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!