Note: This program first aired on June 15, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We’ve reached the end of this series with a look the general trends of the human impacts on the Maine landscape.
If we had to summarize what happened to Maine as a result of human impact we could summarize it thusly: forest, farm forest. As we have said previously, the post glacial biotic community here is forest, at least in the current climatic regime. That is no surprise, follow the 45th parallel around the globe and what you will find is temperate forest world wide. The composition of that forest has changed over the past 12,000 years, but in the face of any disturbance, trees are what grow back here. We can consider European settlers as a major disturbance to this forest community, their direct impact peaked in the late 19th century, that is the “farm” part of the forest, farm, forest pattern. Since that time for a number of social and economic reasons, the forests have returned (not that they ever actually went that far in the first place).
European settlers first arrived in Maine in the early 17th century and for nearly 150 years enjoyed a precarious existence, eeking out a living tied to the coast line and coastal rivers and estuaries and warring with each other, and with the native population, which had been decimated soon after the Europeans arrived by European pathogens. By 1670 a whopping 3500 English settlers lived along the coast and coastal rivers, west of Penobscot Bay, with additional French settlers to its east. The low population numbers and the instability caused by constant conflict kept the impact of these new human colonists fairly low. Deforestation was strongly limited to the immediate coast, and up river valleys, and consisted of clearing for subsistence agriculture, and targeted harvesting of oak for barrels and white pines for ship masts.
Here is where geopolitics influences ecology. Once the American Revolution settled things and relative stability spread across the area, the population of “not yet Maine” grew dramatically. With this population increase came significant increases in land clearing and forest harvesting. It is important to take a moment and parse out these various levels of human impact. Land cleared for agriculture could be one of three things; land cleared for pasturing grazing animals, land cleared for hay fields and land cleared for tillage and the planting of crops. At its peak in the late 19th century, Maine was as much as 15% cleared for these uses (a number that has only gone back down from that time). Cutting in the forest was targeted at white pine for lumber, hemlock for the tanneries, oaks for barrels and ship building, and other hard woods for fuel wood, for both Maine and the Boston market. Forestry at that time was targeted, clear cutting was not part of the system, and wouldn’t be until the late 20th century.
This boom continued in the 19th century, until the time of the civil war and just after. At that point farms began to be abandoned with amazing speed, as the rail roads opened up the mid west (and their deep rich more easily farmed soils) and the economic center of gravity in America shifted from the Northeast westward. Forestry changed as well, as the best lumber logs became harder and harder to find, it was only the advent of the pulp and paper industry in the late 1800’s that kept the forest industry alive in the state. So Maine, the pine tree state, achieved its cleared land maximum just after the civil war, and has been growing trees back ever since. Even the spruce bud worm out break of the 1970’s and the change to industrial land ownership and subsequent clear cutting in the second half of the 20th century have not managed to change the undeniable fact, in Maine, if you turn your back, a tree will sprout. It was only through constant back breaking vigilance that Maine was as cleared as it was in the mid 19th century.
Will it always be so? I doubt it. If there is one constant on Earth it is change. Now our actions will be influencing the Maine landscape well beyond clearing for agriculture or cutting trees in the forest. As climate changes in the coming decades, I doubt that Maine will become a treeless landscape, but a thousand years from now? Who knows? The biotic community we see around us is a result of the average temperature and the amount of water that falls from the sky, both factors that stand to be significantly altered by changing climate. So it seems that the tale of humans’ impact on the Maine landscape isn’t finished after all.
And thus concludes our look into the deep history of the Maine landscape. The tale may not be done, but this series is. Keep your eyes open and read the signs in your own neighborhood. You will be amazed, as I have been, where that story takes you.
References:
If this kind of history floats your boat, I can’t recommend this book enough: Andrew Barton and friends The Changing Nature of the Maine Woods. Readable and excellent!
A classic: William Cronin’s Changes in the Land. This one looks at the impact of Native Americans on the primeval forest, and is definitely weighted more towards southern New England. Lots of good information though.
Colin Woodard’s The Lobster Coast provides a comprehensive overview of the history of the people in Maine, particularly along the coast. Though “lobster” is the unifying theme, there is a tremendous amount of environmental history is this book. Very readable, super interesting.
Curious about the future? Check out this report from the UMaine Climate Change Institute: “ Maine’s Climae Future” http://climatechange.umaine.edu/files/Maines_Climate_Future.pdf
Wednesday, June 19, 2013
The History of Maine: Part 11 First Humans in Maine
Note: This program first aired on June 1, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We’ve reached the third and final (for now) chapter in Maine’s deep history, and it is one we all have a hand in. Before we dive in though, lets quickly recap first two aspects of the existence of the Maine landscape.
About 600 million years ago the very faintest hints of the beginning of Maine were in the air. As a result of the constant movement of the Earth’s tectonic plates, coastal sediments were accreted on to the edge of the North American plate, and volcanic activity added new igneous rock to the mix as well, both above and below the surface. Additionally, little bits of continental plate that likely originated on the ancient European continent also became plastered to the edge of North America. By bits and pieces, the crust of Maine was formed. Two million years ago, the northern hemisphere entered an ice age, and continental glacier after continental glacier advanced over the Maine landscape, scouring and smoothing, and dumping the rocky sediment that challenges gardeners throughout the state.
The last glacier retreated from the state between 15,000 and 13,000 years ago, and the Maine landscape was likely recolonized with Arctic tundra plants, which were then rapidly outcompeted by a forest community as climate continued to warm. Paleopollen analysis indicates that the post glacial environment was mainly forested (with a variety of community compositions). So the types and sizes of trees may have been somewhat different, but after the glaciers left, Maine didn’t look shockingly different than it does today, at least to the casual observer. Botanists and foresters may argue this point, but I think it is important to understand that in the big picture, Maine is solidly a forest community.
It is likely that as soon as there was large game in the post glacial forest, people were here as well. There is archeological evidence to support this. And while there were multiple native cultures that flourished at various times during this post glacial epoch, current thinking posits that the native populations here had a relatively low population density (relative to southern New England or the southeast US), due to an almost entirely hunter gatherer lifestyle. Agriculture was used in more southern regions, and in far southern Maine, but not in the majority of the state. So, early native populations had some kind of impact on the Maine environment, but it was strongly limited by the low population density and lifestyle. Geographically, evidence suggests that the native populations were concentrated on the coast, so for vast stretches of Maine, while there may have been human activity in the past 10,000 years, it left little impact.
