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.

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!

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). also includes an excellent overview of the last ice age in Maine

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.


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
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!

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.


Caldwell, D. W. Roadside geology of Maine
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). also includes an excellent overview of the last ice age in Maine

Nice bigger picture  material from PBS (written quite a while ago by a UMaine researcher):

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:

Explore UMaine’s Climate Change Institute especially this link to the basics of ice core research: