Sunday, May 4, 2014

Groundtruth: Finding Annual Moraines in My Backyard

An annual glacial moraine.
Twenty-one thousand years ago glaciers covered the northern part of the globe.  Their end point in New England can be seen by following the line of Long Island, New York across to Cape Cod and Nantucket.  This relatively straight line is an artifact of the melting edge of the glacier dropping rocks and sediments delivered from the northern part of the globe, year after year.  As the pile of rocks got deeper and deeper it laid the foundation for these scenic places in New England.  Over time the Earth warmed and the glacier receded back to Maine, but it wasn't consistent.  Each winter the glacier moved forward a bit, and each summer it backed off.  Yard by yard and year by year it retreated to its current (not-so) stronghold at the North Pole.

LiDAR image of annual glacial moraines in West Falmouth.The
image above represents about a third of a mile from north to south.

A month or so ago, I ran into a former student who was researching the glacial history of Maine at Bowdoin, and he introduced me to LiDAR (Light Detection And Ranging) hillshade images that are available for large portions of Maine, including my own backyard.  What he revealed was that with the current imaging technology, we could see not only the large scale features, like Cape Cod, but the yearly inchings of the glacier. Unfulfilled with the images on the screen, I went out into the woods to groundtruth the pictures I saw.  Small hills and low areas I have walked over for years without really thinking about their origin were shown to be moraines dropped each summer by the ice sheet as it receded northward.
Looking up a moraine from a lowpoint in the terrain.

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