In terms of human impact, European colonization was the turning point. It began in earnest in 1607 with the failed Popham colony, and the various French and English 17th century trading and fishing outposts. These early developments were entirely coastal, as transportation was water based. Just think about that for a moment. There were no roads, just overland foot paths. If you wanted to get anywhere you got in a boat. Think about how access to transportation, in this case, the water, must have guided settlement, and in fact, limited it to the thinnest of strips right along coastal waterways and eventually, up major rivers as well. European’s ties to the water, as well as conflict with the native populations kept the majority of human impact along the coast for nearly 200 years.
We’ll look at what happened next, next week, when we finish the chapter of the human influence on the Maine landscape, and perhaps even peer into the future.
References:
Terrific new book by Andrew Barton and friends The Changing Nature of the Maine Woods. Readable and excellent!
A classic: William Cronin’s Changes in the Land. This one looks at the impact of Native Americans on the primeval forest, and is definitely weighted more towards southern New England. Lots of good information though.
Recently Maine Public Broadcasting did an interesting call in show about the first people in Maine. Listen to it here: http://www.mpbn.net/OnDemand/AudioOnDemand/MaineCalling/tabid/288/ctl/ViewItem/mid/3682/ItemId/28278/Default.aspx
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We’ve reached the third and final (for now) chapter in Maine’s deep history, and it is one we all have a hand in. Before we dive in though, lets quickly recap first two aspects of the existence of the Maine landscape.
About 600 million years ago the very faintest hints of the beginning of Maine were in the air. As a result of the constant movement of the Earth’s tectonic plates, coastal sediments were accreted on to the edge of the North American plate, and volcanic activity added new igneous rock to the mix as well, both above and below the surface. Additionally, little bits of continental plate that likely originated on the ancient European continent also became plastered to the edge of North America. By bits and pieces, the crust of Maine was formed. Two million years ago, the northern hemisphere entered an ice age, and continental glacier after continental glacier advanced over the Maine landscape, scouring and smoothing, and dumping the rocky sediment that challenges gardeners throughout the state.
The last glacier retreated from the state between 15,000 and 13,000 years ago, and the Maine landscape was likely recolonized with Arctic tundra plants, which were then rapidly outcompeted by a forest community as climate continued to warm. Paleopollen analysis indicates that the post glacial environment was mainly forested (with a variety of community compositions). So the types and sizes of trees may have been somewhat different, but after the glaciers left, Maine didn’t look shockingly different than it does today, at least to the casual observer. Botanists and foresters may argue this point, but I think it is important to understand that in the big picture, Maine is solidly a forest community.
In terms of human impact, European colonization was the turning point. It began in earnest in 1607 with the failed Popham colony, and the various French and English 17th century trading and fishing outposts. These early developments were entirely coastal, as transportation was water based. Just think about that for a moment. There were no roads, just overland foot paths. If you wanted to get anywhere you got in a boat. Think about how access to transportation, in this case, the water, must have guided settlement, and in fact, limited it to the thinnest of strips right along coastal waterways and eventually, up major rivers as well. European’s ties to the water, as well as conflict with the native populations kept the majority of human impact along the coast for nearly 200 years.
We’ll look at what happened next, next week, when we finish the chapter of the human influence on the Maine landscape, and perhaps even peer into the future.
References:
Terrific new book by Andrew Barton and friends The Changing Nature of the Maine Woods. Readable and excellent!
A classic: William Cronin’s Changes in the Land. This one looks at the impact of Native Americans on the primeval forest, and is definitely weighted more towards southern New England. Lots of good information though.
Recently Maine Public Broadcasting did an interesting call in show about the first people in Maine. Listen to it here: http://www.mpbn.net/OnDemand/AudioOnDemand/MaineCalling/tabid/288/ctl/ViewItem/mid/3682/ItemId/28278/Default.aspx
Monday, May 27, 2013
The History of Maine Part 10: Sea Level and the Coastline
Note: This program first aired on May 25, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week looking at some of the landscape features the glaciers left as they retreated 13,000 years ago here in Maine. That period of deglaciation had another profound impact on the Maine landscape, particularly the one most people know Maine for: the coast.
When we look at the location of the coast line, we are really looking at the edge of the water relative to the elevation of the landscape. If the landform is low, the water can move in, if the land form is high, the water is pushed out. There are two factors here that can change, the amount or volume of the water (the more water there is, the more three dimensional space it takes up, the higher it rises relative to the land, if all things are equal), and the level of the land (this is trickier to get your head around, but just remember that the crust of the Earth is floating, just like a ship at sea—the heavier the boat, the lower it floats. Solid crust floats on the more plastic layers of the Earth below.)
When we look at these two things relative to glaciation, we see the complicated and variable history of Maine’s coastline in the past 13,000 years. When glaciers form, they form from water, generally liquid water that evaporates into the atmosphere and then falls back to Earth in solid form. Where does all that liquid come from? The ocean, so overall we see a drop in global sea level when large continental glaciers form, because so much liquid water is taken up out of the ocean. At the height of the last glacial advance, it is thought that sea levels in our neck of the woods were 300 to 400 feet lower than today as a result of this (a level that would have easily exposed the continental shelf, had it not been covered by ice).
The amount of water isn’t the only thing that can change however, remember, the actual relative level of the land can as well. Remember the ship I mentioned, when it is heavy is floats lower in the water, when it is empty it floats at a higher level. The crust that underlies the state of Maine is like that ship. And what could make the crust heavier than normal? How about 10,000 feet of solid ice? That would do it. The continental glacier was so large and massive, it caused the crust to become less buoyant and float a bit lower on the underlying mantle. The crust was literally depressed into the Earth. As a side note, this is happening today in Antarctica, much of that continent is actually below sea level as a result of the weight of its ice cap.
