meltwater outwash, often in outlining ridges called moraines
What Glaciers Do, Erosion and Yosemite
When your tour guide, Dr. Alley, was a much younger man (the year I graduated from high school, 1976), I traveled with my sister Sharon and cousin Chuck on a camping tour of the great national parks of the American west (in Chuck’s 1962 Ford Galaxy 500 land boat). At Yosemite, we hiked from the valley up to Glacier Point. 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 well worth the climb. 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.
We were a bit disheartened by the crowd at Glacier Point-the view is also accessible by the Glacier Point Road. While we 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 quick-sitters asked “Anything out there?” To which the ‘energetic’ one replied “Nah, just a bunch of rocks. Let’s go check out the gift shop.” At last report, the gift shop had been removed. Regardless, it must be a sad person indeed who would prefer a gift shop to Yosemite.
To anyone with open eyes, Yosemite Valley-the “Incomparable Valley”-is well worth inspection. It is carved from 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. Such a fate eventually awaits the Cascades volcanoes, some millions of years in the future.
The Sierra Nevada was raised and tilted along the great fault to its east, and looks down on Death Valley and the rest of the Great Basin. Earthquake activity, and faults cutting recent sediments, 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 are most rocks, but granite does eventually break down, and some streams have managed to exploit weaknesses and cut deep channels through the range. These streams include the Tuolomne River, which carved the mighty Hetch Hetchy valley, now dammed so that a valley the equal of Yosemite is lost under water. 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.
Which Way Did It Flow?
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 pile of pancake batter spreads across a griddle, moving away from where the pile is highest. In the same way, a glacier moves from where its upper surface is highest to where its upper surface is lowest. In the figure below, the pressure at point A (the weight of the material above point A) is larger than at point B. There thus is a net push from A to B. Whether the diagram shows pancake batter, or the ice sheet on Antarctica, this push causes the material to deform and flow.
If you make a pile of pancake batter on a waffle iron, some of the batter may flow along the low grooves and then move up to cover the bumps, but the flow will always 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. For example, pieces of Canada are strewn across northeast Pennsylvania, and were brought across Lake Ontario and New York by the ice-age ice sheet. Ice at 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.
The total imbalance in pushes is larger for thicker ice than for thin. A very thin ice mass will not deform fast enough for the motion to be measured, and so is not considered to be a glacier. Typically, ice thicker than about 50 m (150 feet) will deform and flow, making a glacier.
Look at the second part of the previous figure. Glaciers move in one or more ways. 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 largest, causing 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 fastest at the surface. Some glaciers are at the melting point at the bottom, warmed by heat flowing out of the Earth beneath. These melted-bed glaciers may move over the material beneath them, either by sliding over those materials (shown in the figure) or, if the materials are soft sediment, by deforming those sediments in a sort of slow landslide (not shown).
Recall that rivers adjust to move sediment and water from one place to another. So do glaciers. The water is supplied, frozen, in the accumulation zone, where snowfall exceeds melting. The frozen water flows to the ablation zone, where melting exceeds snowfall (ablation means wearing away), 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-the upper and lower surfaces of the ice slope in the same direction.
When we talk about the advance and retreat of a glacier, we are referring to the position of its terminus, where the glacier ends by melting or calving. A glacier is advancing when it is getting longer, and retreating when it is getting shorter. Notice that ice almost always continues flowing from the accumulation zone to the terminus whether the glacier is advancing or retreating-retreat occurs when ice loss by melting or calving is faster than new ice is supplied, and advance occurs when ice is supplied more rapidly than it is removed by melting or calving.
Permanent deformation-flow-within ice may seem strange-after all, ice is a solid. But, as for 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 is nearly warm enough to melt, and so can flow slowly. As a general rule, materials heated more than halfway from absolute zero to their melting point can flow slowly, and flow becomes easier the closer the temperature is 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 “hot” and is able to flow. For more on this, and on the occurrence of crevasses as well as flow, see the Enrichment.
Glacier Tracks
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 much more rapidly than streams or wind erode. Consider for a moment the Great Lakes of the U.S. and Canada. The lakes were carved by glaciers. The bedrock beneath Lakes Superior and Michigan is well below sea level, and was carved that deep 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 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. Evidently, the glaciers have been much better at their “job” than the streams have been. The same can be said for many other places. It is not too extreme to say that the regions glaciated 20,000 years ago and free of ice today still preserve a glacial landscape.
Ice moving over bedrock “plucks” rocks free, and 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. Subglacial streams sweep away the loose pieces, and may cut into the rock as well.
Plucked and abraded rocks show clearly that glaciers were present, but so do big features, as
seen in Yosemite and elsewhere. If you could make a cut across a typical stream valley in the mountains, you would see that it usually is shaped like the letter “V”. The narrow stream cuts downward, and then mass-movement processes remove material from the walls, giving a “V”. However, glaciers are quite wide, and can erode across a broad region. Glaciated valleys exhibit a characteristic “U” shape. Yosemite Valley, with its near-vertical walls and near-horizontal floor, is a classic “U”, not a “V”.
The steeper a stream is, the faster it erodes. If a main river cuts down rapidly, then the side streams that flow into it will become very steep, and will cut downward very rapidly. In this way, even a small side stream can “keep up” with the main stream as it erodes downward, and stream processes usually produce “rapids”, rather than waterfalls where the side streams must plunge over cliffs to reach the main stream. Glaciers are different. A main glacier often fills its valley, the ice burying most or all of the rock. The ice from a side glacier then does not drop steeply down into the main glacier because there is no drop. So the side glacier is not steeper than the main glacier. The main glacier has more ice and rock and water than the side glacier, and so the main glacier erodes down more rapidly. When the ice melts, a “hanging valley” remains-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.)
Glaciers make many other erosional features. At the head of a glacier, where it freezes onto rocks and pulls them free, a bowl can be carved into the side of a mountain. If bowls chew into a mountain from opposite sides until they meet, a knife-edged ridge is left-the Garden Wall of the continental divide in Glacier National Park, which we’ll meet in the next chapter. 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.
Glaciers also leave distinctive deposits. Streams, waves and wind all sort rocks by size, leaving too-big ones behind and carrying away smaller ones. Glaciers don’t care how big the rocks are that the ice carries, so a deposit put down directly from ice may have the tiniest clay particles up to house-sized boulders. Such a deposit is called 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.
Glacier National Park is the southern half 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 wolves and grizzly bears, mountain goats balanced on cliffs, 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 continental divide snakes along the Garden Wall, in many places a knife-edge ridge left as glaciers gnawed into the backbone of the continent from the east and the west. 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 also has many active glaciers, probably more than 50, although fewer remain than at the turn of the century, the number is decreasing as modern warmth melts many away, and all are very small (see the photo on the next page). Glacier National Park is 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 Northern Pacific Railroad happy when they were promoting tourism in Glacier National Park (via the Northern Pacific Railroad, of course).
Historical photos from the United States Geological Survey archives, showing Grinnell Glacier as it looked in 1910 (top), and again in 1997(bottom) after much of the ice had melted away. http://www.nrmsc.usgs.gov/research/grinnell.htm
Evidence of Ice Ages
Today, permanent ice covers 1/10 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, from roughly 20,000 years ago, are spread across almost one-third of the modern land surface-in places such as Wisconsin, and northern Pennsylvania, and Yosemite, and Glacier, 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 landscape that is glacially dominated. 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 still 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. (The biggest difference is that icebergs and meteorites simply do not make features that even vaguely resemble those actually observed, but let’s look at some other differences.)
If huge ice really 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 would still be rising after the melting of the ice, while 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 so would not have concentrated its weight in one place, and 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.
The water for huge ice sheets would have been supplied by evaporation from the oceans, 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 at the time 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 where there is much sunlight. Dead samples of such corals from about 20,000 years ago can be found where they grew, down the sides of islands more than 300 feet. Other evidence also points to lower sea level in the recent past-the Chesapeake Bay, for example, is a river valley that was drowned by rising waters.
How Many Ice Ages?-An Ocean of Clues
So, much evidence shows that ice ages occurred. Piled tills separated by soils demonstrate that the ice has come and gone many times. But how many times? On land, the glacial record is confused-often, an advancing glacier will erode the evidence of a previous one. A pile of four tills separated by soils may record four advances, or forty. In many places in the deep oceans, sediment has been piling up without erosion for millions of years. If there were a marker of glaciation in the marine sediments, we could tell how many glaciations have occurred. If there were a way to date these sediments, we could tell when the glaciation happened. Fortunately, we can identify glaciations using shells in marine sediments, and we can date them. Here, you will learn to identify the glaciations, and later you will get the low-down on dating.
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 have a quick 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.
When sea level drops during an ice age as water vapor is changed to snow and then to ice sheets, the oceans have lost a lot of water. More light water than heavy water has been lost from the oceans, so they are isotopically a little bit heavier than normal. When ice melts, that light water from the ice sheets is returned to the ocean and makes it lighter.
These changes are very small. If we round off the numbers a little, we can say that in the modern ocean, 1 of each 500 water molecules is heavy, which is the same as saying that 1000 or each 500,000 water molecules are heavy. When the ice sheets were big, had you weighed a whole lot of molecules in the ocean, you would have found that about 1001 of each 500,000 water molecules were heavy. This is a tiny change, the water was still water, but sophisticated modern instruments are so good that such a change is very easy to measure. (And yes, the instruments weigh WAAAY more than 500,000 molecules, to obtain good statistics.)
Many plants and animals that grow in the ocean build shells of calcium carbonate (the stuff of limestone) or of silica, both of which contain oxygen. These shells record the isotopic composition of the water in which they grow because the oxygen in the carbonate or silica is obtained from the water. Critters growing during big-ice times grow isotopically heavy shells, and critters growing during small-ice times grow isotopically light shells. When the critters die, their shells pile up in layers on the sea floor with the youngest ones on top. A core collected from these sediments is a history of the ice volume on Earth. Just date the core, pull out the shells, analyze them isotopically, and there is the answer. With enough care, knowledge, and instrumentation, dedicated workers can obtain consistent, reproducible data that tell a wonderful, clear story.
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.
The Cold of Space
More remarkable, these cycles were predicted, and only decades later did technology become good enough to test the prediction and show that it worked. During the 1920s and 1930s, a Serbian mathematician named Milutin Milankovitch calculated how the sunshine received at different places and seasons on the Earth has changed over long times. 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 north pole tilts a little more and then a little less with a 41,000-year periodicity, and the orbit changes from more-nearly round to more squashed or elliptical with a 100,000-year periodicity. With modern computers, these changes are relatively easy to calculate for many millions of years; for Milankovitch, it was the labor of a lifetime.
These orbital wiggles have little effect on the total sunshine received by the planet, but they do move the sunshine from north to south, poles to equator, or summer to winter 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 on 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. (Meanwhile, the south had slightly cooler summers and warmer winters than recently, because the Earth was closer to the sun during southern winters and farther from the sun during southern summers than today.) The intense summer sunshine in the north 9000 years ago made mountain glaciers smaller then. As the summer sunshine decreased in the north, those glaciers expanded slowly for several thousand years, culminating in the Little Ice Age of the 1600s to 1800s; strong melting of glaciers since then is probably mostly the result of human-caused warming. (We will discuss this later in the course.)
