A Post-Glacial World

          Everywhere you look, New England’s landscape shows what it went through when a mile-thick sheet of ice came down from the north.

Whether or not America has become post-racial, there’s no doubt about New England:  here, we’re post-glacial. The mind-bendingly powerful forces that shaped this region during the last Ice Age both dwarf our human affairs and have played a big role in shaping them. Everything from our signature crops to our stone walls to our favorite recreation spots are products of the massive ice sheets that moved across the land. In addition to those that are the handiwork of man, the built environment of New England is full of structures and landmarks made by nature. Heck, Plymouth Rock itself is a glacial erratic, carried south to its famous resting place by the ice.

Erratics are a good way to understand what happened as the climate cooled and the Ice Age began. Up in present-day Canada, annual snowfalls stopped melting away during the summers, piling up year after year, century after century, until, as Williams College geology professor David Dethier explains, the snowpack got to about 100 feet thick, whereupon it began to act like a liquid, flowing away from its thickest points in the northern interior and creeping south.

Along the way, it broke off and picked up immense numbers of rocks and boulders, grinding some into gravels and sand and carrying others along like so much multi-ton luggage. As National Geographic writer Hannah Holmes puts it, 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 23,000 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 Mounts Katahdin and Washington, which would have made it over a mile thick. It thinned towards 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 land forms are as distinctive as their wonderful names. 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 them, and the perhaps the most famous pond in the world, Walden, is a kettle hole as well.

Inside the melting glacier, all sorts of things were going on. Gravelly riverbeds formed on top, in the middle, and at the bottom, winding their way along until one day, the ice gone, they found themselves lying on the ground. These sinuous piles, called eskers, can go on for miles (Vermont’s Passumpsic Valley esker is 24 miles long, and in some places over 150 feet thick and 200 feet wide). In Cambridge’s Mount Auburn Cemetery, Indian Ridge Path runs atop an esker that is happily protected from the fate of many of its brethren, which have fallen victim to progress. A mile of two-lane highway requires some 45,000 tons of sand and gravel, while the average six-room house’s foundation eats up 90 tons. Eskers are easily mined for this so-called aggregate and many have disappeared.

Drumlins are little hills shaped like an inverted spoon, the steeper side towards the direction the ice came, and are probably the result of ice and water movement over piles of sediment that formed as the glacier melted. Boston Harbor holds a “swarm” of drumlins, about 30 of more than 200 in the greater Boston area. The American Revolution got going in earnest on two nearby drumlins, Breed’s and Bunker Hills, and Amherst College sits atop one today.

The edges of the retreating glaciers were chaotic zones of melting ice, water, and rock material of every size; depressions and crevasses filled with layers of sediment which then, like the eskers, found themselves stranded on the ground when the ice disappeared—kame is a kind of catch-all phrase for these dumped lumps. Visit Author’s Ridge in Concord, Massachusetts, where you’ll find the remains of Louisa May Alcott, Nathaniel Hawthorne, and Henry David the kettle-pond guy, all safety stashed on the kame formation of Sleepy Hollow Cemetery.

Some features have disappeared. When mounting debris coming off the ice sheet got caught between a rock (Rocky Hill, Connecticut, south of Hartford) and a hard place in the Connecticut River valley, a dam formed, backing up to form prehistoric Hitchcock Lake, a vast body of water that eventually extended all the way up to West Burke, Vermont, 250 miles away. When the dam burst, the lake drained, leaving a series of remnant shorelines and terraces on the hillsides high above the current Connecticut River.

Sometimes, the waters flowed unimpeded from the glacier front, forming vast outwash plains of sand, gravel, and boulders. Williams’ Dethier has a particular soft spot for one of these areas, the blueberry barrens of Downeast Maine, which make the perfect growing environment for the sweet native fruit, whose low bushes grow without competition in the poor soil. So replete is the state with glacial features of all types that the University of Maine has put together the Ice Age Trail, with nearly 50 stops stretching from Calais to Cadillac Mountain on Mount Desert Island.

Given what just happened on the campaign trail, that might be just the right trip to take.