So that is how the stage was set when the last ice age began to end here in Maine, about 13,000ish years ago. Just like a trampoline with some one sitting on it, the Earth’s crust is deformed by the weight of the glacier, and there was less water in the ocean. That less water in the ocean bit was changing fast though, as the glacier was melting quite rapidly. All of that melt water returned to the ocean, which rose quite rapidly in response. And as the glacial melted, it relived the pressure on the underlying crust, and just like a trampoline when you bounce off it, the crust rebounded. These two events didn’t happen simultaneously though, the glacier melted a great deal faster than the Earth rebounded, which meant, the ocean water easily covered the newly exposed but still depressed landscape of Maine. This event is called the Marine Incursion and at its maximum the coastline was as far inland as Medway, near Baxter State Park. Soon though the Earth’s rebound caught up with the rising ocean, and then some (again, think of our trampoline; a bouncer gets pushed up above the equilibrium level before coming back down). At the height of the crust’s rebound, sea level was much lower than it is today, as the land rose up above the level of the ocean. During this period, about 11,000 years ago, much of the Gulf of Maine, in particular Georges Bank on the edge of the continental shelf, was dry land. Terrestrial fossils, including trees and mammoth tusks have been recovered from Georges Bank, and the Gulf of Maine was a shallow inland sea cut off from the Atlantic Ocean. Soon thereafter though, the rebound eased and even subsided a bit, and the rising ocean caught up. The present day coastline was more or less established in the last few thousand years, as the rapid changes that resulted from deglaciation ended and temporary equilibrium was restored.
Maine has what is called a drowned coastline, and I hope now you can see why. Next week we will look at the third major force that shapes the Maine landscape, us.
References:
D. W. Caldwell Roadside Geology of Maine 1998
Harry Thurston The Atlantic Coast: A Natural History 2011
David L. Kendall Glacier and Granite 1987
The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf
Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week looking at some of the landscape features the glaciers left as they retreated 13,000 years ago here in Maine. That period of deglaciation had another profound impact on the Maine landscape, particularly the one most people know Maine for: the coast.
When we look at the location of the coast line, we are really looking at the edge of the water relative to the elevation of the landscape. If the landform is low, the water can move in, if the land form is high, the water is pushed out. There are two factors here that can change, the amount or volume of the water (the more water there is, the more three dimensional space it takes up, the higher it rises relative to the land, if all things are equal), and the level of the land (this is trickier to get your head around, but just remember that the crust of the Earth is floating, just like a ship at sea—the heavier the boat, the lower it floats. Solid crust floats on the more plastic layers of the Earth below.)
When we look at these two things relative to glaciation, we see the complicated and variable history of Maine’s coastline in the past 13,000 years. When glaciers form, they form from water, generally liquid water that evaporates into the atmosphere and then falls back to Earth in solid form. Where does all that liquid come from? The ocean, so overall we see a drop in global sea level when large continental glaciers form, because so much liquid water is taken up out of the ocean. At the height of the last glacial advance, it is thought that sea levels in our neck of the woods were 300 to 400 feet lower than today as a result of this (a level that would have easily exposed the continental shelf, had it not been covered by ice).
The amount of water isn’t the only thing that can change however, remember, the actual relative level of the land can as well. Remember the ship I mentioned, when it is heavy is floats lower in the water, when it is empty it floats at a higher level. The crust that underlies the state of Maine is like that ship. And what could make the crust heavier than normal? How about 10,000 feet of solid ice? That would do it. The continental glacier was so large and massive, it caused the crust to become less buoyant and float a bit lower on the underlying mantle. The crust was literally depressed into the Earth. As a side note, this is happening today in Antarctica, much of that continent is actually below sea level as a result of the weight of its ice cap.
So that is how the stage was set when the last ice age began to end here in Maine, about 13,000ish years ago. Just like a trampoline with some one sitting on it, the Earth’s crust is deformed by the weight of the glacier, and there was less water in the ocean. That less water in the ocean bit was changing fast though, as the glacier was melting quite rapidly. All of that melt water returned to the ocean, which rose quite rapidly in response. And as the glacial melted, it relived the pressure on the underlying crust, and just like a trampoline when you bounce off it, the crust rebounded. These two events didn’t happen simultaneously though, the glacier melted a great deal faster than the Earth rebounded, which meant, the ocean water easily covered the newly exposed but still depressed landscape of Maine. This event is called the Marine Incursion and at its maximum the coastline was as far inland as Medway, near Baxter State Park. Soon though the Earth’s rebound caught up with the rising ocean, and then some (again, think of our trampoline; a bouncer gets pushed up above the equilibrium level before coming back down). At the height of the crust’s rebound, sea level was much lower than it is today, as the land rose up above the level of the ocean. During this period, about 11,000 years ago, much of the Gulf of Maine, in particular Georges Bank on the edge of the continental shelf, was dry land. Terrestrial fossils, including trees and mammoth tusks have been recovered from Georges Bank, and the Gulf of Maine was a shallow inland sea cut off from the Atlantic Ocean. Soon thereafter though, the rebound eased and even subsided a bit, and the rising ocean caught up. The present day coastline was more or less established in the last few thousand years, as the rapid changes that resulted from deglaciation ended and temporary equilibrium was restored.
Maine has what is called a drowned coastline, and I hope now you can see why. Next week we will look at the third major force that shapes the Maine landscape, us.
References:
D. W. Caldwell Roadside Geology of Maine 1998
Harry Thurston The Atlantic Coast: A Natural History 2011
David L. Kendall Glacier and Granite 1987
The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf
Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php
Monday, May 20, 2013
The History of Maine Part 9: The Glacial Landscape
Note: This program first aired May 11, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week talking about the patterns of glaciation in this past ice age.
As I said last week, each time an ice sheet advances, it essentially wipes out the evidence left by the ice sheet before it, so while there were several cycles of advance and retreat here in Maine, we really only see evidence from the last one.
The ice sheet originated in northern Canada, and spread under its own weight as far south as Long Island New York. It reached its maximum extent some time between 24,000, and 28,000 years ago, mere nanoseconds on the geologic timescale. As a glacier moves over the landscape, it scours the earth, scraping away all the soil and loose debris down to the bed rock. Much of that material gets incorporated into the base of the glacier, frozen into the ice, and acts as giant grit sand paper, further scouring and smoothing the bedrock below. So the first effect of the glacier on Maine’s landscape is one of smoothing, rounding and eroding. The repeated scourings that resulted from the cycles of advance and retreat have given us the rolling landscape we see here today.