Summer in the northern hemisphere appears to be key to controlling ice ages, probably because the northern hemisphere is mostly land and can grow big ice sheets, but the southern hemisphere is mostly water, already has ice on Antarctica, and 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. Survival of this snow requires cool, short summers. As summers have cooled around Hudson Bay, ice has grown; as summers have warmed, ice has melted. 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 reduced summer sunshine, allowing ice to grow, the southern hemisphere often was receiving extra sunshine, yet ice also grew in the south during many of those times. How Canada told the glaciers of Patagonia and Antarctica to grow was for a long time a great puzzle, which we will revisit. (The answer involves the global warming from atmospheric carbon dioxide.)
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. Naturally, the Earth’s orbit right now is in a rather strange state, and we should be looking forward to another 20,000 years or more with little change before we begin the 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.
Permafrost and Periglaciation
Meanwhile, what of things back in central Pennsylvania and in the many other places that were not quite reached by the ice-age ice? As you might imagine, with the world cold enough to grow ice to cover New York, Chicago, Minneapolis, and Seattle as well as much of Europe, the climate was colder everywhere then than it has been more recently.
If you climb the ridges of central Pennsylvania, perhaps up in the Seven Mountains just southeast of State College (go up Bear Meadows Road past the ski area, for a start), you may notice several interesting things geologically. Beneath the hemlocks and rhododendron, the soils and streams and hillslopes have more in common with the high meadows of Trail Ridge Road in Rocky Mountain National Park, or with the coast of Greenland, than they do with the modern climate of State College. 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. (These areas are also called “periglacial”, because they may occur around the glacier, or on its perimeter.)
Consider the following features of the Seven Mountains.
• Many of the headwaters streams have the braided pattern that forms when lots of big rocks are supplied rapidly. Large rocks are evident across the beds of these braided streams. But, the sediment is not “active”. Trees grow on the bars. Even huge floods, such as the winter flood of 1996, do not move the rocks beneath them. Something in the past delivered much coarse sediment to the streams, and then that delivery stopped. Meanwhile, along Trail Ridge Road today, freeze-thaw processes in the tundra break loose large blocks of rock that can be moved to streams.
• The highest points on the ridges of the Seven Mountains are composed of resistant sandstone bedrock, but near the surface the rock has been broken into huge blocks, of the sort that are worked loose by freeze-thaw activity. On flat places such as Big Flat, these blocks sometimes are patterned, with higher and lower, coarser and finer regions a few yards (or meters) across. (These features were described by geologists during times when logging and fire had removed the thick vegetation; the features are very hard to see and almost impossible to photograph today, but can be found during careful bush-whacking.) Meanwhile, similar features occur along Trail Ridge Road and in other permafrost regions, where expansion-contraction processes during the seasonal freezing and thawing of the upper layer sort and stir the rocks and soil into such patterns.
• Stripes or fields of sandstone blocks extend from the ridges down across other rock types to the streams. The blocks are often aligned, as they would be in a creeping soil mass. The coarsest blocks typically are on top with finer material beneath, and patterns such as those on Big Flat may be present but elongated as if they were creeping downhill. Yet the blocks are not now creeping downhill; trees grow on top, and have not been knocked over or bent by landsliding or soil creep. Meanwhile, when the top of permafrost melts on Trail Ridge Road or elsewhere, the water cannot drain out through the rocks beneath because the spaces between them are plugged by ice. The water is trapped in the thawed layer, which then is capable of creeping on very gradual slopes. “Solifluction” or soil-flow lobes thus are common extending downhill.
• Bear Meadows is a young feature, probably formed during the coldest part of the most recent ice age, and probably dammed by a debris-flow or soil-flow lobe extending down from the ridge above it. Bear Meadows is one of the few natural wetlands of any size in central Pennsylvania. The meadows provide a favorite blueberry-picking spot for people-and bears-in the region. The plants of Bear Meadows are quite interesting and varied. Examination of a core pulled from the mud that fills the bog shows that the bottom is almost free of organic material-just silt. 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. A nearly barren tundra of the Trail Ridge Road type, with a solifluction lobe that dammed a stream, followed by warming, would have produced the sediments we see.
• Other permafrost indicators can be found near State College, including some down in the valleys.
The conclusion is nearly inescapable-Trail Ridge Road in Rocky Mountain today is an excellent picture of what the Seven Mountains looked like during the ice age. Permafrost is common across much of Canada and Siberia and 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.
An Important Aside: Is This Story-Telling Science?
Perhaps more meaningful than the conclusion of past Pennsylvania permafrost is the underlying reasoning. Some people today, including important government officials, claim that “historical” geology is not really science, does not use the scientific method, does not produce scientific results, and so should be ignored.
But, consider how the process works. Go up to Bear Meadows, start up toward the ridge above, and look around carefully. You see that big rocks are present, of a type that is quite different from the bedrock directly beneath.
Many hypotheses are possible to explain this observation-space aliens dropped the big rocks; or bulldozers pushed the rocks into place; or, the rocks came screaming down from uphill in a giant landslide; or, they came creeping down slowly; or, … you could think of others. Each hypothesis leads to predictions. If a bulldozer pushed the big rocks in, we should find the bulldozer tracks, and we should be able to trace back in historical records to whom was driving the bulldozer, and why. The first settlers, who arrived before bulldozers were invented, should have found hillslopes free of big rocks. If the big rocks came from uphill, we should be able to find a source of such rocks uphill. Landslides start with big falls or slumps from particular places, so a landslide should have a big scar at its head, whereas creep slowly collects rocks as they are worked loose and carries them along, lining them up as they go.
