Along the way, it broke off and picked
up immense numbers of rocks and boulders, grinding some into gravel and sand
and carrying others along like so much
multiton luggage. A graphic description of
the scene comes from National Geographic’s Hannah Holmes, who writes that the
“southbound ice sheet slid over the loose
blocks like molasses over spilled sugar
and dragged them along.” As the ice sheet
finally began to melt around 2 3,000 years
ago, it dropped its bags as it retreated.
Harvard University’s Louis Agassiz, observing the many exposed boulders scattered across the Berkshires,
was the first to suggest, in 1871, that they
must have been carried there by ice. Not
far away, in present-day Madison, New
Hampshire, sits one of the largest, if not
the largest, erratics in all of North America. The Madison Boulder is 23 feet high,
37 feet thick, and 85 feet wide. Estimated
weight: 12 million pounds, or what scientists might call a wicked heavy lift.
The power of the ice was matched by
its sheer volume. There’s evidence that
it covered Mount Katahdin in Maine and
Mount Washington in New Hampshire,
each a mile or so tall, making the ice more
than a mile thick. It thinned toward its
melting southern limits, where warmer
temperatures halted its progress. The ice
kept coming but couldn’t advance across
the land anymore, its edge acting instead
like the end of a conveyor belt, continuously depositing loads of rock debris as it
turned to water.
The resulting landforms are as distinctive as their off beat names — terminal
moraines, kettle-hole ponds, eskers. The
largest are the terminal moraines, which
formed along the southernmost edges. Perhaps you’ve heard of some of them: Cape
Cod, Martha’s Vineyard, Nantucket, Block
Island, and Long Island. Their sandy soils
later held bogs, ideal for cranberries. Occasionally, a massive chunk of ice would fall
from the rotting face of the glacier into the
accumulating sand and gravel. Later, when
it melted, a kettle-hole pond would form.
Beautiful Watchaug Pond in Charlestown,
Rhode Island, is one, as are Spy and Fresh
ponds in Arlington and Cambridge, Massachusetts, respectively. Cape Cod is full of
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