By picking up all that surficial material, the glacier is also a very effective earth mover. Rocks originating in bedrock in one part of the state, can be found on the tops of mountains on the other side of the state, confusing many a geologist in the days before the ice age was understood. We can think of the second effect of the glacier as being that of a redistributor of the surface sediments and materials of a Maine.
When the glacier melted, it dropped all of that material in place, so most of Maine is covered with a layer of what is called glacial till, essentially gravel, sand and rocks, broken up to varying degrees and transported in the glacial ice. A glacier is in retreat when the leading edge is melting faster than snow is accumulating at the center. The leading edge melts back and leaves all that till behind. A glacier advances when the snow accumulation rate is higher than the melting rate at the leading edge. Sometimes however, the rate of melting equals the rate of snow accumulation, and the glacier appears to stand still. It isn’t really stationary, as it is still advancing, but the leading edge is melting at the same rate, so the edge of the glacier is stationary relative to the land around it. When this happens the glacier is still melting and dumping till, but it isn’t moving back when it does this, meaning, a big ridge of till piles up at the foot of the glacier. That pile of till is called a moraine, and they are very common on the Maine landscape. My house is built into the end of one. Moraines tend to be hills or ridges full of unsorted till, meaning you find all sizes of materials in them, in no logical pattern. Giant boulders coexist with fine sand or gravel. They were simply dropped where the ice melted, with virtually no sorting of any kind.
Another thing that happens when a glacier melts is the formation of rivers of melt water within and underneath the ice. All that water has to go some were, and channels form in cracks in the ice, that follow gravity and eventually make it to the ground, melting their way out to the snout of the glacier. As the ice melts, it releases its sediment burden, but in this case, the sediment is released into moving water. The energy of the moving water provides a mechanism to sort the sediment; it takes more energy to move a large boulder than it does to move a small grain of sand. Deposits from this melt water are called eskers, and they are characterized by well sorted sediment, making them a favorite for gravel pit operators. They also tend to be quite prominent and run for long distances on top of the surrounding landscape. Many roads in Maine run along the tops of eskers as a result.
We’ll finish today with everyone’s favorite depositional feature of our glacial landscape, glacial erratics. These are large boulders dropped by the glacier as it melted, that stand out prominently on the landscape, instead of being buried in glacial till. In some cases they were deposited as part of glacial till and either by the luck of the draw or erosion of the surrounding sediment, ended up on the surface. In other cases though they were deposited by icebergs. Yes, at many points in the retreat of the last glacier in Maine, the sea came right up to the melting edge of the glacier (much like some of the glaciers in western Greenland today). The melting edge of the glacier could actually float on the sea water, and chunks of it would break off. These chunks or ice bergs still held the rocks, boulders and gravel that makes up glacial till, and as they melted the rocks rained down onto the bottom of the sea. Large boulders deposited this way are called drop stones.
Those are some of the depositional features we see around us here in Maine that resulted from the retreat of the last glacier not so very long ago. We’ll leave off there today, but join us in the coming weeks as we finish up the glacier chapter of the story of Maine’s landscape history.
References
D. W. Caldwell, Roadside Geology of Maine
David L. Kendall Glaciers and Granite: A guide to Maine’s landscape and geology, 1987 North Country Press, Unity Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week talking about the patterns of glaciation in this past ice age.
As I said last week, each time an ice sheet advances, it essentially wipes out the evidence left by the ice sheet before it, so while there were several cycles of advance and retreat here in Maine, we really only see evidence from the last one.
The ice sheet originated in northern Canada, and spread under its own weight as far south as Long Island New York. It reached its maximum extent some time between 24,000, and 28,000 years ago, mere nanoseconds on the geologic timescale. As a glacier moves over the landscape, it scours the earth, scraping away all the soil and loose debris down to the bed rock. Much of that material gets incorporated into the base of the glacier, frozen into the ice, and acts as giant grit sand paper, further scouring and smoothing the bedrock below. So the first effect of the glacier on Maine’s landscape is one of smoothing, rounding and eroding. The repeated scourings that resulted from the cycles of advance and retreat have given us the rolling landscape we see here today.
By picking up all that surficial material, the glacier is also a very effective earth mover. Rocks originating in bedrock in one part of the state, can be found on the tops of mountains on the other side of the state, confusing many a geologist in the days before the ice age was understood. We can think of the second effect of the glacier as being that of a redistributor of the surface sediments and materials of a Maine.
When the glacier melted, it dropped all of that material in place, so most of Maine is covered with a layer of what is called glacial till, essentially gravel, sand and rocks, broken up to varying degrees and transported in the glacial ice. A glacier is in retreat when the leading edge is melting faster than snow is accumulating at the center. The leading edge melts back and leaves all that till behind. A glacier advances when the snow accumulation rate is higher than the melting rate at the leading edge. Sometimes however, the rate of melting equals the rate of snow accumulation, and the glacier appears to stand still. It isn’t really stationary, as it is still advancing, but the leading edge is melting at the same rate, so the edge of the glacier is stationary relative to the land around it. When this happens the glacier is still melting and dumping till, but it isn’t moving back when it does this, meaning, a big ridge of till piles up at the foot of the glacier. That pile of till is called a moraine, and they are very common on the Maine landscape. My house is built into the end of one. Moraines tend to be hills or ridges full of unsorted till, meaning you find all sizes of materials in them, in no logical pattern. Giant boulders coexist with fine sand or gravel. They were simply dropped where the ice melted, with virtually no sorting of any kind.
Another thing that happens when a glacier melts is the formation of rivers of melt water within and underneath the ice. All that water has to go some were, and channels form in cracks in the ice, that follow gravity and eventually make it to the ground, melting their way out to the snout of the glacier. As the ice melts, it releases its sediment burden, but in this case, the sediment is released into moving water. The energy of the moving water provides a mechanism to sort the sediment; it takes more energy to move a large boulder than it does to move a small grain of sand. Deposits from this melt water are called eskers, and they are characterized by well sorted sediment, making them a favorite for gravel pit operators. They also tend to be quite prominent and run for long distances on top of the surrounding landscape. Many roads in Maine run along the tops of eskers as a result.