So, you look for evidence that supports or refutes each of your hypotheses. The early settlers complained about the big rocks, old cabins are built on the big rocks, so the bulldozer hypothesis won’t work. There is no evidence for a landslide scar anywhere, despite evidence for lots of different “stripes” of big rocks extending downhill from a ridgetop source where bedrock of the same type as the big rocks sticks out. You quickly come to the realization that the rocks look like a soil-creep deposit from above; the predictions from each of your other hypotheses fail, but each of the predictions from the soil-creep hypothesis is supported by additional data that you collect for testing purposes.
Then, you 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 you look for a time in the past when tree roots were not holding the rocks in place. You collect more information-the big rocks are on top of smaller rocks and soil, not on the bottom, the big rocks are often standing on edge, the rocks show patterning of coarse and fine, etc. Other geologists are scanning the whole planet, laboring over centuries, and among the many things these geologists report are the conditions of creeping hillslopes in the tropics, the deserts, the temperate zones, and the poles. You talk to other geologists, devote a decade of your life to careful study, and eventually learn that the things you see on the slopes of the Seven Mountains 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. So you take a core in the bog, and find that the bog started in a very cold time (the deepest pollen you find 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 is true that no one has used a backhoe to take the dam apart to look for a space-alien-constructed dilithium-crystal foundation, so maybe the space-alien hypothesis has not been completely falsified and the science could be improved; but, there comes a point of diminishing returns….)
Next, you ask whether this makes sense. You have tentatively concluded that the hillslopes of Pennsylvania record 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. And 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 sunshine and that is recorded in ice-core bubbles from the time.
Now, a modern geologist who tells the “story” of this chapter-Pennsylvania hikers twist their ankles on permafrost deposits-actually has a lot more evidence than the little sketch provided here. The libraries of information collected by centuries of Earth scientists are woven together in a sophisticated, carefully tested, highly reliable whole. This great tapestry of knowledge still has gaps, dropped stitches and moth-bitten places, and the ragged edge where knowledge runs out into the unknown that so excites us as scientists. But the science of the tapestry is well-woven and exceptionally strong. We can only hope that the misguided attacks on this science come from ignorance and not malice, because ignorance is more easily changed.
he 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 guide books somehow have decided that Bear Meadows is 10,000 years old, the Meadows are clearly much older, having formed during the last ice age. 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.
When Is the Next Ice Age?
In the text, we noted that the history of ice ages generally has involved 90,000 years of cooling, followed by 10,000 years of warming, then repeat. The rate of cooling initially is slow, and some people prefer to refer to 10,000 years of warmth followed by cooling. 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 drop into the next ice age. Some people have suggested that humans have already headed off that ice age, or that global warming is a good thing because it will head off the ice age.
The 100,000-year pacing of a 90,000-year/10,000-year world is linked to 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. But, there exists a slower modulation that takes about 400,000 years. The orbit goes nearly round, a little squashed, nearly round, more squashed, nearly round, even more squashed, nearly round, not as squashed, nearly round, barely squashed, repeat, with the nearly-rounds spaced 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 should be our natural future; another 20,000 years of warmth (or maybe 40,000 years) before the next ice age starts. However, human burning of fossil fuels may extend the warmth beyond the next 20,000-40,000 years.
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.
C. From where the glacier’s upper surface is high to where the glacier’s upper surface is low.
The great ice sheet of Greenland spreads from its central dome, so the ice on the south side is moving south, the ice on the north side is moving north, the east-side ice moves east and the west-side ice moves west. Ice flows down many mountains, such as Mount Rainier, but ice came across the Great Lakes and up into the US. Thus, ice flows from where its upper surface is high to where its upper surface is low.
Points Earned: 1/1
Correct Answer: C
Your Response: C
In the picture above, the ice that modified the rock moved:
B. From left to right, striating the surfaces the ice reached first and plucking blocks loose from the far sides of bumps.
Indeed, ice sandpapers and striates the rocks it hits first, and then plucks blocks loose from the other side. And the striae go in the direction that the ice moved.
Points Earned: 1/1
Correct Answer: B
Your Response: B
3. Evidence that glaciers were much bigger about 20,000 years ago than they are now includes:
A. 20,000-year-old deceased shallow-water corals occur in growth position far below the surface on the sides of oceanic islands.
Correct Answer: A
In a glacier, the ice moves fastest:
B. At the upper surface, where ice meets air.
The ice at the surface rides along on that beneath but deforms a bit on its own, and so goes fastest. The fast-food ketchup-packet model in which the mid-depth ice goes fastest would require that the upper and lower pieces be especially strong and rigid (which they aren’t; and, it might require someone huge stomping on the glacier). The bed is held back by friction with the rock. And ice lacks the sentience needed to attempt to avoid commercials.
Points Earned: 1/1
Correct Answer: B
Your Response: B
5. Which of these materials is “hottest” in the sense that it is most likely to flow rather than to break (note that K stands for Kelvin, an absolute temperature scale in which zero is absolute zero and higher numbers mean warmer temperatures):
A. A material at 1000oK that melts at 2000oK.
B. A material at 100oK that melts at 200oK.
C. A material at 250oK that melts at 300oK.
Correct Answer: C
Ice Is Nice: Yosemite, Glacier, Rocky Mountain, Bear Meadows, and NE Greenland
• Glacier=pile of ice and snow that flows
• Forms if snow exceeds melt enough to make a pile
• Takes water (as ice) and sediment from accumulation zone (snow exceeds melt) to ablation zone (melt exceeds snow) or to calve icebergs
• Flows in downhill direction of the upper surface (where ice meets air), even if that means the bottom flows uphill
• Think of pancake batter flowing on a waffle iron
Slip Sliding Away
• Glacier moves by deformation within ice, and if bed warmed to freezing point, by sliding over substrate or deforming sediment there
• Most deformation deep, but top fastest because rides along on deeper layers
• Ice deforms because almost hot enough to melt
• Glaciers erode by plucking rocks loose, sand-papering bed, and by subglacial streams
• Thawed-bed glaciers, especially those with surface meltwater reaching the bed, change landscape more rapidly than streams, etc.
