As you drive around Greenwich, you can't help but notice the excessive number of stones - of all shapes and sizes - haphazardly strewn across all parts of town, or in remnants of beautiful old stone walls most likely built over 100 years ago.   Each spring, when the ground has thawed, if it looks like there is a whole new "crop" of stones that seemed to spring up from the ground - that is exactly what happened.  Our farming predecessors called these "New England potatoes" and they had to clear the new stones every spring before they could plant the intended food.  

Clyne Ledges.

If you spot a giant boulder that appears as though it could have come from some place else-as if it had been randomly dropped out of the blue-your eyes are not deceiving you. The boulder did come from some place else and in geology terms it is called a glacial erratic. It could have come from hundreds of miles away-possibly Vermont or New Hampshire-and it was dropped out of the blue: a giant many-shades-of-blue ice sheet that covered the entire Northeast for thousands of years during the last Ice Age, about 18,000 years ago.  The story behind the stones and the unique natural landscapes of Greenwich is a fascinating one. These distinctive landscapes have been shaped by some of the strongest forces of nature on earth.

Quick Summary: The Continental Ice Sheet That 
Once Covered Greenwich

Much of what we see today in the varied natural landscapes of Greenwich is the result of the same geological forces that transformed the entire northeastern United States: a giant continental ice sheet that spread from the Arctic, covering Canada, and the northern regions of the United States.  Spreading southward, it covered the entire Northeast, which is why New England landscapes have common characteristics that are different from regions below New York.  All of the mountain ranges were covered—even Mt. Washington which was much higher before the ice sheet. The ice sheet covered the Greenwich area for thousands of years—was at its height 18,000 years ago—then as a result of a dramatic shift to warmer temperatures in the northern hemisphere, it melted and retreated, leaving in its wake the geography that we see today. By 12,000 years ago, Connecticut was entirely free of ice but its landscape was a barren wasteland of rocky glacial debris.

The Long Island Sound, a basin created by the enormous weight of the ice sheet, became a freshwater glacial lake separated from the Atlantic Ocean by an extensive ridge of piled up debris and sediment (called a terminal moraine). This ridge formed what is now Long Island, and extended all the way to Cape Cod. Remnants of this ridge still remain, but cannot be seen because it is under water (sailors know this from their navigation charts). What can be seen is now Block Island, Plum Island, Fishers Island, Nantucket, Martha's Vineyard, and all of the assorted smaller islands in between.

How We Know What We Know: Ice Sheets Still Exist 

There are two active ice sheets in the world today that are still giant-sized: in Greenland and Antarctica. As geologists continue to scientifically observe and measure these two ice masses, they continue to add to their understanding of what happened historically. The ice sheet that covers over 80% of Greenland (the largest island in the world) is of particular interest to us in North America because it is what remains of the continental ice sheet that once covered Greenwich so long ago. It has remained stationary over Greenland for the thousands of years since, allowing the people of Greenland to live and work around the fringes of the island.  Imagine living with a mile-thick blue ice sheet always behind you.

Glacial erratics.

Understanding the Ice Sheet: A Fascinating Process of Nature
Introduction
The ice sheet is a unique phenomenon of nature - an enormous, dynamic mass of ice that is constantly on the move, swooping up everything in its path. The life cycle of an ice sheet is directly correlated to the earth's climate cycles: cooling periods produce polar ice caps of glaciers and ice sheets; warming periods cause ice sheets to melt and retreat. What has caused the earth's climate to change is not well understood.  But geologists do know that there have been four such cycles which produced ice sheets from both poles in the last 100 million years.  Of the four cycles, geologists know that two continent-sized ice sheets reached as far south as Connecticut: the first was 150,000 years ago and the second was the one that occurred 18,000 years ago. It is amazing that we still see vestiges of this last ice sheet so prevalent in our landscape. 

Ice sheets are one of the strongest geological forces to change the shape of landforms on earth. During every stage of its life cycle—whether it is spreading, remaining stationary, or melting/retreating, the ice sheet is transforming the land beneath it. And given that all of this takes place over many thousands of years, the effects on the landscape can be dramatic indeed.