We’ll finish today with everyone’s favorite depositional feature of our glacial landscape, glacial erratics. These are large boulders dropped by the glacier as it melted, that stand out prominently on the landscape, instead of being buried in glacial till. In some cases they were deposited as part of glacial till and either by the luck of the draw or erosion of the surrounding sediment, ended up on the surface. In other cases though they were deposited by icebergs. Yes, at many points in the retreat of the last glacier in Maine, the sea came right up to the melting edge of the glacier (much like some of the glaciers in western Greenland today). The melting edge of the glacier could actually float on the sea water, and chunks of it would break off. These chunks or ice bergs still held the rocks, boulders and gravel that makes up glacial till, and as they melted the rocks rained down onto the bottom of the sea. Large boulders deposited this way are called drop stones.
Those are some of the depositional features we see around us here in Maine that resulted from the retreat of the last glacier not so very long ago. We’ll leave off there today, but join us in the coming weeks as we finish up the glacier chapter of the story of Maine’s landscape history.
References
D. W. Caldwell, Roadside Geology of Maine
David L. Kendall Glaciers and Granite: A guide to Maine’s landscape and geology, 1987 North Country Press, Unity Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
The Maine Geological Survey makes its surficial geology map available on line, in a printable 11x17 inch format. It has lots of good info on the results of the last glacial advance and retreat, and it’s free!
http://www.maine.gov/doc/nrimc/mgs/pubs/online/surficial/surficial11x17.pdf
The History of Maine Part 8: Ice Age Patterns
Note: This program first aired May 4, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week talking about the basic mechanics of continental glaciers, from their formation to their plastic-y deformation and movement.
The most recent ice age on Earth began about two and a half million years ago, and marked the beginning of what geologists refer to as the Quaternary Period. This ice age was the culmination of a gradual cooling trend in Earth’s overall climate that began about 75 million years ago, while dinosaurs still roamed the Earth. On the geologic time scale, two and a half million years is chump change, and when compared to the time required for opening and closing of an ocean or the subduction of a plate, it is hard to believe anything significant could happen to a landscape in such a short time. But believe it. The effects of the ice age absolutely define the hardscrabble nature of Maine’s land forms and biotic communities today.
The ice age that began two and a half million years ago* is actually a series of repeating expansions and retreats of continental glaciers, which originate at high latitudes, and spread into temperate or mid latitudes at their furthest reach. The pattern has been generalized as 100,000 year cycles, about 60-90 thousand years of glaciation, followed by a warmer period lasting 10 to as much as 40 thousand years, but there is certainly variability. These warmer periods are called interglacials, and we are in one right now—if we weren’t, Maine winters and summers would be a whole lot colder than we currently experience.
It is difficult to say with certainty just how many times the continental glacier that originated in northern Canada advanced over the landscape of Maine, as each glacial advance essentially obliterates all evidence left by the glacial advance before it. Billions and billions of pounds of one to two mile thick ice quite literally wipes the slate clean every where it goes. But we do have a very clear picture of the global climate during this time, which gives us a good sense of cooler periods and warmer periods, which are relatively safe to correlate with periods of glacial advance and retreat in the northern hemisphere. The climate record is recorded in the ice sheets that persist today (in Antarctica, in Alaska, in Greenland). Scientists drill into these ice sheets and retract a core (essentially a long skinny tube) of ice. Remembering that glaciers are formed from repeated snow fall, researchers can actually discern the layers of snow, compressed but still distinct, and like scientists looking at tree rings, actually reconstruct the climate history with amazing resolution. Tiny bubbles of gas from the fossil atmosphere and dust, ash and other atmospheric particles are clues that aid this research.
While all this glacial activity was going on, and the Earth was over all a bit cooler and probably a bit drier as well, something else was happening, something we are pretty interested in. Importantly for us, this past ice age coincides with the period of hominid evolution, us. While it is impossible to say that humans evolved because of the ice age, it is true that we evolved in a relatively uncommon climatic regime. In the big picture, Earth has been much much warmer than it is now, humans evolved during a cool spell. While it is hard to predict with any certainty just how hot it will get as a result of human influenced climate change, the possibility that keeps climate scientists up at night is that it will get warmer than humans, ANY humans have ever experienced. That is the kind of climate shift that marks the end of the evolutionary line for many a species. We see it over and over again in the fossil record. That is why many people define climate change as an existential threat, it is estimated that 99% of every species that has lived on Earth is extinct, primarily due to the fact that they were not able to evolve fast enough to adapt to rapidly changing climate regimes. Ice ages come and ice ages go, changing the shape of the land along the way, but probably more importantly, ice ages are a symptom of instability in the global thermostat, with significant repercussions for everything that lives on Earth.
We digressed a bit today, but join us next week as we get back on track and look at the direct impact of the glaciers on the landscape of Maine.
References:
Caldwell, D. W. Roadside geology of Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php
Nice bigger picture material from PBS (written quite a while ago by a UMaine researcher):
http://www.pbs.org/wgbh/nova/earth/cause-ice-age.html
Some scholarly business, though there has been a flurry of research in the past two decades on ice sheets, so this is likely a bit dated: Richmond, G.M. and D.S. Fullerton, 1986, Summation of Quaternary glaciations in the United States of America. Quaternary Science Reviews. vol. 5, pp. 183-196
On Extinction—From the Bristol University late Triassic Website: http://palaeo.gly.bris.ac.uk/palaeofiles/triassic/extinction.htm
Explore UMaine’s Climate Change Institute http://climatechange.umaine.edu/ especially this link to the basics of ice core research:
http://climatechange.umaine.edu/icecores/IceCore/Ice_Core_101.html
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week talking about the basic mechanics of continental glaciers, from their formation to their plastic-y deformation and movement.