Ages of Ice
• Recent (about 20,000-year-old), unique glacier tracks across broad areas now far from ice suggest past ice age(s)
• Ice-age hypothesis predicts land rising where ice was, sinking around, and that is indeed observed
• Ice-age hypothesis predicts sea level was lower when ice big, and indeed observe dead shallow-water corals of that age in growth position deep, flooded river valleys, etc.
Ice-Age Records
• Isotopically lighter water evaporates more easily
• Bigger ice-->isotopically heavier ocean and shells
• Shell-isotopic history from ocean-mud cores shows biggest ice every 100,000 years, smaller wiggles about 41,000 and 19,000 years apart
• Predicted by Milankovitch before observed--these are wiggle-spacings in Earth’s orbit
• Ice grows globally when little northern sunshine
• Orbitally changing sun controls northern ice, which affects CO2, which controls southern ice
Bear Meadows
• Ice sheets today about 10% of land area; at height of ice age covered about 30% of modern land; central PA just beyond edge of Canadian ice
• Rocky Mountain, coastal NE Greenland National Parks have permafrost--soil at some depth frozen year-round
• Permafrost freeze-thaw and enhanced creep (summer melt can’t drain down, so soil soggy and creep easy) make distinctive features
• Those features exist but are not forming in central PA
• So, we were really cold in the ice age
Glacier Tracks
• Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock
• Smooths upglacier, plucks downglacier sides of bumps
• Glaciers make valleys with “U”-shaped cross-sections, often with side-valley floors hanging above main-valley floor; streams make “V” shape without hanging valleys
• Glaciers gnaw bowls called cirques into mountains
• Glaciers deposit all-different-size-pieces till and washed-by-meltwater outwash, often in outlining ridges called moraines
What Glaciers Do, Erosion and Yosemite
When your tour guide, Dr. Alley, was a much younger man (the year I graduated from high school, 1976), I traveled with my sister Sharon and cousin Chuck on a camping tour of the great national parks of the American west (in Chuck’s 1962 Ford Galaxy 500 land boat). At Yosemite, we hiked from the valley up to Glacier Point. 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 well worth the climb. 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.
We were a bit disheartened by the crowd at Glacier Point-the view is also accessible by the Glacier Point Road. While we 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 quick-sitters asked “Anything out there?” To which the ‘energetic’ one replied “Nah, just a bunch of rocks. Let’s go check out the gift shop.” At last report, the gift shop had been removed. Regardless, it must be a sad person indeed who would prefer a gift shop to Yosemite.
To anyone with open eyes, Yosemite Valley-the “Incomparable Valley”-is well worth inspection. It is carved from 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. Such a fate eventually awaits the Cascades volcanoes, some millions of years in the future.
The Sierra Nevada was raised and tilted along the great fault to its east, and looks down on Death Valley and the rest of the Great Basin. Earthquake activity, and faults cutting recent sediments, 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 are most rocks, but granite does eventually break down, and some streams have managed to exploit weaknesses and cut deep channels through the range. These streams include the Tuolomne River, which carved the mighty Hetch Hetchy valley, now dammed so that a valley the equal of Yosemite is lost under water. 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.
Which Way Did It Flow?
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 pile of pancake batter spreads across a griddle, moving away from where the pile is highest. In the same way, a glacier moves from where its upper surface is highest to where its upper surface is lowest. In the figure below, the pressure at point A (the weight of the material above point A) is larger than at point B. There thus is a net push from A to B. Whether the diagram shows pancake batter, or the ice sheet on Antarctica, this push causes the material to deform and flow.
If you make a pile of pancake batter on a waffle iron, some of the batter may flow along the low grooves and then move up to cover the bumps, but the flow will always 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. For example, pieces of Canada are strewn across northeast Pennsylvania, and were brought across Lake Ontario and New York by the ice-age ice sheet. Ice at 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.
The total imbalance in pushes is larger for thicker ice than for thin. A very thin ice mass will not deform fast enough for the motion to be measured, and so is not considered to be a glacier. Typically, ice thicker than about 50 m (150 feet) will deform and flow, making a glacier.
Look at the second part of the previous figure. Glaciers move in one or more ways. 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 largest, causing 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 fastest at the surface. Some glaciers are at the melting point at the bottom, warmed by heat flowing out of the Earth beneath. These melted-bed glaciers may move over the material beneath them, either by sliding over those materials (shown in the figure) or, if the materials are soft sediment, by deforming those sediments in a sort of slow landslide (not shown).
Recall that rivers adjust to move sediment and water from one place to another. So do glaciers. The water is supplied, frozen, in the accumulation zone, where snowfall exceeds melting. The frozen water flows to the ablation zone, where melting exceeds snowfall (ablation means wearing away), 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-the upper and lower surfaces of the ice slope in the same direction.
When we talk about the advance and retreat of a glacier, we are referring to the position of its terminus, where the glacier ends by melting or calving. A glacier is advancing when it is getting longer, and retreating when it is getting shorter. Notice that ice almost always continues flowing from the accumulation zone to the terminus whether the glacier is advancing or retreating-retreat occurs when ice loss by melting or calving is faster than new ice is supplied, and advance occurs when ice is supplied more rapidly than it is removed by melting or calving.