Ice Sheets Are Heavy 
An ice sheet develops a slick underbelly which allows it to spread over the ground. As it moves, it picks up everything that is not bedrock—boulders, trees, vegetation, clay, soil, sediment, etc.—which gets folded into the body of the ice sheet.  This becomes one huge scraping and grinding machine as the ice sheet spreads over the land, scouring away all in its path - even the tops of mountains. Snow continues to add to the mass of the ice sheet which ranges from one to two miles thick! The weight of a two-mile-thick slab of ice carrying millions of boulders and debris is difficult to imagine. But physical evidence shows that it weighed enough to crush down the earth's crust (where it could) by 300-600 feet.  This is how the Long Island Sound basin was created. 

Ice Sheet Eventually Stopped 
An ice sheet will continue to spread until it meets up against a warmer climate, whether from warmer ocean currents or warmer regional land temperatures. At this point the forward movement of the ice sheet is halted. It reaches an equilibrium and remains stationary until a change in climate conditions that is significant enough to cause a difference - either more spreading or begin melting. During the time that the ice sheet is stationary, it is still actively moving the processional of boulders and debris coming from inland along the underbelly. Geologists suggest thinking of it as a one-way conveyor belt—the boulders from the underbelly get moved up ward into the body of the ice sheet. The area where this moving and grinding process produces the greatest accumulation of boulders and stones is at the southernmost edge of the ice front—exactly the area of the ice sheet that sat over Greenwich for thousands of years.  When the ice sheet finally began to melt, the large boulders at the top were sprung loose and plummeted to the ground (out of the blue, as it were). And once the ice melted out, all of the millions of stones that were inside the body of the ice front were left in place - one gigantic unsorted pile of rocks.

Melting and Retreating Also Transformed Landscape
As the ice sheet continued to melt and retreat inland, the water forced its way through the land to the ocean, causing severe eroding.  In our area, the best example of this is Mianus River Gorge. The force of the water also rerouted the Mianus River into Cos Cob, to empty into the Long Island Sound.  Sediment carried by melting water filled the Long Island Sound basin with a 300 foot thick layer. This is why the Long Island Sound is relatively shallow, averaging only 65 feet in depth. Sediment also covered the piles of stones and boulders left behind in this area, setting the stage for vegetation, forests, fertile soil, and eventually humans.

Ice Sheet Dramatically Affected Sea Level
Much of the water from the Atlantic Ocean was taken up by the ice sheet to make ice. As a result, the sea level was significantly reduced—300 feet lower than it is today! The Continental Shelf was even exposed.  As the climate warmed and the ice sheet melted, the water went back into the ocean causing the sea level to rise. As a result, the Atlantic Ocean eventually flooded into the Long Island Sound basin giving Connecticut its distinctive "drowned" coastline, and it also submerged most of the terminal moraine ridge off the coast of Rhode Island up to Cape Cod. Although the sea level has continued to rise slightly over time, it still has remained close to the level since the time the ice sheet finished melting in the northeast 8,000 years ago.

Post-Glacial Times  
The post-glacial history of this area is equally fascinating. It explains why we have those "New England Potatoes" that became the building material of the old stone walls. We plan to do a special focus on local stone walls (the old ones), with some spectacular examples located on Greenwich Land Trust properties. 

We invite you to experience the beautiful natural landscapes of Greenwich in person by becoming a member of the Land Trust and volunteering or participating in our activities. We welcome one and all. Please join today!

Historic Stone Walls Page
Photographs of GLT's Historic Stone Wall Event
Donate to GLT today!

Sources: The research for this geological history of Greenwich was based on a variety of sources, both oral and written. Few people know more about the natural history of this area than our Stewardship Director and environmental science expert, Dan Barrett, and he has been one of the most inspiring sources.  When Dan leads a team of volunteers through a Land Trust property on a stewardship mission, it very often turns into a captivating natural history lesson. Among many written sources about geological history, two in particular provided the most complete information about Connecticut specifically: a book, Roadside Geology of Connecticut and Rhode Island by James W. Skehan (2008) and a booklet, The Face of Connecticut -People, Geology and The Land by Michael Bell (1997) which is available free on line Other excellent source material about Connecticut's geological history is available on line through the Connecticut Department of Environmental Protection.  National Geographic Magazine provided outstanding information about the ice sheets in Greenland and Antarctica especially in an article, The Big Thaw (June 2007 Issue).