The most recent ice age on Earth began about two and a half million years ago, and marked the beginning of what geologists refer to as the Quaternary Period. This ice age was the culmination of a gradual cooling trend in Earth’s overall climate that began about 75 million years ago, while dinosaurs still roamed the Earth. On the geologic time scale, two and a half million years is chump change, and when compared to the time required for opening and closing of an ocean or the subduction of a plate, it is hard to believe anything significant could happen to a landscape in such a short time. But believe it. The effects of the ice age absolutely define the hardscrabble nature of Maine’s land forms and biotic communities today.
The ice age that began two and a half million years ago* is actually a series of repeating expansions and retreats of continental glaciers, which originate at high latitudes, and spread into temperate or mid latitudes at their furthest reach. The pattern has been generalized as 100,000 year cycles, about 60-90 thousand years of glaciation, followed by a warmer period lasting 10 to as much as 40 thousand years, but there is certainly variability. These warmer periods are called interglacials, and we are in one right now—if we weren’t, Maine winters and summers would be a whole lot colder than we currently experience.
It is difficult to say with certainty just how many times the continental glacier that originated in northern Canada advanced over the landscape of Maine, as each glacial advance essentially obliterates all evidence left by the glacial advance before it. Billions and billions of pounds of one to two mile thick ice quite literally wipes the slate clean every where it goes. But we do have a very clear picture of the global climate during this time, which gives us a good sense of cooler periods and warmer periods, which are relatively safe to correlate with periods of glacial advance and retreat in the northern hemisphere. The climate record is recorded in the ice sheets that persist today (in Antarctica, in Alaska, in Greenland). Scientists drill into these ice sheets and retract a core (essentially a long skinny tube) of ice. Remembering that glaciers are formed from repeated snow fall, researchers can actually discern the layers of snow, compressed but still distinct, and like scientists looking at tree rings, actually reconstruct the climate history with amazing resolution. Tiny bubbles of gas from the fossil atmosphere and dust, ash and other atmospheric particles are clues that aid this research.
While all this glacial activity was going on, and the Earth was over all a bit cooler and probably a bit drier as well, something else was happening, something we are pretty interested in. Importantly for us, this past ice age coincides with the period of hominid evolution, us. While it is impossible to say that humans evolved because of the ice age, it is true that we evolved in a relatively uncommon climatic regime. In the big picture, Earth has been much much warmer than it is now, humans evolved during a cool spell. While it is hard to predict with any certainty just how hot it will get as a result of human influenced climate change, the possibility that keeps climate scientists up at night is that it will get warmer than humans, ANY humans have ever experienced. That is the kind of climate shift that marks the end of the evolutionary line for many a species. We see it over and over again in the fossil record. That is why many people define climate change as an existential threat, it is estimated that 99% of every species that has lived on Earth is extinct, primarily due to the fact that they were not able to evolve fast enough to adapt to rapidly changing climate regimes. Ice ages come and ice ages go, changing the shape of the land along the way, but probably more importantly, ice ages are a symptom of instability in the global thermostat, with significant repercussions for everything that lives on Earth.
We digressed a bit today, but join us next week as we get back on track and look at the direct impact of the glaciers on the landscape of Maine.
References:
Caldwell, D. W. Roadside geology of Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
Super nerds unite: check out the Maine Ice Age Trail, and you too can tour Hancock and Washington counties looking at gravel pits (I’ve done it, its awesome). http://iceagetrail.umaine.edu/ also includes an excellent overview of the last ice age in Maine http://iceagetrail.umaine.edu/content/iceageinmaine/iceageinmaine.php
Nice bigger picture material from PBS (written quite a while ago by a UMaine researcher):
http://www.pbs.org/wgbh/nova/earth/cause-ice-age.html
Some scholarly business, though there has been a flurry of research in the past two decades on ice sheets, so this is likely a bit dated: Richmond, G.M. and D.S. Fullerton, 1986, Summation of Quaternary glaciations in the United States of America. Quaternary Science Reviews. vol. 5, pp. 183-196
On Extinction—From the Bristol University late Triassic Website: http://palaeo.gly.bris.ac.uk/palaeofiles/triassic/extinction.htm
Explore UMaine’s Climate Change Institute http://climatechange.umaine.edu/ especially this link to the basics of ice core research:
http://climatechange.umaine.edu/icecores/IceCore/Ice_Core_101.html
Monday, April 22, 2013
The History of Maine Part 7: How Glaciers Work
Note: This program first aired on April 20, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week shifting focus in our exploration of Maine’s landscape history, by moving from tectonic forces to the power of ice. Today we take some time to look at the basic mechanics of an ice sheet.
Approximately 2 million years ago, the North American continent, in essentially the same global position it is in now, entered an ice age. When we say ice age, we mean that ice covered a significant percentage of the Earth’s surface. In the case of this past ice age, at its peak, ice covered about 32% of the land surface and 30% of the oceans, significantly more than is covered today. While the cause of an initiation of an ice age is still under scientific investigation, when climatic conditions are just right, glaciers will form and behave in a well documented manner.
The climatic conditions that support the growth of glaciers are high winter snow fall, combined with cool summer temperatures. You see, glaciers, whether they form in the mountains or in the middle of a continental land mass, are formed from snow and only snow. The climate must be cool enough, at least regionally, that precipitation falls as snow, in at least the winter. The summers, if we can call them that, must be cool enough that all that snow doesn’t go away. At its simplest, a glacier starts as a multi year accumulation of snow.
The glaciers that define an ice age are continental glaciers, meaning they form large sheets of ice that cover virtually everything on a continent. Think of the ice coverage of the continent of Antarctica today. They form when it snows in the winter, and that snow doesn’t melt in the summer, over and over again, on a very large scale. After a few years, that multi year snow transforms physically to something called firn (f I r n). As snow ages it undergoes metamorphism, the snow crystals, or flakes, break down and become more rounded, and start to bond together-that’s firn, an intermediate stage between fresh snow and ice. More snow piles on top, and the weight of that snow presses down on the older snow below and accelerates this process.
As the firn snow gets more and more compressed, by the weight of the accumulating snow on top of it, the air pockets trapped by the original snow fall (think fluffy powder snow) get more and more compressed, and are slowly forced out of the compacting snow pack. It may take up to one hundred years, but this compaction gradually changes the original snow (fluffy, white!) into a solid blue material we would all look at and recognize as ice.