Permanent deformation-flow-within ice may seem strange-after all, ice is a solid. But, as for 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 is nearly warm enough to melt, and so can flow slowly. As a general rule, materials heated more than halfway from absolute zero to their melting point can flow slowly, and flow becomes easier the closer the temperature is 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 “hot” and is able to flow. For more on this, and on the occurrence of crevasses as well as flow, see the Enrichment.
Glacier Tracks
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 much more rapidly than streams or wind erode. Consider for a moment the Great Lakes of the U.S. and Canada. The lakes were carved by glaciers. The bedrock beneath Lakes Superior and Michigan is well below sea level, and was carved that deep 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 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. Evidently, the glaciers have been much better at their “job” than the streams have been. The same can be said for many other places. It is not too extreme to say that the regions glaciated 20,000 years ago and free of ice today still preserve a glacial landscape.
Ice moving over bedrock “plucks” rocks free, and 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. Subglacial streams sweep away the loose pieces, and may cut into the rock as well.
Plucked and abraded rocks show clearly that glaciers were present, but so do big features, as
seen in Yosemite and elsewhere. If you could make a cut across a typical stream valley in the mountains, you would see that it usually is shaped like the letter “V”. The narrow stream cuts downward, and then mass-movement processes remove material from the walls, giving a “V”. However, glaciers are quite wide, and can erode across a broad region. Glaciated valleys exhibit a characteristic “U” shape. Yosemite Valley, with its near-vertical walls and near-horizontal floor, is a classic “U”, not a “V”.
The steeper a stream is, the faster it erodes. If a main river cuts down rapidly, then the side streams that flow into it will become very steep, and will cut downward very rapidly. In this way, even a small side stream can “keep up” with the main stream as it erodes downward, and stream processes usually produce “rapids”, rather than waterfalls where the side streams must plunge over cliffs to reach the main stream. Glaciers are different. A main glacier often fills its valley, the ice burying most or all of the rock. The ice from a side glacier then does not drop steeply down into the main glacier because there is no drop. So the side glacier is not steeper than the main glacier. The main glacier has more ice and rock and water than the side glacier, and so the main glacier erodes down more rapidly. When the ice melts, a “hanging valley” remains-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.)
Glaciers make many other erosional features. At the head of a glacier, where it freezes onto rocks and pulls them free, a bowl can be carved into the side of a mountain. If bowls chew into a mountain from opposite sides until they meet, a knife-edged ridge is left-the Garden Wall of the continental divide in Glacier National Park, which we’ll meet in the next chapter. 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.
Glaciers also leave distinctive deposits. Streams, waves and wind all sort rocks by size, leaving too-big ones behind and carrying away smaller ones. Glaciers don’t care how big the rocks are that the ice carries, so a deposit put down directly from ice may have the tiniest clay particles up to house-sized boulders. Such a deposit is called 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.
Glacier National Park is the southern half 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 wolves and grizzly bears, mountain goats balanced on cliffs, 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 continental divide snakes along the Garden Wall, in many places a knife-edge ridge left as glaciers gnawed into the backbone of the continent from the east and the west. 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 also has many active glaciers, probably more than 50, although fewer remain than at the turn of the century, the number is decreasing as modern warmth melts many away, and all are very small (see the photo on the next page). Glacier National Park is 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 Northern Pacific Railroad happy when they were promoting tourism in Glacier National Park (via the Northern Pacific Railroad, of course).
Historical photos from the United States Geological Survey archives, showing Grinnell Glacier as it looked in 1910 (top), and again in 1997(bottom) after much of the ice had melted away. http://www.nrmsc.usgs.gov/research/grinnell.htm
Evidence of Ice Ages
Today, permanent ice covers 1/10 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, from roughly 20,000 years ago, are spread across almost one-third of the modern land surface-in places such as Wisconsin, and northern Pennsylvania, and Yosemite, and Glacier, 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 landscape that is glacially dominated. 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 still 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. (The biggest difference is that icebergs and meteorites simply do not make features that even vaguely resemble those actually observed, but let’s look at some other differences.)
If huge ice really 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 would still be rising after the melting of the ice, while 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 so would not have concentrated its weight in one place, and 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.
The water for huge ice sheets would have been supplied by evaporation from the oceans, 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 at the time 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 where there is much sunlight. Dead samples of such corals from about 20,000 years ago can be found where they grew, down the sides of islands more than 300 feet. Other evidence also points to lower sea level in the recent past-the Chesapeake Bay, for example, is a river valley that was drowned by rising waters.
How Many Ice Ages?-An Ocean of Clues
So, much evidence shows that ice ages occurred. Piled tills separated by soils demonstrate that the ice has come and gone many times. But how many times? On land, the glacial record is confused-often, an advancing glacier will erode the evidence of a previous one. A pile of four tills separated by soils may record four advances, or forty. In many places in the deep oceans, sediment has been piling up without erosion for millions of years. If there were a marker of glaciation in the marine sediments, we could tell how many glaciations have occurred. If there were a way to date these sediments, we could tell when the glaciation happened. Fortunately, we can identify glaciations using shells in marine sediments, and we can date them. Here, you will learn to identify the glaciations, and later you will get the low-down on dating.
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 have a quick 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.
When sea level drops during an ice age as water vapor is changed to snow and then to ice sheets, the oceans have lost a lot of water. More light water than heavy water has been lost from the oceans, so they are isotopically a little bit heavier than normal. When ice melts, that light water from the ice sheets is returned to the ocean and makes it lighter.
