BELFAST BAY (May 29): Editor's note: This is part of a continuing series of articles that take an in-depth look at the historic, social, economic and environmental forces shape the land and people of Belfast Bay. The stories will appear at intervals in the coming months.

The slow process of erosion over hundreds of millions of years largely created the landscape of Belfast Bay. But astonishingly rapid changes began to occur about 2 million years ago, when the northern latitudes entered the Ice Age.

Daniel Belknap, chairman of the earth sciences department at the University of Maine, said the arrival of the glaciers was cataclysmic. Until very recently, in geological terms, Maine was covered by the same coastal plain soil that is found in New Jersey and Delaware.

The ice sheets sculpted the landscape, fracturing loose bedrock and wiping away soil and vegetation, only to deposit the material somewhere else. The glaciers widened and deepened river channels as they moved generally southward, then blocked rivers and created new hillocks with rocks and soil as they retreated.

The Marsh Stream in Frankfort is one example, according to Belknap. The winding stream is probably the former course of the Penobscot River, which was blocked by material left behind by the glacier.

Despite the dramatic arrival of the ice, Belknap said the glaciers were really just a bit of cosmetic surgery on the face of Maine. "In the end, it's just a retouching of the landscape, no more than a few meters of change [in a landscape that has seen a mile or more of rock eroded over 200 million years]," he said.

Still, the ice sheets had profound impact on Maine. The red clays of Georgia are tens of millions of years old, while Maine's soil has largely developed over the last 15,000 years. The landscape is sharp and irregular — jagged bedrock, a bony coastline.

Evidence of earlier glaciers was largely wiped away by succeeding ice sheets. The glaciers are believed to have followed one another every 100,000 to 200,000 years, spurred by changes in the Earth's orbit, aspect and rotation called Milankovitch Cycles.

Three factors are at work: Over a 100,000-year period, the Earth's orbit around the sun can be more elliptical or more circular, changing the distance from our planet to the star at the summer and winter solstices. In addition, the tilt of the Earth on its axis changes on a 41,000-year cycle, which alters the amount of light that falls on the northern latitudes, and the distribution of light between winter and summer.

Finally, the planet "wobbles" as it spins, in 23,000-year cycles, which changes whether the northern hemisphere experiences the winter solstice when it is closer to, or farther from, the sun.

Put it all together and there is a substantial variation in the amount of solar radiation that reaches Belfast Bay and other parts of the northern hemisphere.

Belknap said the most recent glacier probably started about 35,000 years ago in northern Labrador and Quebec, and then grew to the south, east and west.

For glaciers to grow, snow must be able to remain on the ground over the summer. That’s caused by a combination of snowfall accumulation, temperature, local weather patterns and the “albedo effect,” in which the growing ice sheet acts like a mirror to reflect solar energy back into space.

The most recent glacier was called the Laurentide Ice Sheet. It ultimately stretched from Labrador to Long Island and the waters of the Gulf of Maine. The glacial edge began to retreat from Southern Maine about 17,000 years ago, and it was gone from Northern Maine some 5,000 years later.

In some places, the glacier was more than a mile high. The grinding force, the crushing weight of ice and rock and soil carried by the ice sheet struck New England like a bulldozer. And, like a mile-high bulldozer driven by a wild-eyed Atlas (the Greek god who carried the heavens on his shoulders), the ice sheet scraped the landscape clean. It peeled away soil and trees, smashed softer bedrock and polished harder material until it shined.

"At the base of the glacier were lots of rocks that acted like sandpaper," said Stephen Dickson, a marine geologist at the Maine Geological Survey. MGS offers a wealth of information about the state's geology on its website, maine.gov/doc/nrimc/mgs/explore/index.htm.

Acadia Mountain and the Camden Hills are typical of places that were scraped clean of soil by the glacier, leaving areas of bare ledge. Fractures caused some sections of bedrock to break loose — either quickly, from the incalculable weight of the glacier, or over centuries, as the freezing and thawing of water in the cracks pried apart the rock. The boulders were picked up by the ice sheet and became part of the southward juggernaut.

The weight of the ice was so great that the underlying bedrock could not support it, and areas of Maine farther inland than Bangor were pressed below sea level. As the glacier retreated, seawater rushed in, flooding most of Waldo County and the rest of the coast. Hills that today are above 300 feet in elevation became islands in a vastly different Gulf of Maine.

At the same time, material carried by the glacier began to be deposited, in several ways, onto the landscape. At the leading edge of the glacier, soil and rock accumulated and were pushed up by the ice, creating ridges called moraines that were left behind when the glacier melted. Long Island, N.Y., is a moraine that marks the southern extent of the Laurentide Ice Sheet.

In other places, melting water inside the glacier created streams that flowed in tunnels through the ice sheet. When the ice melted, sand and gravel in the streambeds were left behind in long, narrow ridges called eskers, horsebacks or whalebacks.

Throughout New England, a blanket of soil and rock — sometimes carried hundreds of miles — dropped haphazardly from the melting ice. The covering is generally thickest at the bottom of valleys, where it was carried by gravity and running water.

The debris that was picked up, carried and deposited by the glacier is called till. It is a mixture of everything from house-sized boulders to microscopic particles of clay. In some place the till is very thin or even nonexistent. In others, it is tens of feet deep. The generations of Waldo County children who cleared stones from farmers' fields can attest to the vast quantity of material dropped by the last glacier.

After the ice sheet retreated, the Maine coast was submerged for as long as 4,000 or 5,000 years as a thick coating of marine clay built up. "Mud coated everything," said Dickson, "and then the crust rebounded and left marine clay and shells fringing the bay."

Although the land rebounded, the sea level remained lower than present because of all the water that was still locked up in the retreating ice sheet. That meant places like Islesboro and Matinicus were part of the mainland for a time, according to Dickson.

The situation also led to the creation of a huge saltwater bay along the southern reaches of the Penobscot River between Rockland and Vinalhaven, which was then connected to the Blue Hill Peninsula. The bay was about 10 miles long, north to south, and four miles wide. It was eventually flooded by rising sea levels.

Dickson said the last 10,000 years, called the Holocene, have been a time of milder temperatures. The forests gradually returned to Maine, taking root in the till left behind by the ice sheet. But evidence of the glacier, and events stretching all the way back to the ancient island chain of Avalon, lies just below the surface.

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