We think of ice as hard and solid, especially when we are first learning to ice skate, but in reality, ice is more malleable, and nowhere do we see this more clearly than in glaciers. Just like the atmosphere has mass and weighs upon us here on the surface of the earth (otherwise known as the bottom of the atmosphere), and the water of the ocean has mass weighs down on the bottom of the sea, the snow that accumulates on a glacier has mass. As that snow accumulates it gets heavier, it piles higher, it literally builds up. Once the ice gets thick enough, once enough snow has accumulated and weighs down on the snow underneath it, the ice that is formed starts to deform; in geological terminology we say the ice has become plastic.
Gravity doesn’t like it when some things (any things) are higher than other things. Gravity wants everything to be in equilibrium, in other words, at the same level. When the things that are higher than other things are solid, like mountains, gravity can’t do anything about it except wait for erosion. But if the things that are higher than their surroundings are fluid, or plastic, they are capable of flow, and will yield (albeit slowly, in the case of a glacier) to the power of gravity. Which is to say, when a continental glacier gets big enough, and thick enough, it will start to flow outward in all directions, sliding on its base, where it is in contact with the land below, and internally deforming (or squishing) in between the surface and base. The glacier will continue to spread as long as snow keeps falling on the interior, and more snow accumulates than melts each year. A glacier is said to be in retreat, when it is melting faster than it is forming. In this way the size of the glacier is directly related to climate, which is the primary reason that so many climate scientists, including several world class ones here in Maine, study the dynamics of the world’s remaining ice sheets in their pursuit of the keys to climate change.
We’ll leave it off there for today, but join us in the coming weeks as we look into the details of the past 2 million years of glacial advance and retreat, and what that has meant for the landscape we see around us today.
Approximately 2 million years ago, the North American continent, in essentially the same global position it is in now, entered an ice age. When we say ice age, we mean that ice covered a significant percentage of the Earth’s surface. In the case of this past ice age, at its peak, ice covered about 32% of the land surface and 30% of the oceans, significantly more than is covered today. While the cause of an initiation of an ice age is still under scientific investigation, when climatic conditions are just right, glaciers will form and behave in a well documented manner.
The climatic conditions that support the growth of glaciers are high winter snow fall, combined with cool summer temperatures. You see, glaciers, whether they form in the mountains or in the middle of a continental land mass, are formed from snow and only snow. The climate must be cool enough, at least regionally, that precipitation falls as snow, in at least the winter. The summers, if we can call them that, must be cool enough that all that snow doesn’t go away. At its simplest, a glacier starts as a multi year accumulation of snow.
The glaciers that define an ice age are continental glaciers, meaning they form large sheets of ice that cover virtually everything on a continent. Think of the ice coverage of the continent of Antarctica today. They form when it snows in the winter, and that snow doesn’t melt in the summer, over and over again, on a very large scale. After a few years, that multi year snow transforms physically to something called firn (f I r n). As snow ages it undergoes metamorphism, the snow crystals, or flakes, break down and become more rounded, and start to bond together-that’s firn, an intermediate stage between fresh snow and ice. More snow piles on top, and the weight of that snow presses down on the older snow below and accelerates this process.
As the firn snow gets more and more compressed, by the weight of the accumulating snow on top of it, the air pockets trapped by the original snow fall (think fluffy powder snow) get more and more compressed, and are slowly forced out of the compacting snow pack. It may take up to one hundred years, but this compaction gradually changes the original snow (fluffy, white!) into a solid blue material we would all look at and recognize as ice.
We think of ice as hard and solid, especially when we are first learning to ice skate, but in reality, ice is more malleable, and nowhere do we see this more clearly than in glaciers. Just like the atmosphere has mass and weighs upon us here on the surface of the earth (otherwise known as the bottom of the atmosphere), and the water of the ocean has mass weighs down on the bottom of the sea, the snow that accumulates on a glacier has mass. As that snow accumulates it gets heavier, it piles higher, it literally builds up. Once the ice gets thick enough, once enough snow has accumulated and weighs down on the snow underneath it, the ice that is formed starts to deform; in geological terminology we say the ice has become plastic.
Gravity doesn’t like it when some things (any things) are higher than other things. Gravity wants everything to be in equilibrium, in other words, at the same level. When the things that are higher than other things are solid, like mountains, gravity can’t do anything about it except wait for erosion. But if the things that are higher than their surroundings are fluid, or plastic, they are capable of flow, and will yield (albeit slowly, in the case of a glacier) to the power of gravity. Which is to say, when a continental glacier gets big enough, and thick enough, it will start to flow outward in all directions, sliding on its base, where it is in contact with the land below, and internally deforming (or squishing) in between the surface and base. The glacier will continue to spread as long as snow keeps falling on the interior, and more snow accumulates than melts each year. A glacier is said to be in retreat, when it is melting faster than it is forming. In this way the size of the glacier is directly related to climate, which is the primary reason that so many climate scientists, including several world class ones here in Maine, study the dynamics of the world’s remaining ice sheets in their pursuit of the keys to climate change.
We’ll leave it off there for today, but join us in the coming weeks as we look into the details of the past 2 million years of glacial advance and retreat, and what that has meant for the landscape we see around us today.
References:
Caldwell, D. W. Roadside Geology of Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
Caldwell, D. W. Roadside Geology of Maine
http://nsidc.org/cryosphere/glaciers/life-glacier.html
The National Snow and Ice Data Center (yes there is such a thing!) All About Glaciers!
When seen from above, it is much easier to see a glacier's fluid nature. From NASA’s Earth Observatory website (a must visit—they have a weekly email list serve for serious nerds, of which I am one). http://earthobservatory.nasa.gov/IOTD/view.php?id=4710
Wednesday, April 17, 2013
The History of Maine Part 6: The Ice Age
Note: This program first aired on April 13, 2013.
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week about 200 million years ago, with the opening of the Atlantic Ocean. Europe and North America split apart and started moving away from each other, at about the same rate as our fingernails grow.