These changes are very small. If we round off the numbers a little, we can say that in the modern ocean, 1 of each 500 water molecules is heavy, which is the same as saying that 1000 or each 500,000 water molecules are heavy. When the ice sheets were big, had you weighed a whole lot of molecules in the ocean, you would have found that about 1001 of each 500,000 water molecules were heavy. This is a tiny change, the water was still water, but sophisticated modern instruments are so good that such a change is very easy to measure. (And yes, the instruments weigh WAAAY more than 500,000 molecules, to obtain good statistics.)
Many plants and animals that grow in the ocean build shells of calcium carbonate (the stuff of limestone) or of silica, both of which contain oxygen. These shells record the isotopic composition of the water in which they grow because the oxygen in the carbonate or silica is obtained from the water. Critters growing during big-ice times grow isotopically heavy shells, and critters growing during small-ice times grow isotopically light shells. When the critters die, their shells pile up in layers on the sea floor with the youngest ones on top. A core collected from these sediments is a history of the ice volume on Earth. Just date the core, pull out the shells, analyze them isotopically, and there is the answer. With enough care, knowledge, and instrumentation, dedicated workers can obtain consistent, reproducible data that tell a wonderful, clear story.
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.
The Cold of Space
More remarkable, these cycles were predicted, and only decades later did technology become good enough to test the prediction and show that it worked. During the 1920s and 1930s, a Serbian mathematician named Milutin Milankovitch calculated how the sunshine received at different places and seasons on the Earth has changed over long times. 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 north pole tilts a little more and then a little less with a 41,000-year periodicity, and the orbit changes from more-nearly round to more squashed or elliptical with a 100,000-year periodicity. With modern computers, these changes are relatively easy to calculate for many millions of years; for Milankovitch, it was the labor of a lifetime.
These orbital wiggles have little effect on the total sunshine received by the planet, but they do move the sunshine from north to south, poles to equator, or summer to winter 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 on 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. (Meanwhile, the south had slightly cooler summers and warmer winters than recently, because the Earth was closer to the sun during southern winters and farther from the sun during southern summers than today.) The intense summer sunshine in the north 9000 years ago made mountain glaciers smaller then. As the summer sunshine decreased in the north, those glaciers expanded slowly for several thousand years, culminating in the Little Ice Age of the 1600s to 1800s; strong melting of glaciers since then is probably mostly the result of human-caused warming. (We will discuss this later in the course.)
Summer in the northern hemisphere appears to be key to controlling ice ages, probably because the northern hemisphere is mostly land and can grow big ice sheets, but the southern hemisphere is mostly water, already has ice on Antarctica, and 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. Survival of this snow requires cool, short summers. As summers have cooled around Hudson Bay, ice has grown; as summers have warmed, ice has melted. 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 reduced summer sunshine, allowing ice to grow, the southern hemisphere often was receiving extra sunshine, yet ice also grew in the south during many of those times. How Canada told the glaciers of Patagonia and Antarctica to grow was for a long time a great puzzle, which we will revisit. (The answer involves the global warming from atmospheric carbon dioxide.)
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. Naturally, the Earth’s orbit right now is in a rather strange state, and we should be looking forward to another 20,000 years or more with little change before we begin the 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.
Permafrost and Periglaciation
Meanwhile, what of things back in central Pennsylvania and in the many other places that were not quite reached by the ice-age ice? As you might imagine, with the world cold enough to grow ice to cover New York, Chicago, Minneapolis, and Seattle as well as much of Europe, the climate was colder everywhere then than it has been more recently.
If you climb the ridges of central Pennsylvania, perhaps up in the Seven Mountains just southeast of State College (go up Bear Meadows Road past the ski area, for a start), you may notice several interesting things geologically. Beneath the hemlocks and rhododendron, the soils and streams and hillslopes have more in common with the high meadows of Trail Ridge Road in Rocky Mountain National Park, or with the coast of Greenland, than they do with the modern climate of State College. 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. (These areas are also called “periglacial”, because they may occur around the glacier, or on its perimeter.)
Consider the following features of the Seven Mountains.
• Many of the headwaters streams have the braided pattern that forms when lots of big rocks are supplied rapidly. Large rocks are evident across the beds of these braided streams. But, the sediment is not “active”. Trees grow on the bars. Even huge floods, such as the winter flood of 1996, do not move the rocks beneath them. Something in the past delivered much coarse sediment to the streams, and then that delivery stopped. Meanwhile, along Trail Ridge Road today, freeze-thaw processes in the tundra break loose large blocks of rock that can be moved to streams.
• The highest points on the ridges of the Seven Mountains are composed of resistant sandstone bedrock, but near the surface the rock has been broken into huge blocks, of the sort that are worked loose by freeze-thaw activity. On flat places such as Big Flat, these blocks sometimes are patterned, with higher and lower, coarser and finer regions a few yards (or meters) across. (These features were described by geologists during times when logging and fire had removed the thick vegetation; the features are very hard to see and almost impossible to photograph today, but can be found during careful bush-whacking.) Meanwhile, similar features occur along Trail Ridge Road and in other permafrost regions, where expansion-contraction processes during the seasonal freezing and thawing of the upper layer sort and stir the rocks and soil into such patterns.
• Stripes or fields of sandstone blocks extend from the ridges down across other rock types to the streams. The blocks are often aligned, as they would be in a creeping soil mass. The coarsest blocks typically are on top with finer material beneath, and patterns such as those on Big Flat may be present but elongated as if they were creeping downhill. Yet the blocks are not now creeping downhill; trees grow on top, and have not been knocked over or bent by landsliding or soil creep. Meanwhile, when the top of permafrost melts on Trail Ridge Road or elsewhere, the water cannot drain out through the rocks beneath because the spaces between them are plugged by ice. The water is trapped in the thawed layer, which then is capable of creeping on very gradual slopes. “Solifluction” or soil-flow lobes thus are common extending downhill.
• Bear Meadows is a young feature, probably formed during the coldest part of the most recent ice age, and probably dammed by a debris-flow or soil-flow lobe extending down from the ridge above it. Bear Meadows is one of the few natural wetlands of any size in central Pennsylvania. The meadows provide a favorite blueberry-picking spot for people-and bears-in the region. The plants of Bear Meadows are quite interesting and varied. Examination of a core pulled from the mud that fills the bog shows that the bottom is almost free of organic material-just silt. 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. A nearly barren tundra of the Trail Ridge Road type, with a solifluction lobe that dammed a stream, followed by warming, would have produced the sediments we see.
• Other permafrost indicators can be found near State College, including some down in the valleys.
The conclusion is nearly inescapable-Trail Ridge Road in Rocky Mountain today is an excellent picture of what the Seven Mountains looked like during the ice age. Permafrost is common across much of Canada and Siberia and 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.
An Important Aside: Is This Story-Telling Science?
Perhaps more meaningful than the conclusion of past Pennsylvania permafrost is the underlying reasoning. Some people today, including important government officials, claim that “historical” geology is not really science, does not use the scientific method, does not produce scientific results, and so should be ignored.
But, consider how the process works. Go up to Bear Meadows, start up toward the ridge above, and look around carefully. You see that big rocks are present, of a type that is quite different from the bedrock directly beneath.
Many hypotheses are possible to explain this observation-space aliens dropped the big rocks; or bulldozers pushed the rocks into place; or, the rocks came screaming down from uphill in a giant landslide; or, they came creeping down slowly; or, … you could think of others. Each hypothesis leads to predictions. If a bulldozer pushed the big rocks in, we should find the bulldozer tracks, and we should be able to trace back in historical records to whom was driving the bulldozer, and why. The first settlers, who arrived before bulldozers were invented, should have found hillslopes free of big rocks. If the big rocks came from uphill, we should be able to find a source of such rocks uphill. Landslides start with big falls or slumps from particular places, so a landslide should have a big scar at its head, whereas creep slowly collects rocks as they are worked loose and carries them along, lining them up as they go.
So, you look for evidence that supports or refutes each of your hypotheses. The early settlers complained about the big rocks, old cabins are built on the big rocks, so the bulldozer hypothesis won’t work. There is no evidence for a landslide scar anywhere, despite evidence for lots of different “stripes” of big rocks extending downhill from a ridgetop source where bedrock of the same type as the big rocks sticks out. You quickly come to the realization that the rocks look like a soil-creep deposit from above; the predictions from each of your other hypotheses fail, but each of the predictions from the soil-creep hypothesis is supported by additional data that you collect for testing purposes.
Then, you 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 you look for a time in the past when tree roots were not holding the rocks in place. You collect more information-the big rocks are on top of smaller rocks and soil, not on the bottom, the big rocks are often standing on edge, the rocks show patterning of coarse and fine, etc. Other geologists are scanning the whole planet, laboring over centuries, and among the many things these geologists report are the conditions of creeping hillslopes in the tropics, the deserts, the temperate zones, and the poles. You talk to other geologists, devote a decade of your life to careful study, and eventually learn that the things you see on the slopes of the Seven Mountains 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. So you take a core in the bog, and find that the bog started in a very cold time (the deepest pollen you find 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 is true that no one has used a backhoe to take the dam apart to look for a space-alien-constructed dilithium-crystal foundation, so maybe the space-alien hypothesis has not been completely falsified and the science could be improved; but, there comes a point of diminishing returns….)
Next, you ask whether this makes sense. You have tentatively concluded that the hillslopes of Pennsylvania record 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. And 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 sunshine and that is recorded in ice-core bubbles from the time.
Now, a modern geologist who tells the “story” of this chapter-Pennsylvania hikers twist their ankles on permafrost deposits-actually has a lot more evidence than the little sketch provided here. The libraries of information collected by centuries of Earth scientists are woven together in a sophisticated, carefully tested, highly reliable whole. This great tapestry of knowledge still has gaps, dropped stitches and moth-bitten places, and the ragged edge where knowledge runs out into the unknown that so excites us as scientists. But the science of the tapestry is well-woven and exceptionally strong. We can only hope that the misguided attacks on this science come from ignorance and not malice, because ignorance is more easily changed.
he 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 guide books somehow have decided that Bear Meadows is 10,000 years old, the Meadows are clearly much older, having formed during the last ice age. 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.
When Is the Next Ice Age?
In the text, we noted that the history of ice ages generally has involved 90,000 years of cooling, followed by 10,000 years of warming, then repeat. The rate of cooling initially is slow, and some people prefer to refer to 10,000 years of warmth followed by cooling. 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 drop into the next ice age. Some people have suggested that humans have already headed off that ice age, or that global warming is a good thing because it will head off the ice age.
The 100,000-year pacing of a 90,000-year/10,000-year world is linked to 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. But, there exists a slower modulation that takes about 400,000 years. The orbit goes nearly round, a little squashed, nearly round, more squashed, nearly round, even more squashed, nearly round, not as squashed, nearly round, barely squashed, repeat, with the nearly-rounds spaced 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 should be our natural future; another 20,000 years of warmth (or maybe 40,000 years) before the next ice age starts. However, human burning of fossil fuels may extend the warmth beyond the next 20,000-40,000 years.
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.