With the exception of a hot spot that New England drifted over during this time, there was little volcanic or tectonic activity between then and now. North America slowly drifted poleward from the equator, and the climate of Maine changed accordingly. The Appalachian Mountains eroded, sending a huge sediment load oceanward, forming the coastal plane we see today on the east coast south of Long Island. The continents, freed from the bondage of Pangea, moved slowly over the surface of the Earth, coming closer and closer to their present day positions. And it is this continental movement and positioning that may have set in motion what happened next.
Up until this point, we have been referring to a timescale of 10s to 100’s of millions of years; big and fairly imprecise chunks of time, inferred from a geologic record of highly metamorphosed rocks, thousands of feet of sediment, and broad brush continuities on a global scale. For this next part of the story, we need to hone our gaze and zoom in quite a bit, this next chapter covers only 3 million years at best. We can call this chapter: The Ice Age, and technically, because there are still ice caps on Greenland and Antarctica, we’re still in it.
Ice ages have occurred throughout Earth’s history, current thinking puts the number at 5. The causes of the ice ages are not well understood, though there are many scientists working on this question, as the understanding of what is called “climate forcing” is directly related to current investigations of climate change. Causes likely include changes in oceanic circulation due to continental drift (as the continents move the ocean basins change shape, and currents can be redirected to higher or lower latitude), changes in atmospheric composition (including key greenhouse gasses like carbon dioxide and methane), fluxuation of solar out put, changes in planetary orbits (called orbital forcing) and changes in atmospheric circulation due to tectonic uplift. Its pretty complex stuff, and the scientists working on this are trying to reconstruct all of these factors and let them run in fantastically complicated computer models, hoping that what the models predict should have happened corresponds with the known geologic record.
So for this last ice age, which stretches back about 2.5 million years to the beginning of the Quaternary Period, one event that coincided with the start of global cooling was the cutting off of the Atlantic from the Pacific when the isthmus of Panama fully formed. This changed circulation patterns in both oceans, especially the Atlantic, and it drove the current we now call the Gulf Stream further north. The Gulf Stream is a warm water current, and you may be wondering how a warm water current moving further north triggers a global ice age. This is a reasonable question, however the concept is that warm air is able to hold more moisture than cold air, and warm water evaporates more readily than cold water. Both of these factors put more moisture into the atmosphere in higher, cooler latitudes, which can then be precipitated out. That increase in precipitation could have yielded the continental glaciers that covered much of the northern hemisphere during periods of the past two and a half million years. Is that exactly what happened? We don’t know, yet. Remember, science is about noticing patterns and then trying to explain them with additional evidence. In the Western scientific tradition, we’ve been noticing and trying to explain glaciers for the past 200 years, and much of the current research is simply about trying to discern the observable patterns at higher and higher resolution. Questions about the current patterns of climate change are driving us to want to know more and more about significant climate change events in the past, but most of these questions are still wide open, which makes the research that much more exciting.
-->
We’re spending a few weeks here on the world around us, tracing the deep history of Maine, from its geological genesis to the current day. We left off last week about 200 million years ago, with the opening of the Atlantic Ocean. Europe and North America split apart and started moving away from each other, at about the same rate as our fingernails grow.
With the exception of a hot spot that New England drifted over during this time, there was little volcanic or tectonic activity between then and now. North America slowly drifted poleward from the equator, and the climate of Maine changed accordingly. The Appalachian Mountains eroded, sending a huge sediment load oceanward, forming the coastal plane we see today on the east coast south of Long Island. The continents, freed from the bondage of Pangea, moved slowly over the surface of the Earth, coming closer and closer to their present day positions. And it is this continental movement and positioning that may have set in motion what happened next.
Up until this point, we have been referring to a timescale of 10s to 100’s of millions of years; big and fairly imprecise chunks of time, inferred from a geologic record of highly metamorphosed rocks, thousands of feet of sediment, and broad brush continuities on a global scale. For this next part of the story, we need to hone our gaze and zoom in quite a bit, this next chapter covers only 3 million years at best. We can call this chapter: The Ice Age, and technically, because there are still ice caps on Greenland and Antarctica, we’re still in it.
Ice ages have occurred throughout Earth’s history, current thinking puts the number at 5. The causes of the ice ages are not well understood, though there are many scientists working on this question, as the understanding of what is called “climate forcing” is directly related to current investigations of climate change. Causes likely include changes in oceanic circulation due to continental drift (as the continents move the ocean basins change shape, and currents can be redirected to higher or lower latitude), changes in atmospheric composition (including key greenhouse gasses like carbon dioxide and methane), fluxuation of solar out put, changes in planetary orbits (called orbital forcing) and changes in atmospheric circulation due to tectonic uplift. Its pretty complex stuff, and the scientists working on this are trying to reconstruct all of these factors and let them run in fantastically complicated computer models, hoping that what the models predict should have happened corresponds with the known geologic record.
So for this last ice age, which stretches back about 2.5 million years to the beginning of the Quaternary Period, one event that coincided with the start of global cooling was the cutting off of the Atlantic from the Pacific when the isthmus of Panama fully formed. This changed circulation patterns in both oceans, especially the Atlantic, and it drove the current we now call the Gulf Stream further north. The Gulf Stream is a warm water current, and you may be wondering how a warm water current moving further north triggers a global ice age. This is a reasonable question, however the concept is that warm air is able to hold more moisture than cold air, and warm water evaporates more readily than cold water. Both of these factors put more moisture into the atmosphere in higher, cooler latitudes, which can then be precipitated out. That increase in precipitation could have yielded the continental glaciers that covered much of the northern hemisphere during periods of the past two and a half million years. Is that exactly what happened? We don’t know, yet. Remember, science is about noticing patterns and then trying to explain them with additional evidence. In the Western scientific tradition, we’ve been noticing and trying to explain glaciers for the past 200 years, and much of the current research is simply about trying to discern the observable patterns at higher and higher resolution. Questions about the current patterns of climate change are driving us to want to know more and more about significant climate change events in the past, but most of these questions are still wide open, which makes the research that much more exciting.
-->
References:
Yep, these references really hit all the key points: same as for “The History of Maine: Part 1”. http://theworldaroundusradio.blogspot.com/2013/02/the-history-of-maine-part-1.html
Good overview of the Quaternary Period from National
Geographic http://science.nationalgeographic.com/science/prehistoric-world/quaternary/
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